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17.4: Factors Affecting Solubility

Created by: CK-12

Lesson Objectives

The student will:

  • define the terms soluble, insoluble, and solubility.
  • describe the factors that affect solid solubility.
  • describe the factors that affect gas solubility.
  • describe how pressure can affect solubility.
  • explain the effect of temperature on the solubility of a solid solute in a liquid solvent.
  • explain the effect of pressure on the solubility of a gaseous solute in a liquid solvent.
  • state Henry's law.

Vocabulary

  • Henry's law
  • insoluble
  • solubility
  • soluble

Introduction

Take some sand and try to dissolve it in a cup of water. What happens? The sand will not dissolve; in other words, it is insoluble. Insoluble means that the substance does not dissolve. If you were to take a teaspoon of table salt or sugar and conduct the same experiment, the result would be different. Salt and sugar are both soluble in water. When a substance is soluble, it means that the substance has the ability to dissolve in another substance.

Solubility is the maximum amount of a substance that will dissolve in a given amount of solvent at a specific temperature. There are two direct factors that affect solubility: temperature and pressure. Temperature affects the solubility of both solids and gases, but pressure only affects the solubility of gases. Surface area does not affect how much of a solute will be dissolved, but it is a factor in how quickly or slowly the substance will dissolve. In this section, we will explore all three of these factors and how they affect the solubility of solids and gases.

The Effect of Temperature on Solubility

Temperature has a direct effect on solubility. For the majority of ionic solids, increasing the temperature increases how quickly the solution can be made. As the temperature increases, the particles of the solid move faster, which increases the chances that they will interact with more of the solvent particles. This results in increasing the rate at which a solution occurs.

Temperature can also increase the amount of solute that can be dissolved in a solvent. Generally speaking, as the temperature is increased, more solute particles will be dissolved. For instance, when you add table sugar to water, a solution is quite easily made. When you heat that solution and keep adding sugar, you find that large amounts of sugar can be added as the temperature keeps rising. The reason this occurs is because as the temperature increases, the intermolecular forces can be more easily broken, allowing more of the solute particles to be attracted to the solvent particles. There are other examples, though, where increasing the temperature has very little effect on how much solute can be dissolved. Table salt is a good example: you can dissolve just about the same amount of table salt in ice water as you can in boiling water.

For all gases, as the temperature increases, the solubility decreases. The kinetic molecular theory can be used to explain this phenomenon. As the temperature increases, the gas molecules move faster and are then able to escape from the liquid. The solubility of the gas, then, decreases.

Looking at the graph below, ammonia gas, \mathrm{NH}_3, shows a sharp decline in solubility as the temperature increases, whereas all of the ionic solids show an increase in solubility as the temperature increases.

A graph for the solubility of oxygen gas, \mathrm{O}_2, would be very similar to the one for \mathrm{NH}_{3(g)}; in other words, oxygen gas would decrease in solubility as the temperature rises. Conversely, the colder the temperature, the greater amount of \mathrm{O}_{2(g)} would be dissolved.

The Effect of Pressure on Solubility

The second factor, pressure, affects the solubility of a gas in a liquid but never of a solid dissolving in a liquid. When pressure is applied to a gas that is above the surface of a solvent, the gas will move into the solvent and occupy some of the spaces between the particles of the solvent. A good example is carbonated soda. Pressure is applied to force the \mathrm{CO}_2 molecules into the soda. The opposite is also true. When the gas pressure is decreased, the solubility of that gas is also decreased. When you open a can of carbonated beverage, the pressure in the soda is lowered, so the gas immediately starts leaving the solution. The carbon dioxide stored in the soda is released, and you can see the fizzing on the surface of the liquid. If you leave an open can of soda out for a period of time, you may notice the beverage becoming flat because of the loss of carbon dioxide.

This gas pressure factor is expressed in Henry’s law. Henry’s law states that, at a given temperature, the solubility of a gas in a liquid is proportional to the partial pressure of the gas above the liquid. An example of Henry’s Law occurs in scuba diving. As a person dives into deep water, the pressure increases and more gases are dissolved into the blood. While ascending from a deep-water dive, the diver needs to return to the surface of the water at a very slow rate to allow for all of the dissolved gases to come out of the blood very slowly. If a person ascends too quickly, a medical emergency may occur due to the gases coming out of blood too quickly. This is called having the “bends.”

