Water and Solutions Chapter 23 Solutions
Transcript Of Water and Solutions Chapter 23 Solutions
8
Water and Solutions
Introduction to Chapter 23
Many of the foods you eat and the products you use (like shampoo) are solutions or other types of mixtures. In this chapter you will learn about solutions and solubility. Since many solutions are critical to the human body, you will also learn how solutions affect health and athletic performance.
23.1 What is a Solution?
Investigations for Chapter 23
Can you identify mixtures as solutions, suspensions, or colloids?
In this Investigation you will construct an apparatus to view the Tyndall effect. The Tyndall effect is a test for determining the characteristics of a mixture. Your tests will tell you whether the mixture is a solution, colloid, or suspension.
23.2 Dissolving Rate
How can you influence dissolving rates?
In this Investigation you will design three methods for dissolving rock salt in water and calculate the dissolving rates for each method. Your tests will determine the effect of changing conditions on solubility, such as heat and stirring.
23.3 Solubility
What factors affect solubility?
When you want to dissolve sugar in water, it helps to heat things up. In this Investigation, you will observe how temperature influences how fast a substance dissolves. Using your observations, you will develop an explanation for how temperature affects solubility. In addition, using carbonated water and a balloon, you will have the opportunity to explore how pressure affects the solubility of a gas in a liquid.
Chapter 23
Solutions
393
Chapter 23: Solutions
Learning Goals
In this chapter, you will: D Categorize mixtures as solutions, suspensions, or colloids. D Define solubility. D Describe saturated, unsaturated, and supersaturated solutions. D Define and calculate dissolving rate. D List factors which influence dissolving rate. D Evaluate the effectiveness of different methods of influencing dissolving rates. D Explain how temperature and pressure influence solubility. D Understand solubility values. D Interpret temperature-solubility graphs.
Vocabulary
alloys atmospheres colloid dissolved dissolving rate
equilibrium hydrated saturated solubility
solubility value solutes solution solvent
supersaturated system Tyndall effect unsaturated
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23.1 What is a Solution?
If you walk down the beverage aisle of your local grocery store, you might be surprised by the many different ways water is bottled for sale. You might see mineral water, spring water, potable water, distilled water, and carbonated (or seltzer) water. To complicate matters more, there may be several brands of each kind of water to choose from! Is there truly any difference between them?
What is in bottled water?
Bottled water contains more than pure H20 Mineral water contains naturally present minerals
Potable water contains additives
Distilled water is nearly
mineral-free
Carbonated water
The types of bottled water mentioned above have unique characteristics. Mineral water, potable water (that is, water suitable for drinking), and carbonated water contain elements other than pure H20.
Mineral water, according to government regulations, must contain at least 250 milligrams per liter of dissolved minerals such as calcium, potassium, and magnesium. The minerals must be present naturally and cannot be added at the bottling plant to make mineral water. Also, the water must be collected from an underground source.
Potable water is bottled from a city or town water source. In addition to minerals, potable water may contain sodium fluoride, chlorine, or other additives. Consumers sometimes purchase this water if they know their home plumbing contains lead pipes or lead solder and they want to avoid ingesting any traces of lead that might be present in their tap water.
Distilled water has had most of the minerals removed. First, the water is boiled. The minerals are left behind when the water molecules enter the gas phase. The water vapor is then collected and cooled to room temperature so that it exists once again as a liquid. Sometimes the water is further purified by processes called deionization and reverse osmosis. Salt water is converted to fresh water using these processes. However, even treated water contains traces of other elements.
Carbonated water contains carbon dioxide gas evenly distributed throughout the liquid to make it bubbly.
Figure 23.1: The variety of
containers for bottled water. Spring water and seltzer are often purchased for drinking. Distilled water is used in steam irons and for performing experiments in the laboratory. Why would distilled water be a good choice for these uses?
23.1 What is a Solution?
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Chapter 23
Types of bottled water are examples of solutions
A solution is homogeneous at
the molecular level
An alloy is a solution of two
or more metals
A solution is a mixture of solute
dissolved in a solvent
In chemistry terms, we call mineral water, potable water, tap water, carbonated water, and even distilled water solutions. A solution is a mixture of two or more substances that is homogeneous at the molecular level. The word homogeneous means the particles in the water are evenly distributed. For example, in mineral water, there are no clumps of hundreds of mineral ions. The particles in a solution exist as individual atoms, ions, or molecules. Each has a diameter between 0.01 and 1.0 nanometer.
