University of Virginia Physics Department Temperature Effects on 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 can still be used to demonstrate physical changes.)

Objectives

Students will

• determine the solubility of salt in water at three different temperatures and graph the results;
• use the following skills: observing, inferring, and graphing.

Background Information

Solutions consist of a solute and a solvent. The solute is usually a solid that dissolves in the solvent. Salts are common solutes and water is by far the most common solvent. Some substances are insoluble in water, but many more substances are soluble in water to some degree. The solubility of a substance in water depends on several factors. One of the most important factors is temperature.

This activity is written to use any salt to finding the temperature dependence of solubility. Table salt or NaCl is cheap and easy for teachers to obtain. The curve for NaCl is nearly flat and does level out around 60°, therefore, it is not the most interesting salt to use. If, however, you have more resources, this activity would be more interesting using another substance with a solubility curve that has a greater slope. We suggest using ammonium chloride or NH ₄ Cl which is easily obtainable from catalogs such as Science Kit for a price around 2.5 kg for $22.95 (2.5 kg should be enough for 10 groups to each do 4 temperatures). (The solubility curve for NH ₄ Cl is much steeper and therefore would provide much more interesting data. Another option is using KCl which also has a nice slope. Potassium chloride or KCl can be obtained from catalogs also for about $29.95 for 2.5 kg (2.5 kg should be enough for 12 groups to each do 4 temperatures). The following curve should help you choose a salt to use to do this activity that meets your classroom needs:

Student Activity

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Note: This experiment requires patience on the part of the student.

Materials

Three 250 mL beakers	Tap water
100 mL graduated cylinder	Hot plate
Weighing boat (or paper)	Stirring rod
Salt (amount determined by kind of salt used)	Thermometer
Ice water	Access to a balance
Graph paper

Procedure

Part I

1. Use the graduated cylinder to add 100 mL of tap water to a 250 mL beaker.
2. Place a weighing boat (or piece of paper) on the balance and record its mass.
3. Add about 75 g of salt to the weighing boat. It does not have to be exactly 75 g, but you should record the mass of the weighing boat and salt to the nearest tenth of a gram.
4. Slowly add about 5 g of the salt from the weighing boat to the beaker containing the 100 mL of tap water. Stir until all the salt is dissolved before adding any more salt.
5. Continue to gradually add salt to the water until no more will dissolve after you have stirred the solution for two minutes. The solution is now saturated.
6. Use a thermometer to record the final temperature of the saturated solution and record in the data table below.
7. Mass the weighing boat and the remaining salt and record.

Part II

1. Use the graduated cylinder to add 100 mL of ice water to a 250 mL beaker. Try to get a few pieces of ice into the 100 mL of ice water.
2. Add more salt to the weighing boat until there is about 75g of salt on the weighing boat. Record this mass to the tenth of a gram.
3. Slowly add about 5 g of the salt from the weighing boat to the beaker containing the 100 mL of ice water. Stir until all the salt is dissolved before adding any more salt.
4. Continue to gradually add salt to the water until no more will dissolve after you have stirred the solution for two minutes. The solution is now saturated.
5. Use a thermometer to record the final temperature of the saturated solution and record in the data table below.
6. Mass the weighing boat and the remaining salt and record.

Part III

1. Use the graduated cylinder to add 100 mL of water to a 250 mL beaker. Place the beaker with the 100 mL of water on a hot plate and heat it until it is about 90°C. Turn off the hot plate, but do not remove the beaker. Your teacher may actually provide the hot water for you.
2. Add salt to the weighing boat until the mass of the salt is about 75g. Record this mass to the nearest tenth of a gram.
3. Slowly add about 5 g of the salt from the weighing boat to the beaker containing the 100 mL of hot water. Stir until all the salt is dissolved before adding any more salt.
4. Continue to gradually add salt to the water until no more will dissolve after you have stirred the solution for two minutes. The solution is now saturated.
5. Use a thermometer to record the final temperature of the saturated solution and record in the data table below.
6. Mass the weighing boat and the remaining salt and record.

Data Sheet

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Hypothesis: In this experiment you are going to determine the amount of a specific salt that will dissolve in cold water, water from the tap, and hot water. Formulate a hypothesis of what you think will happen in this experiment.       Data: Mass of Weighing Boat (g) Initial Mass of Weighing Boat and Salt (g) Final Mass of Weighing Boat and Salt (g) Final Temperature (oC) Tap Water                 Cold Water                 Hot Water                 Calculations: Subtract the final mass of the weighing boat and salt from the initial mass of the weighing boat and salt for the tap water and record in the table below. This is the amount of salt that dissolved in the tap water. Repeat step 1 for the cold water trial and the hot water trial and record in the table below. Mass of Salt in Solution (g) Tap Water     Cold Water     Hot Water     Graph:  Graph the mass of salt in the solution vs. the final temperature of the solution. Put temperature on the X-axis and the mass of salt in solution on the Y-axis.

 

Assessment

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(Please answer in complete sentences):

1. How do your results compare to your hypothesis? Was your hypothesis correct?

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2. How does temperature affect the solubility of a substance?

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3. In the experiment, what was the solvent and what was the solute?

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4. From your graph, how much salt would dissolve at 50°C?

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5. From your graph, what temperature of water is required to dissolve 25 g of salt?

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Extensions

Have each group do a fourth temperature and then collect a class list of data and analyze using Excel or graphing calculators. The class then should discuss outliers and why some data points are inaccurate or should be discounted. For directions on how to make graphs on Excel, click here.

Students with special needs

Click here for information on laboratories with students with special needs.