University of Virginia
Physics Department

## More on Temperature and Solubility

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.5 (Due to the revised 2003 SOLs, this activity no longer corresponds directly to PS.5, but still can be used to demonstrate physical changes.)

Objectives

Students will

• understand that the solubility of a solute is dependent on the temperature of the solvent;
• understand that the solubility of a substance in water is a characteristic physical property only at a range of temperatures;
• use the following skills: observing, inferring, measuring, comparing and contrasting.

Motivation for Learning

Driving Question

Does temperature effect the solubility of solutes?

Demonstration

The two solids used in this experiment are sodium chloride and potassium nitrate. Both are white solids and have similar solubilities at room temperature.

Procedure
1. Before students arrive in class mass out 10.0 grams of each chemical on separate pieces of paper and place 10 mL of water in two separate test tubes.
2. When the students arrive in class, place the salt in one of the test tubes and the potassium nitrate in the other. Stopper and shake the test tubes until no more of each chemical will dissolve.
3. Show the students that the two solids have slightly different solubilities at room temperature. Ask the students if these chemicals will dissolve similarly at different temperatures.

Background Information

The solubility of a substance indicates how much of a substance will dissolve in a given volume of water. The substance being dissolved is the solute. The substance dissolving is called the solvent.

Solubility is a characteristic physical property of both the solute and the solvent. It is usually expressed in complex units of grams of solute per 100 mL of solvent. If the student knows the solubility of a substance in a determined amount of solvent, he or she can easily calculate the minimum amount of solute needed for the amount of solvent being used. For example, if a solid powder has a solubility of 10g/100 mL of water, then 1.0 grams of the same solid would need 10 mL of water to dissolve. However, most chemicals change their solubility with the temperature of the solvent and they do so in a non-linear fashion. The true solubility of a solute in a particular solvent must include its solubility at a range of temperatures. Two chemicals may dissolve the same in water at one temperature but very differently at another temperature.

### Student Activity

To print out the Student Copy only, click here.

Materials

 Sodium chloride (10g) Graduated cylinder Potassium nitrate (10g) Hot plate 2 Test tubes (20 x 150 mm) 2- #2 solid stoppers Test tube rack Stirring rod 2 Beakers (250 mL) Safety glasses

Procedure

1. Place 10 mL of water into each of the two test tubes.
2. Add 10.0 grams of sodium chloride to one and 10.0 grams of potassium nitrate to the other.
3. Place both test tubes in a large beaker of water and stir the solutions with the stirring rod until both have completely dissolved.
4. Heat the solutions on the hot plate and stir continuously until the water in the beaker is boiling.
5. While heating, try to observe if the solubilities of the solids are changing equally as the temperature increases.
6. When the water has boiled, remove both test tubes and place each into a beaker of cold water and observe the changes taking place in each test tube.

Data Sheet

To print out the Data Sheet only, click here.

 While the water in the beaker was heating, did both the solids in the test tubes dissolve at the same rate or at different rates?     Describe what happened when the two test tubes were placed in the beaker of cold water.     If 5 grams of potassium nitrate (KNO3) were dissolved in 10 mL of water at room temperature, how much could be dissolved in 100 grams of water at the same temperature?     Solubility depends on three factors: the nature of the solvent, the nature of the solute and the temperature of the solution. Which factor was the variable in this experiment?

Extensions

An interested class can determine the solubility of potassium nitrate at various temperatures by assigning each group of students a different temperature for the water. The data from each group can be collected and a solubility curve for potassium nitrate constructed.

Assessment

Pass out a solubility graph of potassium nitrate and other common chemicals. Have students answer questions concerning the solubilities of the solids at different temperatures using the information on the graph.