Although we often think of solutions as mixtures of solids in liquids, solutions exist in every phase, be it solid, liquid, or gas. Carbonated water is a solution of a gas in a liquid. Fourteen-karat gold is a solution of two solids, silver and gold. “Fourteen-karat” means that 14 out of every 24 atoms in the solution are gold atoms. Likewise, ten-karat means that 10 out of every 24 atoms in the solution are gold. Solutions of two or more metals are called alloys.
Scientists generally refer to the component of the mixture that is present in the greatest amount as the solvent. The remaining components are called the solutes.
When the solute particles are evenly distributed throughout the solvent, we say that the solute has dissolved.
Table 23.1: Different solutions
Solution
Solvent
Solute(s) State of solution
air
nitrogen (gas) O2, CO2, He, H2, gas
H20, etc. (gases)
carbonated water water (liquid) CO2 (gas)
liquid
saline solution water (liquid) salt (solid)
liquid
rubbing alcohol alcohol (liquid) water (liquid) liquid
sterling silver silver (solid) copper (solid) solid
What is a nanometer? A nanometer is one-billionth of a meter. It is represented by writing “nm.” In addition to particles, wavelengths of light are measured in nanometers. For example, the range of wavelengths of visible light is 400 to 700 nm.
Figure 23.2: Solutions are made
when solutes dissolve in solvents. Here, salt is the solute, and water is the solvent.
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Colloids and suspensions
Colloid particles are larger than those in true solutions, but
smaller than those in suspensions
Mixtures such as mayonnaise, egg whites, and gelatin are colloids. They look like solutions, but the particles in these mixtures, at one to 1,000 nanometers, are larger than those found in solutions. True solutions contain single atoms and molecules (less than 1 nanometer in size). By comparison, colloid particles are formed of clusters of atoms or molecules. Nevertheless, colloid particles are too small (1-1,000 nanometers) to settle to the bottom of their container. Instead, they stay evenly distributed throughout the mixture because they are constantly tossed about by the movement of the liquid particles.
Suspensions settle upon standing
You may have noticed that when you step into a pond or lake to go swimming, you suddenly make the water cloudy. Your feet cause the mud on the bottom of the pond or lake to mix with the water. However, if you stand very still, eventually the water becomes clear again. This is because the individual mud particles sink. In suspensions like muddy water, the particles are greater than 1,000 nanometers in diameter. Atoms and molecules are much smaller than 100 nanometers. Suspensions are mixtures that settle upon standing (figure 23.3). Suspensions can be separated by filtering.
The Tyndall effect
It isn’t possible to separate colloids by filtering. However, there is a way to visually distinguish colloids from true solutions. It is called the Tyndall effect. If you shine a flashlight through a jar of a translucent colloid, the particles scatter the light, making the beam visible. Fog is an example of a colloid. This is why an automobile’s headlight beams can be seen on a foggy evening.
Table 23.2: Properties of solutions, suspensions, and colloids
Approximate size Solute particles
of solute particles
settle
Can be separated by filtering
Particles scatter light
solutions 0.01 - 1.0 nm
no
no
no
colloids 1.0 - 1,000 nm no
only with special equipment yes, if transparent
suspensions >1000 nm
with time
yes
yes, if transparent
Figure 23.3: Mayonnaise is a
colloid. Water and silt make a suspension.
Figure 23.4: The Tyndall effect
helps you tell the difference between colloids and solutions. Here, the beam of the flashlight is visible as it shines through the colloid in the beaker. The beam would not be visible if the beaker contained a solution.
23.1 What is a Solution?
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Chapter 23
23.2 Dissolving Rate
On long backpacking trips, hikers must make sure that they have a safe, reliable source of drinking water. Drinking from an icy cold mountain stream may seem appealing to a hot, tired backpacker, but it could bring the trip to an unpleasant end. Most streams, rivers, and lakes in the United States contain Giardia and other microorganisms that can cause serious intestinal disturbances.
Consequently, wise backpackers always carry water-treatment supplies. One of the safest and least expensive ways to purify the water is to add an iodine tablet. Iodine tablets are effective against many microorganisms, bacteria, and viruses. These tablets are especially useful because they add so little weight to the pack and can be used with refreshingly cold water.
What could a thirsty backpacker do to get an iodine tablet to dissolve faster? In this section, you will learn about factors that affect the dissolving rate of various substances.
