2: Solutions 2.02: The Dissolving Process Wiva k12 Chemistry

Examine a solubility curve.

A solubility curve shows that the solubility of various solid solutes is affected by the temperature of the solvent. A solubility curve is a tool that describes the effect that temperature, for example, has on the amount of solute that can dissolve in the solvent before it becomes saturated. Here is the solubility curve for the solid potassium nitrate (KNO3). In this case the curve shows the maximum amount of solute that will dissolve in the water at various temperatures.

The formation of a solution depends on several factors.

Dissolution rate is a measure of how fast a solute dissolves in a solvent. Solubility is a measure of how much of a solute can dissolve in a solvent under certain conditions. This lesson focuses on both of these important aspects of solution formation.

Dissolution and solubility are closely related, so use the terms carefully.

Dissolution rate is a measure of how quickly (or slowly) the substance dissolves. There is another equally important aspect of solution formation, and this is how much of a solute can dissolve in a solvent. Solubility is the usual maximum amount of a solute that will dissolve in a solvent under specific conditions of temperature and pressure. Some solutes have little or no solubility in water. Substances like oil or mercury do not dissolve in water; as such, they are said to be insoluble in water. You may have heard the phrase "oil and water don't mix." You can now say that oil is insoluble in water.

Dissolution and solubility are important aspects of solutions.

Dissolution rate, or dissolving rate, is a measure of how quickly or slowly a solute dissolves in a solvent. Temperature, surface area, and pressure can all affect the rate of solution formation—that is, the rate of dissolving. Solubility, on the other hand, is a measure of how much solute will dissolve in a solvent under certain conditions. A solution that can allow no more dissolution of solute is called a saturated solution. A solute that will not dissolve in a particular solvent is said to be insoluble. Factors that can affect solubility are temperature and sometimes pressure.

A change in temperature affects solubility.

If you put salt in water, you will finally come to a point where the solution becomes saturated. A saturated solution is one in which no more solute can dissolve in the solvent under specific conditions. For example, 100 g of water at 10°C can dissolve no more than 35 g of NaCl. The solution is a saturated one. But 100 ml water at 99°C can dissolve up to 40 g of NaCl before it is saturated. Solubility is usually expressed as grams of solute per 100 ml of solvent. Gas solutions behave differently with regard to saturation. When a gas is dissolved in a liquid solvent, an increase in temperature reduces solubility—the opposite of what happen with most solids in liquids. The gas molecules are moving so quickly that, at a higher temperature, they will leave the liquid at a high rate. For example, carbon dioxide, the gas that makes soda fizz, has a solubility in water of about 0.2 g/L at 10°C. Raising the temperature to 60°C, though, drops the solubility below 0.1 g/L.

Temperature also affects how quickly a solution will form.

If you were to add a spoonful of powdered cocoa to a glass of cold milk and then stir it, you would find that the cocoa dissolves to make chocolate milk. But if you added the same amount of cocoa to a glass of warm milk, the cocoa would dissolve faster, even though the surface areas of the two spoonfuls of cocoa were the same. The rate at which a solution forms increases as the temperature of the system increases. You can understand this phenomenon by studying the collision of molecules. The faster the solvent molecules move, the more collisions will occur between the solvent and the solute. Try this by using the Temperature Test activity. Be sure to select the "particle view" to see how changing the temperature affects the speed of the molecules and therefore the rate of collision. Then find out how this affects how fast the salt cubes dissolve. Other factors that affect how fast a solute will dissolve are stirring and particle size. Stirring speeds up the dissolving process. And substances made of larger molecules, for example, in general dissolve more slowly than those with smaller particles.

The solubility of a gas solute varies with pressure.

In everyday circumstances, pressure changes have little impact on solutes that are solids or liquids. However, if the solute is a gas, pressure plays a key role. The release of carbon dioxide from a just-opened bottle of soda is an example of pressure changing the solubility of the carbon dioxide. When a carbonated beverage is bottled, the pressure of the system is relatively high. Opening the bottle causes a dramatic drop in pressure of the system. As a result, the carbon dioxide will bubble—or fizz—out of the liquid. The reason is that under high pressure, the carbon dioxide is much more soluble in water than at lower pressure.