This video serves a blackboard lecture on the factors that affect solubility (6c): http://www.youtube.com/watch?v=1Hbkco6hyJ4 (4:33).

The Effect of Surface Area on the Rate of Dissolving

One other factor to consider affects the rate of solubility. If we were to increase the surface area of a solid, then it would have been broken into smaller pieces. We would do this to increase how quickly the solute would dissolve in solution. If you were to dissolve sugar in water, a sugar cube will dissolve slower than an equal amount of tiny pieces of sugar crystals. The combined surface area of all of the sugar crystals have a much greater surface area than the one sugar cube and will have more contact with the water molecules. This allows the sugar crystals to dissolve much more quickly.

If you were working in a lab, you might be asked to make a solution of copper(II) sulfate. Copper(II) sulfate comes in several forms: large blue crystals and fine blue crystals (see Figure below). When you set equal amounts of both forms in test tubes filled with 10 \ \mathrm{mL} of water, you will notice after 5 minutes that more of the fine crystals will have dissolved (and the solution will be a darker blue) than the test tube with the large crystals. You can also take two samples of the fine crystals and put them into separate test tubes. This time, place a stopper on one of the test tubes and carefully shake it while letting the other test tube sit still. By shaking, you are again increasing the surface area by increasing the how much of the fine crystals will come in contact with the water. The result will still be the same as before: the test tube with the greater surface area will go into solution at a faster rate. Note, however, that although maximum solubility is achieved more quickly with greater surface area, the concentration of the solute at maximum solubility will be exactly the same.

Forms of copper(II) sulfate pentahydrate.

Lesson Summary

  • When a substance can dissolve in another it is said to be soluble; when it cannot, it is said to be insoluble
  • Temperature affects the solubility of both gases and solids. With solids, generally the solubility increases with increasing temperature. With gases, the solubility tends to decrease with increasing temperature.
  • Pressure only affects the solubility of gases. Henry’s law states that, at a given temperature, the solubility of a gas in a liquid is proportional to the partial pressure of the gas above the liquid.
  • Increasing the surface area increases the rate of solubility of a solid because a larger number of molecules have contact with the solvent.

Review Questions

  1. What are the factors that affect solubility?
  2. What is Henry’s law?
  3. Is it ever possible to have ionic solids decrease solubility with increasing temperature?

Consider the following factors for problems 4-5.

i. temperature
ii. pressure
iii. surface area
  1. Which of the factors listed above would affect the solubility of sodium sulfate?
    1. i, ii, and iii
    2. i and ii
    3. i and iii
    4. ii and iii
  2. Which of the factors listed above would affect the solubility of methane?
    1. i, ii, and iii
    2. i and ii
    3. i and iii
    4. ii and iii
  3. If you crush a cube a sugar before putting it in your cup of coffee, how have you affected its solubility?
    1. Crushing it has really no affect on solubility because we have not heated it at all.
    2. Crushing it has increased the surface area so it speeds up the dissolving process but doesn't change maximum solubility.
    3. Crushing it has really no affect on solubility because we have not stirred it at all.
    4. Crushing it has increased the surface area so it increases the maximum solubility.
  4. Why do people add chlorine to their swimming pools on a hot day?
  5. Explain why crushed table salt at room temperature dissolves faster than rock salt.
  6. Under which of the following sets of conditions would the solubility of \mathrm{CO}_{2(g)} be lowest? The pressure given is the pressure of \mathrm{CO}_{2(g)} above the solution.
    1. 5.0 \ \mathrm{atm} and 75^\circ\mathrm{C}
    2. 1.0 \ \mathrm{atm} and 75^\circ\mathrm{C}
    3. 5.0 \ \mathrm{atm} and 25^\circ\mathrm{C}
    4. 1.0 \ \mathrm{atm} and 25^\circ\mathrm{C}
    5. 3.0 \ \mathrm{atm} and 25^\circ\mathrm{C}
  7. An aqueous solution of \mathrm{KCl} is heat from 15^\circ\mathrm{C} to 85^\circ\mathrm{C}. Which of the following properties of the solution remain the same: i. molality, ii. molarity, iii. density?
    1. i only
    2. iii only
    3. i and ii only
    4. ii and iii only
    5. i, ii, and iii

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CK.SCI.ENG.SE.2.Chemistry.17.4

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