Molecular motion and dissolving rate
Stirring a mixture speeds dissolving
Stirring exposes fresh solvent to newly exposed
solute
One of the simplest ways to increase the dissolving rate of the iodine tablet is to stir the water or shake the water bottle. To understand why this method works, we need to take a look at what is happening on a molecular level.
A solute such as iodine can dissolve when solvent molecules collide with clumps of solute particles. In this case, water molecules collide with iodine ions in the tablet. Stirring helps this process by increasing the rate of the collisions. The result of collisions between molecules is that solute particles are “pulled” into solution and surrounded by solvent molecules.
Stirring a solution does more that just increase collisions between solute and solvent molecules. Stirring moves the molecules around. Solvent molecules that have collided with the solute have to get out of the way so that new ones can collide with the exposed surface of the solute.
Figure 23.5: Water from lakes,
rivers, and streams must be treated to make it safe to drink. A microscopic view of two Giardia in water is shown in the upper left corner of the figure. Giardia is a parasite that lives in the small intestine of wild animals. You can become infected with Giardia if you drink untreated water that contains infective cysts, a life stage of Giardia that permits this parasite to spread from animal to animal or from animals to humans.
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Surface area and dissolving rate
Crushing a solute tablet increases the surface area exposed to the solvent
If the hiker were to crush the iodine tablet, the dissolving rate would increase dramatically. When the tablet is whole, many billions of iodine atoms remain completely surrounded by other iodine atoms, but the atoms inside the tablet are protected from the water molecules. Crushing the tablet increases the surface area that is available for solvent molecules to interact with solute molecules.
Here is a 1-centimeter cube. This means that each edge of the cube is 1 centimeter wide. The area of each face of the cube is 1 cm by 1 cm or 1 cm2. Cubes have six faces. Therefore, the total surface area of the cube is: 6 × 1 cm2 = 6 cm2.
Every time the piece is divided the surface area
increases
What do you think happens to surface area if we cut the cube in half? Now, in addition to the original 6 cm2, you have added two additional faces of 1 cm2 each, for a total surface area of 8cm2.
Cut the halves of the cube in half, and you have four new faces for a total surface area of 12 cm2.
Stack up the four pieces and make a vertical cut down the center. You have added two new faces, for a total surface area of 14 cm2.
Cut each of your new stacks in half, to add four more faces. Now you have 18 cm2, or three times your original surface area. Imagine how much greater the surface area would be if you crushed the cube into a powder!
Disinfecting with iodine
Iodine, like all halogens (fluorine, chlorine, and bromine), has seven valence electrons. It reacts with substances to gain an electron and satisfy the “octet rule”— the need for eight electrons in the outermost energy level of an atom.
Halogens are so reactive that
they can be used as
disinfectants. However, of the
halogens, iodine (the only one
existing as a solid at room
temperature) and chlorine (in
combination with other
elements) are the most easily
used in this way. In the
proper
concentrations,
chlorine and iodine tablets
dissolved in water can kill
microorganisms like Giardia
without being toxic to the
person drinking the water.
Fluorine and bromine are not useful for sterilizing water. Fluorine is a highly reactive gas, and bromine is a liquid that is very damaging to skin.
23.2 Dissolving Rate
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Chapter 23
Timed-release capsules: making less medicine more effective
Timed-release medicine
What is microencapsulation?
Timed-release capsules mean that
you can take less medicine to feel
better
Timed-release capsules are
safer and more cost-effective
Have you ever taken cold or allergy medicine in the form of a clear capsule with multi-colored round beads inside? If so, you are familiar with timed-release medicine. Understanding dissolving rates made the invention of this type of medicine possible.
There are several ways to manufacture timed-release medication. One of the most common is called microencapsulation. Using this method, pharmaceutical manufacturers divide a dose of medicine into tiny particles. Some of these particles are placed into the capsule unchanged. These particles of medicine become active in the patient’s body soon after the capsule is swallowed. The remaining particles are coated with a polymer usually derived from gelatin, cellulose, or silicone. The coating dissolves slowly in the stomach, releasing the medicine inside over a period of time. By changing the thickness or varying the coating material, manufacturers alter the dissolving rate of the coating. This allows them to control the amount of time it takes for the medicine to be released in the body. The different colored beads inside the capsule show the different coatings used to encapsulate the medicine particles.
