Physical Properties of Solids - Solubility Test

Solubility test

Question to investigate: Can you identify the unknown crystal by the amount that dissolves in water?

• 1. Have students weigh equal amounts of the crystals.

     1. The amounts of crystal and water used in this solubility test are specific and should be used because they give clear results. There are a variety of methods students could use to weigh equal amounts of each crystal. They could construct a balance themselves, or they could use any scale that can weigh 0.5 grams.

     2. Procedure

        1. If the cups are not already labeled, use your masking tape and pencil to label five small cups salt, sugar, epsom salt, MSG, and unknown. Also label five larger clear plastic cups in the same way. You should have two labeled cups (one large, one small) for each type of crystal.

        2. Using a balance, get 0.5 grams of salt into the newly-labeled small cup for salt.

        3. Weigh the other four crystals in the same way so that you have equal amounts of all five crystals in their small labeled cups.

        4. Discuss the variables that need to be controlled in the solubility test.

           1. How might we mix the crystals into water to compare how they dissolve?

              • How many cups do we need?

              • Should the cups all have the same amount of water?

              • What else about the water should be the same?

              • What is a good way to mix the crystals into water in each cup?

        5. Students dissolve the crystals in water.

• • • • 1. The following procedure is also listed in Activity sheet 2.3—Solubility test. The amount of water used in the procedure is specific and should be used because it gives clear results. Swirling the crystals in water is a good way of mixing them to help them dissolve. The class works together at the same step so that all groups pour their crystal samples into the water at the same time. Teacher tells students when to swirl the water and crystals and when to stop and observe. There will be three 20-second intervals.

        2. Procedure

           1. Carefully fill a large clear plastic cup with hot tap water and then place 1 teaspoon of hot tap water into each labeled empty clear plastic cup.

           2. Match up each pair of cups so that each cup of crystal is near its corresponding cup of water. With the help of your lab partners, listen for your teacher’s instructions, and pour the weighed amount of each crystal into its cup of water at the same time.

           3. With the help of your lab partners, swirl each cup at the same time and in the same way for about 20 seconds and observe. Swirl again for 20 seconds and observe and then for 20 more seconds and make your final observations.

           4. Slowly and carefully pour the solution from each clear plastic cup back into its small empty cup. Try not to let any undissolved crystal go into the small cup. Compare the amount of crystal remaining in each clear plastic cup.

        3. Students should use their observations during the solubility test to help them answer the questions about the possible identity of the unknown in part 3 of the Physical Properties of Solids Assignment Sheet.

1. From http://www.inquiryinaction.org/