Factors That Affect Solubility

Transcription

1 Imagine you are making lemonade by stirring lemonade mix into a glass of water. One spoonful of mix would likely dissolve easily in the water, producing a solution. (A solution is a mixture of two or more substances spread evenly together.) What would happen if you added another spoonful of mix to the solution? Depending on how much water is in the glass, the additional mix may take longer to dissolve. If you add another spoonful or two, you would reach the point where the lemonade mix no longer dissolves in the water. The lemonade mix that does not dissolve settles to the bottom of the glass. Why is there a limit to how much lemonade mix dissolves in water? Water is one of the most abundant liquids on Earth. It is also one of the best solvents. (A solvent is a substance in which another substance dissolves.) Water dissolves many different types of substances. Solutions in which water is used as the solvent are known as aqueous solutions. Why is water such an effective solvent? An important reason is that water molecules are polar. In other words, they have opposite, partial charges. A water molecule (H 2 O) consists of two hydrogen atoms (H), each of which shares its electron with a central oxygen atom (O). The shared electrons are attracted more to the oxygen atom, so it acquires a partial negative charge. Meanwhile, the hydrogen atoms acquire partial positive charges. What does this have to do with solubility? When other polar molecules or ionic compounds are mixed with water, their positively charged components are attracted to the oxygen atoms in water molecules, and their negatively charged components are attracted to the hydrogen atoms in water molecules. (Ions are particles with electric charges.) The chemical bonds joining some compounds are strong enough to withstand the attractive forces from water molecules. Other compounds, however, are broken apart. This is what happens when a substance dissolves. In contrast, nonpolar substances such as vegetable oil do not dissolve in water. Instead, the denser substance sinks beneath the less dense substance. A water molecule is polar. The oxygen atom (red) has a partial negative charge (δ ), and each hydrogen atom (white) has a partial positive charge (δ+). Ionic compound: a compound formed between a negatively charged ion and a positively charged ion 1

2 The temperature of a solution also affects the solubility of a solute. Typically, at higher temperatures, the solubility of solid and liquid solutes increases. You can dissolve more lemonade mix in hot water than in cold water. In contrast, at higher temperatures gases are less soluble in liquids. If you were to leave a can of soda in a car on a very warm day, it might explode. The soda is a mixture of gas (carbon dioxide) and liquid (water). At warmer temperatures the gas particles move more quickly, so it is easier for them to escape from solution. As they collide with greater force against the walls of the can, pressure builds inside the can. Eventually, the pressure may cause the can to explode. Changes in pressure do not drastically affect the solubilities of solids and liquids. However, pressure changes can have an enormous effect on the solubilities of gases in liquids. At high pressures, gases are more soluble than at low pressures. This is a serious concern for scuba divers. Air is a mixture of gases primarily nitrogen and oxygen. As we breathe air, these gases are dissolved in our blood. Deep underwater, the pressure is much greater than at the surface, so a scuba diver s blood contains more dissolved nitrogen and oxygen. The body cannot use the extra nitrogen, so as the diver begins to ascend, the extra gas is forced out of the blood, forming bubbles that go into the bloodstream and to tissues and muscles. Divers who ascend too quickly may suffer from decompression sickness, or the bends : pain or weakness throughout one s body and, in some cases, shortness of breath. To avoid getting the bends, scuba divers should surface slowly. This allows nitrogen gas dissolved in their blood to come out of solution in smaller amounts over a longer period of time. One definition for an acid is a substance that lowers the ph of a solution as the acid s concentration is increased. A base, on the other hand, is a substance that increases the ph of a solution as the base s concentration is increased. But different acids have different effects on ph. Some acids, such as hydrochloric acid, are strong and have a large effect on ph. Others, such as acetic acid, are weak and change ph only a little. This has to do with the substance s percent dissociation. Strong acids react completely, becoming completely percent dissociation: a measurement of how much of a dissolved compound breaks into its constituent ions in solution. disassociated. When an acid dissociates, it releases hydrogen ions into solution. This in turn lowers the ph. Because strong acids completely dissociate, they release many more hydrogen ions than a weak acid. Weak acids do not completely dissociate. Some weak acids have less than 0.1% dissociation! Thus they affect ph much less than strong acids. The same concept holds true for strong and weak bases. The only real change is that bases release hydroxide, which decrease the amount of hydrogen in solution. Weak bases still have less than 100% dissociation, while strong base have 100% dissociation. 2

3 The following graph shows the effect of temperature on a solute dissolving in a solvent. The x-axis shows the temperature of the solvent (in C). The y-axis shows the mass of solute (in g) that dissolves at each temperature. What can you conclude about the relationship between temperature and solubility for these substances? Identifying Electrolytes and Nonelectrolytes A common way to classify aqueous solutions is to determine whether they are able to conduct an electric current. An electrolytic solution conducts electricity. These solutions contain electrolytes, substances that separate into ions when dissolved in a solvent. The ions are electric charges, which can move within the solution, allowing it to conduct electricity. A non-electrolytic solution does not contain electrolytes and so does not conduct electricity. The diagram at right shows a conductivity meter. Do you think the meter is connected to an electrolytic or a non-electrolytic solution? If an electrolytic solution is connected to the conductivity meter, electric current will flow to the bulb and produce light. If a non-electrolytic solution is connected to the meter, the light bulb will not turn on. Sodium chloride (NaCl) is a strong electrolyte. When NaCl dissolves in water, the sodium ions (Na + ) and chloride ions (Cl ) separate from each other. The ions disperse evenly throughout the water, which becomes an electrolytic solution. Here is the chemical equation to describe sodium chloride dissolving in water: In contrast, substances such as ethanol (C 2 H 5 OH) and normal table sugar, or sucrose (C 12 H 22 O 11 ), are nonelectrolytes. A granule of sugar large enough for us to see consists of many individual sugar molecules joined together. When sucrose dissolves in water, the connections between the different sugar molecules are what break apart, not the sugar molecules themselves. The resulting solution contains evenly dispersed water and sugar molecules. Because ions are not produced, the solution does not conduct electricity. Here is the chemical equation to describe sucrose dissolving: 3