Temperature Test

Objects dropped into solutions with different temperatures will take different amounts of time to dissolve. First we'll drop a cube of salt into a beaker of water at room temperature. The particle view shows a leisurely speed of the molecules. Once we add the cube of salt, and the saline solution is formed, there is a slight increase in the speed of the molecules and we can plot the time it took for the salt cube to dissolve. Now let's empty the beaker and refill it with a fresh supply of water. We'll turn on some heat and drop in another cube of salt, and plot the results. With only a small temperature increase, the dissolve speed increased significantly as did the speed of the molecules colliding. We'll empty the beaker one more time, refill it with fresh water and increase the heat still more before adding the salt cube. This temperature increase resulted in an even faster dissolve rate and the collision speed of the particles increased significantly.

Take a closer look at the factors that control the dissolving process.

Suppose you put a cube of salt in a glass of water. As you can see here, the molecules of water bombard the surface of the salt cube. This bombardment breaks off small particles of the salt cube, which then mix in with the water. The process continues until the salt molecules are mixed among the water molecules. That describes the dissolving process, or dissolution, of a salt cube. The molecules of the solvent bombard the molecules of the solute, breaking down the solid. Substances dissolve as a result of the collision of molecules. Random molecular motion leads to collisions, which cause the solute to break up. That breaking-up process is also called dissolving.

Surface area affects how fast a given amount of solid solute will dissolve.

Think back to the salt cube in water. As the water molecules bombarded the sides of the cube, the salt dissolved. Imagine what would happen if the salt cube had been broken into several pieces and then placed in the water. When a mass of salt is in many smaller pieces, the water molecules have more surface area on which to work, or to bombard. The process of dissolving the salt happens faster—that is, the rate of dissolving increases. Now imagine that you could grind the salt into a fine powder. The salt would dissolve much faster than if a whole cube had been placed in the water. For a given mass of a substance, the smaller the particle size, the faster the substance will dissolve. Explore the on-screen activity to find out how changing the surface area affects the rate at which the salt dissolves.

When a solute dissolves in a solvent, a solution is formed.

Two important aspects of solution formation are dissolution and solubility. Let's begin with dissolution, the breaking-up of a solute in a solvent If you went to the ocean and looked at the water, you would not see individual grains of salt. But if you tasted the water, it would taste salty. Seawater is a solution containing 3.5% dissolved salts. The ions that result from the dissolving process are primarily ions of sodium, chlorine, potassium, and magnesium. Many of the salts from which the ions were formed were once on land as solids. When rain dissolved the salts, their ions were washed into rivers and then into the sea. Seawater is a solution; a solution is formed when a solute is dissolved in a solvent. But how does a substance dissolve? And what factors influence the dissolving process?

5. Which of the following explains the relationship between surface area of a solid solute and its dissolution in a solvent? a.) As surface area increases, the time necessary for complete dissolution decreases. b.) As surface area decreases, the time necessary for complete dissolution decreases. c.) Surface area does not affect the time necessary for complete dissolution.

a.) As surface area increases, the time necessary for complete dissolution decreases.

2. What happens to the solubility of most solids as the temperature of their solvent is increased? a.) The solubility increases. b.) The solubility decreases. c.) The solubility stays the same.

a.) The solubility increases.

4. What relationship between temperature and solubility does this graph reveal? a.) The solubility of a solid increases with an increase in temperature. b.) The solubility of a gas decreases with an increase in temperature. c.) The solubility of a gas increases with an increase in temperature. d.) the solubility of a solid decreases with an increase in temperature.

a.) The solubility of a solid increases with an increase in temperature.

solubility

amount of solute it takes to saturate a certain quantity of solvent (at a given temperature); how much of a substance can dissolve in another substance

1. Which of the following would increase the dissolution rate of most solids in a liquid? a.) decreasing the temperature of the solvent b.) grinding the solid into smaller pieces c.) stirring much more slowly d.) adding more solid to the liquid

b.) grinding the solid into smaller pieces

3. Pressure has the most influence on the solubility of which type of solute? a.) solid b.) liquid c.) gas

c.) gas

saturated solution

one in which no more solute can dissolve in the solvent under specific conditions

insoluble

said of a solute that is incapable of dissolving in a specific solvent

dissolution

the process of dissolving