Problem 35 - Chemistry The Central Science In Si Units

Question

Indicate whether each statement is true or false: (a) The higher the temperature, the more soluble most gases are in water. (b) The higher the temperature, the more soluble most ionic solids are in water. (c) As you cool a saturated solution from high temperature to low temperature, solids start to crystallize out of solution if you achieve a supersaturated solution. (d) If you take a saturated solution and raise its temperature, you can (usually) add more solute and make the solution even more concentrated.

Step-by-Step Solution

Answer

(a) False, (b) True, (c) True, (d) True.

Step 1: Understanding Gas Solubility in Water

Statement (a) claims that the solubility of gases in water increases with temperature. Generally, the solubility of gases in water decreases as temperature increases because higher temperatures provide gas molecules with more energy to escape the liquid. Therefore, this statement is false.

Step 2: Understanding Ionic Solids Solubility in Water

Statement (b) claims that the solubility of ionic solids in water increases with temperature. In most cases, the solubility of ionic solids does increase with temperature, as higher temperatures provide energy to break ionic bonds in solids. Therefore, this statement is true.

Step 3: Crystallization of Solutes at Lower Temperatures

Statement (c) discusses cooling a saturated solution and the crystallization process. When a saturated solution is cooled, solubility decreases, and if the solution becomes supersaturated, excess solid begins to crystallize out. Therefore, this statement is true.

Step 4: Effect of Temperature on Saturated Solutions

Statement (d) states that increasing the temperature of a saturated solution allows the addition of more solute. This is true because higher temperatures often increase the solubility of solids, allowing more solute to dissolve. Therefore, this statement is true.

Key Concepts

Gas Solubility

Gas solubility refers to how well a gas dissolves in a liquid, such as water. Many students might think that, like sugar or salt, gases dissolve better at higher temperatures. However, this isn't usually the case. When the temperature rises, gas molecules gain more energy. This energy makes them move more vigorously and escape from the liquid more easily.
As a result, for most gases, solubility decreases with an increase in temperature. Human activities, like heating water in industrial processes, can reduce the amount of dissolved oxygen in bodies of water, affecting aquatic life.

• Lower temperatures generally mean higher gas solubility.
• Higher temperatures mean gases escape more readily from the solution.

This explains why cold sodas keep their fizz longer than warm ones, as the carbonation (a gas) remains dissolved better when cold.

Ionic Solubility

Ionic solubility deals with how ionic compounds, like table salt, dissolve in water. In these compounds, positive and negative ions bond together. To dissolve, these bonds need to break, which usually requires energy. Hence, solubility for most ionic solids increases with temperature.
With higher temperatures, the movement of water molecules becomes more vigorous, helping break the ionic bonds easier and allowing more ions to dissolve.

• Heat increases molecular movement, aiding the dissolving process.
• More ions in solution mean better solute conductivity.

However, not all ionic compounds follow this trend, so exceptions do exist. For instance, the solubility of calcium sulfate doesn't increase significantly with temperature.

Temperature Effects

Temperature plays a crucial role in solubility. Different substances respond differently to temperature changes, affecting how they dissolve in solvents. For gases, increasing temperature often means lesser solubility, while for solids, like salts, more solute can typically dissolve at higher temperatures.
When a saturated solution is heated, it often allows more solute to dissolve. This is because increased kinetic energy facilitates the breaking of solute bonds.

• High temperatures can dissolve more solid solute.
• Conversely, gases often become less soluble with heat.

Understanding these temperature effects is important in practical applications, like preventing the formation of gas bubbles in industrial processes or maximizing the dissolution of solutes.

Supersaturation

Supersaturation occurs when a solution contains more dissolved solute than it normally would at a given temperature. This condition is typically achieved by dissolving solute at a high temperature and then slowly cooling it without disturbing the solution. Supercooling can be delicate. Even a slight disturbance can cause the excess solute to crystallize out.
This property is used in making rock candies, where sugar is dissolved at a high temperature and then allowed to cool undisturbed.

• Supersaturated solutions are unstable and can crystallize unexpectedly.
• Crystallization releases energy, often as heat, termed the exothermic process.

Understanding supersaturation is key in chemical industries where precise control over solute concentration is crucial.