4 Do not confuse molecules that are polar and compounds that are electrolytes. Polar molecules have partial charges one end is partly positive, and one end is partly negative but they are not composed of ions, which have full positive or negative charges. Like electrolytes, polar molecules dissolve in water. Unlike electrolytes, however, polar molecules do not break apart into ions when they dissolve, so they do not produce electrolytic solutions. For example, table sugar is a polar molecule that dissolves in water and is a nonelectrolyte. In contrast, table salt is an ionic compound that dissolves in water and is an electrolyte. Types of Solutions Recall the glass of lemonade at the beginning of this companion. Initially, a small amount of lemonade mix was added to the water. The resulting solution was unsaturated, because additional solute (the mix) could be dissolved in the solvent (the water). More lemonade mix was added until eventually no more mix would dissolve. At this point, the solution was saturated. Any mix added after this point settled on the bottom of the glass. It is possible to dissolve more solute in a saturated solution by increasing the solution s temperature or the pressure acting on it. Such a solution becomes supersaturated. When a supersaturated solution is brought to normal pressure or temperature conditions, the additional solute precipitates from the solution. Adding more solute or disturbing the solution for example, by tapping on the container can also trigger this precipitation. Many supersaturated solutions form crystals. In an unsaturated solution (A), more solute will dissolve in the solvent. In a saturated solution (B), no more solute will dissolve in the solvent. In a supersaturated solution (C), additional solute has precipitated out of solution to form crystals. Rates The speed at which a solute dissolves in a solvent or the rate of dissolution depends on three factors: the temperature of the solution, the agitation (or mixing) of the solution, and the surface area of the solute. If any of these three factors change, the rate of dissolution changes. Surface Area: When the surface area of a solute increases, the dissolution rate also increases. Suppose you had a spoonful of sugar granules and the same amount of sugar pressed into a cube. The spoonful of sugar has a greater total surface area than the cube, because each granule in the spoonful contacts the solvent separately. So, the spoonful of sugar will dissolve at a faster rate than the sugar cube. A spoonful of sugar granules (left) has a greater surface area than sugar cubes (right) and a faster dissolution rate. 4

5 Solution Temperature: Heating a solution causes the particles in the solution to move more quickly. As a result, the solvent and solute particles collide more frequently and with greater force. Therefore, when the temperature of a solution increases, the rate of dissolution increases. Lemonade mix dissolves more quickly in hot water than in cold water. Agitation: Agitating a solution increases dissolution rate. If you stir lemonade mix into water, the mix dissolves more quickly. A washing machine spins as it cleans your clothes to help the detergent dissolve more quickly in the water. Getting Technical: Lithium-Ion Batteries Many modern electronic devices including computers, cell phones, and portable music players are powered by rechargeable, lithium-ion batteries. Inside these batteries, the movement of lithium ions and electrons generates electric current. A battery is made of three parts: a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode of a lithium-ion battery contains lithium cobalt oxide (LiCoO 2 ). The negative electrode is made of carbon (C). The electrolytic solution contains various lithium salts such as LiBF 4, LiClO 4, or LiPF 6. Because they are electrolytes, these lithium salts dissociate into positively charged lithium ions (Li + ) and negatively charged polyatomic ions (BF 4, ClO 4, or PF 6 ). As the battery charges, the lithium ions move to the negative electrode. As the battery is used, the lithium ions move back to the positive electrode. The result is an electric current able to power electric devices. Inside a lithium-ion battery, lithium ions move between the electrodes, creating an electric current. 5

6 What do you know? The following terms are associated with electrolytes or nonelectrolytes. Write each term in the correct place on the Venn diagram on the next page. Place terms associated with both electrolytes and nonelectrolytes in the middle. Electrolyte/Nonelectrolyte Terms MgCl 2 dissolved in water Will dissolve in water Conductor of electricity HOCH 2 CH 2 OH dissolved in water Does not dissociate in solution Can be polar substances Dissociates into ions Lights a bulb when connected to a conductivity meter Not an electrical conductor Does not light a bulb connected to a conductivity meter Electrolytes Nonelectrolytes 6

7 Experimenting with Solubility To help your child learn more about the factors that affect solubility and the rate of dissolution, gather the following materials: 6 sugar cubes 2 glasses Measuring cup Spoon Water (both warm and cold) Thermometer Stopwatch Your child will observe what happens in three different scenarios and compare the results of each set of experiments. First, your child should pour one cup of warm water into one glass and a cup of cold water into a second glass. After recording the water s temperature in each glass, add one sugar cube to each glass at the same time and start the stopwatch. Your child should record which sugar cube dissolved faster. Second, pour one cup of water at the same temperature into two separate glasses. Crush one of the sugar cubes into smaller pieces. Then, add the crushed pieces to one glass of water and a complete sugar cube to the other glass at the same time and start the stopwatch. Your child should record which sugar sample dissolved faster. Finally, pour one cup of water at the same temperature into two separate glasses. Then, add one sugar cube to each glass at the same time and start the stopwatch. Your child should stir the solution in one glass gently with the spoon, but leave the solution in the other glass unstirred. Record which sugar cube dissolved faster. Here are some questions to discuss with your child: In each separate example, which sugar dissolved faster? Explain why. How is the rate of dissolution different than the solubility of a substance? What was being tested in this experiment? How could you test the other term? Combining all of the data, how could you minimize the dissolving time of a sugar cube? Explain. 7