Timed-release capsules have several advantages. First, they make smaller doses of medicine more effective. Previously, when a patient swallowed a dose of medicine, there would be a peak level in the body followed by a steady decline until the next pill was swallowed. To maintain the minimum effective amount of the medicine in the body at all times, the peak level had to be significantly higher than the ideal dose. This meant the patient had to consume larger quantities of the medicine, thereby increasing the risk of side effects. Timed-release capsules help to ensure a steady supply of the minimum effective dose.
Timed-release capsules are safer and more convenient for the patient. For example, a patient who previously had to take a pill four times a day might switch to a once-a-day, timed-release capsule. Taking only one capsule reduces the chances of forgetting how much medicine you have taken.
Figure 23.6: Grinding substances
to make medicines is an ancient practice that is still used today. Powdered substances dissolve quickly.
Figure 23.7: Timed-release
capsules contain microencapsulated medicine that appear as tiny beads within the capsule. The different dissolving rates of the beads allow you to take less medicine to feel better.
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Chapter 23
23.3 Solubility
The human body is mostly water. For every hour of vigorous exercise, you may lose as much as a halfgallon of your body’s water supply through sweating and exhaling! You also lose small amounts of salts, lactic acid, and urea when you sweat. Lactic acid and urea are the breakdown products of sugar and proteins, respectively. The more you exercise, the more water and salts you lose, and the more you break down sugar and protein. You can replenish lost fluid by drinking water. To quickly replace salts and sugar as well, many athletes consume sports drinks. Sweat and sports drinks are both examples of solutions—both are mostly water with dissolved substances. In this section, you will learn about the factors that affect how solutes dissolve in solutions.
Using systems to talk about solutions
Systems are collections of
matter and processes that can
be studied
Systems can be open or closed
A solute and a solvent make up a
system
Your body is a system. A system is a collection of matter and processes that take place in a certain space and can be observed and studied. Your body with its numerous metabolic activities is an excellent example of a system that is “open.” This means that your body constantly interacts with its environment by taking in and releasing substances. For example, you eat food, exhale carbon dioxide, and give off heat.
Scientists often find open systems difficult to work with because it is hard to control the variables in open systems. Scientists have an easier time studying closed or nearly closed systems. A reaction that takes place in a stoppered test tube is a good example of a closed system.
The system of a solution includes the solute and the solvent. The kind of container that holds the solute and solvent is not important. For the rest of this section, the factors that affect the system of a solute and a solvent will be discussed.
Figure 23.8: With vigorous
exercise, you can lose up to a half-
gallon of water per hour by sweating
and exhaling.
23.3 Solubility
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Chapter 23
What happens when a solute dissolves in a solvent?
NaCl is an example of
a solute
The bond between Na and Cl is ionic
Water molecules bond with each other
Water molecules hydrate Na and Cl
Hydration continues until the
solution is saturated
Let’s use sodium chloride (NaCl) as an example of a solute being dissolved in the solvent, water. If you look closely at a single crystal of NaCl, you will notice that it is a cube. Millions of sodium (Na) and chlorine (Cl) atoms, each too small to see with your naked eye, are a part of a single crystal of NaCl.
Within a crystal of NaCl, ionic bonds are formed between Na and Cl. The oxidation state of Na is (1+), and the oxidation state of Cl is (1-). Na and Cl are a good match because their oxidation states add up to zero: (1+) + (1-) = 0. This means that a single NaCl molecule is neutral and stable. However, the Cl end of the molecule is more negatively charged than the Na end because the Cl has attracts an electron away from Na to form the ionic bond.
Although a water molecule does not have ionic bonds, the molecule does have a partially charged positive end and a partially charged negative end. Water molecules weakly connect to each other by matching their partial positive end to the partial negative end of a neighboring molecule. These links between water molecules are called hydrogen bonds.
When a NaCl crystal is mixed with water, a reaction occurs. The partially charged ends of the water molecules are attracted to Na and Cl in the crystal. The process results in the formation of Na+ and Cl- ions that are completely surrounded by water molecules. When this happens, we say the ions are hydrated and write “aq” next to the ions. “Aq” stands for aqueous which refers to a water solution. Hydrated Na(aq)+ and Cl(aq)- ions are able to freely move in a solution.
As one layer of molecules on a NaCl crystal is brought into solution by water molecules, another layer is exposed. If the conditions are right, this process continues until the entire crystal is dissolved. It seems, in fact, to have disappeared! All that has happened, however, is that Na+ and Cl- ions from NaCl have been separated and completely surrounded by water molecules.
Figure 23.9: When a crystal of
NaCl molecules is mixed with water,
hydrated Na+ and Cl- ions are
formed.
