Water and Solubility

This activity created in partnership with AGI.

Water is a necessity in our lives. We use it every day in many different ways: to drink, wash, cook, and help get rid of waste. We provide plants with water so they can grow. We travel on water. In some parts of the world, water is used to generate electricity. And when there is not enough water in a particular area, living things suffer. Water is a very valuable substance.

Water takes many forms on the Earth: ocean, lake, river, stream, groundwater, ice, and water vapor in the atmosphere. All this water makes up Earth’s hydrosphere.

Video © AGI

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Water has special properties that make it essential to life on Earth. Watch this video to learn about some of water’s properties and uses. Pay particular attention when the video discusses how water dissolves different substances. When the video is over, in your notebook write your ideas about why water is so important.

One of water’s most valuable properties is its ability to dissolve many substances. Without this property, many things on Earth would be different. For example, the oceans wouldn’t be salty. Plants and animals, which depend on moving dissolved substances into and out of their cells, would not exist.

A water molecule
Image courtesy of U.S. Geological Survey.

The water molecule has a positive charge on one side and a negative charge on the other.

Water is made up of two hydrogen atoms bonded to one oxygen atom (H20). It is able to dissolve many materials because it is a polar, or charged molecule. This means that one side of the molecule (the hydrogen side) tends to be more positive than the other (the oxygen side), as shown in the diagram of the water molecule.

Our Experiment

In this activity, you will learn more about water’s role in dissolving some common substances. This is called solubility, a substance’s ability to be dissolved by water. You will also investigate how the solubility of substances might change if the water is at different temperatures.

Tools and materials

Four transparent drinking cups, at least 250 mL (8.5 oz)
Four substances for solubility testing:
- Vegetable oil
- Food coloring or ink
- Powdered iced tea, fruit drink mix, or colored sugar
- Sand or gravel
Spoon or stick for stirring
Water at room temperature
Hot tap water
Very cold water
Notebook and pen
Paper towels
Alcohol thermometer (optional)

What to do

Fill each of the four drinking cups just above the halfway level with room temperature water. Place your four substances for testing nearby.

In your notebook, write a prediction of what you think will happen if you add oil to the water in the first cup. Explain your prediction.

Fill each of the four drinking cups just above the halfway level with room temperature water. Place your four substances for testing nearby

Add a small amount of oil to the water in the first cup; then wait a moment. Observe what happens. Record your observations in your notebook.
Stir the water, and wait a minute or two. Record your observations in your notebook.

Stir the water, and wait a minute or two. Record your observations in your notebook.

Repeat steps 2, 3, and 4 for the each of the three other substances, using the other three cups.
Once you have completed testing of all four substances in water at room temperature, think about the following questions, and write your thoughts in your notebook:
1. Which substance(s) dissolved in the water? Why do you think this happened?
2. Which substance(s) did not dissolve? Why do you think this occurred?
3. What is the effect of stirring?
Pour the liquid in each cup down the drain and place the residue sand or gravel into a trash can. Then wash and dry out each of the four cups.
Fill each of the four drinking cups just above the halfway level with very cold water. Repeat steps 2 through 5 for each substance. Pay particular attention to how much stirring you do.
After you have tried all four substances in very cold water, write down what happened for each substance. Note any effect you observe that might be caused by the water temperature or any other factor, such as the amount of stirring.
Clean out the cups in same way as in step 7.
Fill each of the four drinking cups just above the halfway level with very hot water from a faucet
Caution: Handle all hot water very carefully. Repeat the activity in steps 2 through 5 for each substance. Pay particular attention to how much stirring you do.
After you have tried all four substances in hot water, write down what happened for each. Note any effect you observe that might be caused by the water temperature or any other factor, such as the amount of stirring.
Think about the following questions for the four substances and the water at different temperatures and write your thoughts in your notebook:
1. How did cold water affect the ability of each substance to dissolve?
2. How did hot water affect the ability of each substance to dissolve?
3. What properties of water does this activity demonstrate?

Digging Deeper

Find out more about: Water and Solubility.

Course:

Science

Result/Solution(s)

From left to right: sand, oil, drink mix, food coloring

In our tests we found that the oil and the sand did not dissolve in the water at any of the three temperatures; however, when stirred, the oil and the sand each mixed with the water temporarily. After a little time, the oil rose to the top of the water in its cup and the sand sank to the bottom of the water in its cup.

The powdered drink mix and food coloring both dissolved in water at all three temperatures. Even after we waited for several minutes, nothing rose to the top or sank to the bottom. (We also found that if we added a lot of the powdered drink mix to the water, a little of it sometimes remained in solid form and sank to the bottom, particularly in the cold water.)

We found that it took more stirring to dissolve the drink mix and food coloring in the cold water than in the hot water. The cold water seemed to slow down the dissolving process, and the hot water seemed to speed it up. For the drink mix in the cold water, it looked as if some particles settled to the bottom of the cup; however, with more stirring, they eventually dissolved.

We also noticed that the oil and food coloring were both already in liquid form, yet only the food coloring dissolved in the water. The oil would mix when stirred, but then it always rose to the top of the water.

The sand and the drink mix were both in solid form, yet only the drink mix dissolved in the water. The sand would mix somewhat when stirred, but then it always sank to the bottom of the cup.

These different behaviors are caused by the electrical characteristics of the molecules of the different substances and by the energy of the water molecules at the different temperatures. The molecules of a nonpolar substance, such as oil, have a uniform outer electrical charge and are repelled by the polar water molecules. The more energetic molecules of hot water help to break the bonds of crystalline substances such as the drink mix so that the individual molecules can be dissolved by the water more readily.

What other common substances can you try to dissolve in water? We encourage further experimentation—you can try such substances as hard candies, crackers, honey, ketchup, flour, or even dirt.

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