BioEd Online

Teacher Background

One of the most important properties of liquid water is its ability to dissolve many different substances. The same forces of attraction among molecules that account for the “stickiness” of water also act as tiny magnets that pull certain types of molecules (such as table salt) apart or allow some substances (alcohol, for example) to mix uniformly with water. In general, molecules that have a positive end and a negative end, or that can separate into components with positive and negative charges, will dissolve in water. Molecules without these characteristics, such as oils, will not dissolve in water.

The uniform mixture that results when one substance (such as table salt) is dissolved completely in another (such as water) is called a solution. Many common items are solutions. Household vinegar, for example, is a solution of acetic acid in water.

The reactions that take place inside living cells depend on the presence of water. Likewise, organisms require water outside of cells to transport nutrients and other substances from place to place, and to carry waste products away. In our daily lives, we take advantage of water’s abilities to dissolve and remove unwanted substances by using it for cleaning and rinsing.

Objectives and Standards

Concepts

• Some liquids and solids will dissolve in water.

• Substances dissolved in water sometimes are invisible.

Science, Health and Math Skills

• Predicting

• Making and recording observations

• Measuring

• Drawing conclusions

Materials and Setup

Materials per Group of Students

• 6 cups, 9-oz clear plastic

• 6 cups, 2-oz clear plastic (or bottle caps)

• 6 spoons (or coffee stirrers)

• Beaker, 100–250 mL

• Tsp each of salt, sugar, flour, oil (clear), diluted food coloring and ground coffee (not instant)

• Water

• Copy of the student sheet

Setup

1. Before beginning, prepare a dilute solution of food coloring by adding several drops of any color to a glass of water. This will be one of the substances tested in this activity.

2. Set all the materials in a central area.

3. Students should work in groups of four.

Procedure and Extensions

1. Distribute a copy of the student page to each group. Have Materials Managers pick up materials for their groups.

2. Show the students a clear glass of water. Ask, Have you ever mixed or stirred something into a glass of water? What happened? Do you think that everything can mix with water? Tell students that they will observe what happens when they mix different things with water.

3. Before student groups begin, have them predict what will happen when they mix each substance with water. You may want to give groups time to discuss criteria for deciding if something has “dissolved.” For example, a substance could be considered dissolved if the water is transparent, not cloudy, after the mixture has been stirred.

4. Have students measure approximately 100 mL of water into each of the six cups. Guide the groups as they conduct their tests, one substance at a time, in separate cups. For each test, ask students to observe the substance. Ask, Is it a liquid or a solid? Next, have students measure about one teaspoon of the substance into one of the cups of water and stir until there is no change in the mixture. Finally, they should note what happened and record their observations.

5. When students have completed their investigations, discuss their observations. Project a transparent copy of the “My Observations” sheet or draw a similar table on the board, and call on each group to share its observations for one of the substances. Expect the following results.
   
   Salt. Will dissolve (disappear), leaving a clear solution.
   
   Sugar: Will dissolve (disappear), leaving a clear solution.
   
   Flour. Will not dissolve; the mixture will be cloudy, because the large flour particles will remain suspended in the water (example of a colloid).
   
   Oil. Will not dissolve; the oil will float on top of the water because the oil is less dense, and because the oil molecules will not mix with the water molecules.
   
   Food coloring. Will dissolve; the resulting transparent liquid will be colored.
   
   Coffee. Part of the coffee will dissolve in the water, coloring it brown; the remainder of the coffee (woody parts of the coffee bean) will not dissolve or disperse through the liquid and will float.

6. Conclude with a discussion of the students’ observations. Ask, Which things disappeared into the water when you stirred? Do you think that they (salt or sugar) are still there? How could you figure this out? Ask about the other substances.

7. Extend the discussion to include students’ ideas about how water’s role as a “dissolver” is useful in daily life. Have students think about things that remain in water after it is used for cleaning, rinsing, etc. Ask, How this might contribute to water pollution? Also ask, How many substances did you dissolve in (or add to) water today?

Extension

Create filtering cups by punching holes in the bottoms of disposable cups. Line the cups with coffee filters or paper towels. Have the student groups pour the contents of each cup used for the activity through the filtering cups and report the results.

Handouts and Downloads