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Water and Solutions
Introduction to Chapter 23
Many of the foods you eat and the products you use (like shampoo) are solutions or other types of mixtures. In this chapter you will learn about solutions and solubility. Since many solutions are critical to the human body, you will also learn how solutions affect health and athletic performance.
23.1 What is a Solution?
Investigations for Chapter 23
Can you identify mixtures as solutions, suspensions, or colloids?
In this Investigation you will construct an apparatus to view the Tyndall effect. The Tyndall effect is a test for determining the characteristics of a mixture. Your tests will tell you whether the mixture is a solution, colloid, or suspension.
23.2 Dissolving Rate
How can you influence dissolving rates?
In this Investigation you will design three methods for dissolving rock salt in water and calculate the dissolving rates for each method. Your tests will determine the effect of changing conditions on solubility, such as heat and stirring.
23.3 Solubility
What factors affect solubility?
When you want to dissolve sugar in water, it helps to heat things up. In this Investigation, you will observe how temperature influences how fast a substance dissolves. Using your observations, you will develop an explanation for how temperature affects solubility. In addition, using carbonated water and a balloon, you will have the opportunity to explore how pressure affects the solubility of a gas in a liquid.
Chapter 23
Solutions
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Chapter 23: Solutions
Learning Goals
In this chapter, you will: D Categorize mixtures as solutions, suspensions, or colloids. D Define solubility. D Describe saturated, unsaturated, and supersaturated solutions. D Define and calculate dissolving rate. D List factors which influence dissolving rate. D Evaluate the effectiveness of different methods of influencing dissolving rates. D Explain how temperature and pressure influence solubility. D Understand solubility values. D Interpret temperature-solubility graphs.
Vocabulary
alloys atmospheres colloid dissolved dissolving rate
equilibrium hydrated saturated solubility
solubility value solutes solution solvent
supersaturated system Tyndall effect unsaturated
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Chapter 23
23.1 What is a Solution?
If you walk down the beverage aisle of your local grocery store, you might be surprised by the many different ways water is bottled for sale. You might see mineral water, spring water, potable water, distilled water, and carbonated (or seltzer) water. To complicate matters more, there may be several brands of each kind of water to choose from! Is there truly any difference between them?
What is in bottled water?
Bottled water contains more than pure H20 Mineral water contains naturally present minerals
Potable water contains additives
Distilled water is nearly
mineral-free
Carbonated water
The types of bottled water mentioned above have unique characteristics. Mineral water, potable water (that is, water suitable for drinking), and carbonated water contain elements other than pure H20.
Mineral water, according to government regulations, must contain at least 250 milligrams per liter of dissolved minerals such as calcium, potassium, and magnesium. The minerals must be present naturally and cannot be added at the bottling plant to make mineral water. Also, the water must be collected from an underground source.
Potable water is bottled from a city or town water source. In addition to minerals, potable water may contain sodium fluoride, chlorine, or other additives. Consumers sometimes purchase this water if they know their home plumbing contains lead pipes or lead solder and they want to avoid ingesting any traces of lead that might be present in their tap water.
Distilled water has had most of the minerals removed. First, the water is boiled. The minerals are left behind when the water molecules enter the gas phase. The water vapor is then collected and cooled to room temperature so that it exists once again as a liquid. Sometimes the water is further purified by processes called deionization and reverse osmosis. Salt water is converted to fresh water using these processes. However, even treated water contains traces of other elements.
Carbonated water contains carbon dioxide gas evenly distributed throughout the liquid to make it bubbly.
Figure 23.1: The variety of
containers for bottled water. Spring water and seltzer are often purchased for drinking. Distilled water is used in steam irons and for performing experiments in the laboratory. Why would distilled water be a good choice for these uses?
23.1 What is a Solution?
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Chapter 23
Types of bottled water are examples of solutions
A solution is homogeneous at
the molecular level
An alloy is a solution of two
or more metals
A solution is a mixture of solute
dissolved in a solvent
In chemistry terms, we call mineral water, potable water, tap water, carbonated water, and even distilled water solutions. A solution is a mixture of two or more substances that is homogeneous at the molecular level. The word homogeneous means the particles in the water are evenly distributed. For example, in mineral water, there are no clumps of hundreds of mineral ions. The particles in a solution exist as individual atoms, ions, or molecules. Each has a diameter between 0.01 and 1.0 nanometer.
Although we often think of solutions as mixtures of solids in liquids, solutions exist in every phase, be it solid, liquid, or gas. Carbonated water is a solution of a gas in a liquid. Fourteen-karat gold is a solution of two solids, silver and gold. “Fourteen-karat” means that 14 out of every 24 atoms in the solution are gold atoms. Likewise, ten-karat means that 10 out of every 24 atoms in the solution are gold. Solutions of two or more metals are called alloys.
Scientists generally refer to the component of the mixture that is present in the greatest amount as the solvent. The remaining components are called the solutes.
When the solute particles are evenly distributed throughout the solvent, we say that the solute has dissolved.
Table 23.1: Different solutions
Solution
Solvent
Solute(s) State of solution
air
nitrogen (gas) O2, CO2, He, H2, gas
H20, etc. (gases)
carbonated water water (liquid) CO2 (gas)
liquid
saline solution water (liquid) salt (solid)
liquid
rubbing alcohol alcohol (liquid) water (liquid) liquid
sterling silver silver (solid) copper (solid) solid
What is a nanometer? A nanometer is one-billionth of a meter. It is represented by writing “nm.” In addition to particles, wavelengths of light are measured in nanometers. For example, the range of wavelengths of visible light is 400 to 700 nm.
Figure 23.2: Solutions are made
when solutes dissolve in solvents. Here, salt is the solute, and water is the solvent.
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Chapter 23
Colloids and suspensions
Colloid particles are larger than those in true solutions, but
smaller than those in suspensions
Mixtures such as mayonnaise, egg whites, and gelatin are colloids. They look like solutions, but the particles in these mixtures, at one to 1,000 nanometers, are larger than those found in solutions. True solutions contain single atoms and molecules (less than 1 nanometer in size). By comparison, colloid particles are formed of clusters of atoms or molecules. Nevertheless, colloid particles are too small (1-1,000 nanometers) to settle to the bottom of their container. Instead, they stay evenly distributed throughout the mixture because they are constantly tossed about by the movement of the liquid particles.
Suspensions settle upon standing
You may have noticed that when you step into a pond or lake to go swimming, you suddenly make the water cloudy. Your feet cause the mud on the bottom of the pond or lake to mix with the water. However, if you stand very still, eventually the water becomes clear again. This is because the individual mud particles sink. In suspensions like muddy water, the particles are greater than 1,000 nanometers in diameter. Atoms and molecules are much smaller than 100 nanometers. Suspensions are mixtures that settle upon standing (figure 23.3). Suspensions can be separated by filtering.
The Tyndall effect
It isn’t possible to separate colloids by filtering. However, there is a way to visually distinguish colloids from true solutions. It is called the Tyndall effect. If you shine a flashlight through a jar of a translucent colloid, the particles scatter the light, making the beam visible. Fog is an example of a colloid. This is why an automobile’s headlight beams can be seen on a foggy evening.
Table 23.2: Properties of solutions, suspensions, and colloids
Approximate size Solute particles
of solute particles
settle
Can be separated by filtering
Particles scatter light
solutions 0.01 - 1.0 nm
no
no
no
colloids 1.0 - 1,000 nm no
only with special equipment yes, if transparent
suspensions >1000 nm
with time
yes
yes, if transparent
Figure 23.3: Mayonnaise is a
colloid. Water and silt make a suspension.
Figure 23.4: The Tyndall effect
helps you tell the difference between colloids and solutions. Here, the beam of the flashlight is visible as it shines through the colloid in the beaker. The beam would not be visible if the beaker contained a solution.
23.1 What is a Solution?
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Chapter 23
23.2 Dissolving Rate
On long backpacking trips, hikers must make sure that they have a safe, reliable source of drinking water. Drinking from an icy cold mountain stream may seem appealing to a hot, tired backpacker, but it could bring the trip to an unpleasant end. Most streams, rivers, and lakes in the United States contain Giardia and other microorganisms that can cause serious intestinal disturbances.
Consequently, wise backpackers always carry water-treatment supplies. One of the safest and least expensive ways to purify the water is to add an iodine tablet. Iodine tablets are effective against many microorganisms, bacteria, and viruses. These tablets are especially useful because they add so little weight to the pack and can be used with refreshingly cold water.
What could a thirsty backpacker do to get an iodine tablet to dissolve faster? In this section, you will learn about factors that affect the dissolving rate of various substances.
Molecular motion and dissolving rate
Stirring a mixture speeds dissolving
Stirring exposes fresh solvent to newly exposed
solute
One of the simplest ways to increase the dissolving rate of the iodine tablet is to stir the water or shake the water bottle. To understand why this method works, we need to take a look at what is happening on a molecular level.
A solute such as iodine can dissolve when solvent molecules collide with clumps of solute particles. In this case, water molecules collide with iodine ions in the tablet. Stirring helps this process by increasing the rate of the collisions. The result of collisions between molecules is that solute particles are “pulled” into solution and surrounded by solvent molecules.
Stirring a solution does more that just increase collisions between solute and solvent molecules. Stirring moves the molecules around. Solvent molecules that have collided with the solute have to get out of the way so that new ones can collide with the exposed surface of the solute.
Figure 23.5: Water from lakes,
rivers, and streams must be treated to make it safe to drink. A microscopic view of two Giardia in water is shown in the upper left corner of the figure. Giardia is a parasite that lives in the small intestine of wild animals. You can become infected with Giardia if you drink untreated water that contains infective cysts, a life stage of Giardia that permits this parasite to spread from animal to animal or from animals to humans.
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Chapter 23
Surface area and dissolving rate
Crushing a solute tablet increases the surface area exposed to the solvent
If the hiker were to crush the iodine tablet, the dissolving rate would increase dramatically. When the tablet is whole, many billions of iodine atoms remain completely surrounded by other iodine atoms, but the atoms inside the tablet are protected from the water molecules. Crushing the tablet increases the surface area that is available for solvent molecules to interact with solute molecules.
Here is a 1-centimeter cube. This means that each edge of the cube is 1 centimeter wide. The area of each face of the cube is 1 cm by 1 cm or 1 cm2. Cubes have six faces. Therefore, the total surface area of the cube is: 6 × 1 cm2 = 6 cm2.
Every time the piece is divided the surface area
increases
What do you think happens to surface area if we cut the cube in half? Now, in addition to the original 6 cm2, you have added two additional faces of 1 cm2 each, for a total surface area of 8cm2.
Cut the halves of the cube in half, and you have four new faces for a total surface area of 12 cm2.
Stack up the four pieces and make a vertical cut down the center. You have added two new faces, for a total surface area of 14 cm2.
Cut each of your new stacks in half, to add four more faces. Now you have 18 cm2, or three times your original surface area. Imagine how much greater the surface area would be if you crushed the cube into a powder!
Disinfecting with iodine
Iodine, like all halogens (fluorine, chlorine, and bromine), has seven valence electrons. It reacts with substances to gain an electron and satisfy the “octet rule”— the need for eight electrons in the outermost energy level of an atom.
Halogens are so reactive that
they can be used as
disinfectants. However, of the
halogens, iodine (the only one
existing as a solid at room
temperature) and chlorine (in
combination with other
elements) are the most easily
used in this way. In the
proper
concentrations,
chlorine and iodine tablets
dissolved in water can kill
microorganisms like Giardia
without being toxic to the
person drinking the water.
Fluorine and bromine are not useful for sterilizing water. Fluorine is a highly reactive gas, and bromine is a liquid that is very damaging to skin.
23.2 Dissolving Rate
399
Chapter 23
Timed-release capsules: making less medicine more effective
Timed-release medicine
What is microencapsulation?
Timed-release capsules mean that
you can take less medicine to feel
better
Timed-release capsules are
safer and more cost-effective
Have you ever taken cold or allergy medicine in the form of a clear capsule with multi-colored round beads inside? If so, you are familiar with timed-release medicine. Understanding dissolving rates made the invention of this type of medicine possible.
There are several ways to manufacture timed-release medication. One of the most common is called microencapsulation. Using this method, pharmaceutical manufacturers divide a dose of medicine into tiny particles. Some of these particles are placed into the capsule unchanged. These particles of medicine become active in the patient’s body soon after the capsule is swallowed. The remaining particles are coated with a polymer usually derived from gelatin, cellulose, or silicone. The coating dissolves slowly in the stomach, releasing the medicine inside over a period of time. By changing the thickness or varying the coating material, manufacturers alter the dissolving rate of the coating. This allows them to control the amount of time it takes for the medicine to be released in the body. The different colored beads inside the capsule show the different coatings used to encapsulate the medicine particles.
Timed-release capsules have several advantages. First, they make smaller doses of medicine more effective. Previously, when a patient swallowed a dose of medicine, there would be a peak level in the body followed by a steady decline until the next pill was swallowed. To maintain the minimum effective amount of the medicine in the body at all times, the peak level had to be significantly higher than the ideal dose. This meant the patient had to consume larger quantities of the medicine, thereby increasing the risk of side effects. Timed-release capsules help to ensure a steady supply of the minimum effective dose.
Timed-release capsules are safer and more convenient for the patient. For example, a patient who previously had to take a pill four times a day might switch to a once-a-day, timed-release capsule. Taking only one capsule reduces the chances of forgetting how much medicine you have taken.
Figure 23.6: Grinding substances
to make medicines is an ancient practice that is still used today. Powdered substances dissolve quickly.
Figure 23.7: Timed-release
capsules contain microencapsulated medicine that appear as tiny beads within the capsule. The different dissolving rates of the beads allow you to take less medicine to feel better.
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Chapter 23
23.3 Solubility
The human body is mostly water. For every hour of vigorous exercise, you may lose as much as a halfgallon of your body’s water supply through sweating and exhaling! You also lose small amounts of salts, lactic acid, and urea when you sweat. Lactic acid and urea are the breakdown products of sugar and proteins, respectively. The more you exercise, the more water and salts you lose, and the more you break down sugar and protein. You can replenish lost fluid by drinking water. To quickly replace salts and sugar as well, many athletes consume sports drinks. Sweat and sports drinks are both examples of solutions—both are mostly water with dissolved substances. In this section, you will learn about the factors that affect how solutes dissolve in solutions.
Using systems to talk about solutions
Systems are collections of
matter and processes that can
be studied
Systems can be open or closed
A solute and a solvent make up a
system
Your body is a system. A system is a collection of matter and processes that take place in a certain space and can be observed and studied. Your body with its numerous metabolic activities is an excellent example of a system that is “open.” This means that your body constantly interacts with its environment by taking in and releasing substances. For example, you eat food, exhale carbon dioxide, and give off heat.
Scientists often find open systems difficult to work with because it is hard to control the variables in open systems. Scientists have an easier time studying closed or nearly closed systems. A reaction that takes place in a stoppered test tube is a good example of a closed system.
The system of a solution includes the solute and the solvent. The kind of container that holds the solute and solvent is not important. For the rest of this section, the factors that affect the system of a solute and a solvent will be discussed.
Figure 23.8: With vigorous
exercise, you can lose up to a half-
gallon of water per hour by sweating
and exhaling.
23.3 Solubility
401
Chapter 23
What happens when a solute dissolves in a solvent?
NaCl is an example of
a solute
The bond between Na and Cl is ionic
Water molecules bond with each other
Water molecules hydrate Na and Cl
Hydration continues until the
solution is saturated
Let’s use sodium chloride (NaCl) as an example of a solute being dissolved in the solvent, water. If you look closely at a single crystal of NaCl, you will notice that it is a cube. Millions of sodium (Na) and chlorine (Cl) atoms, each too small to see with your naked eye, are a part of a single crystal of NaCl.
Within a crystal of NaCl, ionic bonds are formed between Na and Cl. The oxidation state of Na is (1+), and the oxidation state of Cl is (1-). Na and Cl are a good match because their oxidation states add up to zero: (1+) + (1-) = 0. This means that a single NaCl molecule is neutral and stable. However, the Cl end of the molecule is more negatively charged than the Na end because the Cl has attracts an electron away from Na to form the ionic bond.
Although a water molecule does not have ionic bonds, the molecule does have a partially charged positive end and a partially charged negative end. Water molecules weakly connect to each other by matching their partial positive end to the partial negative end of a neighboring molecule. These links between water molecules are called hydrogen bonds.
When a NaCl crystal is mixed with water, a reaction occurs. The partially charged ends of the water molecules are attracted to Na and Cl in the crystal. The process results in the formation of Na+ and Cl- ions that are completely surrounded by water molecules. When this happens, we say the ions are hydrated and write “aq” next to the ions. “Aq” stands for aqueous which refers to a water solution. Hydrated Na(aq)+ and Cl(aq)- ions are able to freely move in a solution.
As one layer of molecules on a NaCl crystal is brought into solution by water molecules, another layer is exposed. If the conditions are right, this process continues until the entire crystal is dissolved. It seems, in fact, to have disappeared! All that has happened, however, is that Na+ and Cl- ions from NaCl have been separated and completely surrounded by water molecules.
Figure 23.9: When a crystal of
NaCl molecules is mixed with water,
hydrated Na+ and Cl- ions are
formed.
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