One of the many ways you can describe a material is by its density. Density is nothing more than the measure of how solid something is. For example, water is less solid than a chunk of concrete, and its density is less than that of concrete.

Scientists use a formula involving mass (how much of the material there is) and volume (how much space the material takes up) to figure out an object's density. The less dense it is, the less tightly packed the particles are, and the more space it tends to take up.

Density is what makes balloons float up in the air, ice cubes float in your drink, and rocks sink to the bottom of a lake. But it can also be tricky! Here's a fun trick that is guaranteed to amaze your friends.

QUESTION

Can you make a grape float in the middle of a glass of water?

MATERIALS

• 4 drinking glasses
• Masking tape
• Marker
• 1 larger glass or measuring cup
• Water and sugar
• Grapes
• A spoon

PROCEDURE

NOTE: Before you do this for an audience, you should practice this by yourself. When you are ready to perform, you should have the glasses already prepared.

1. Using the masking tape and marker, label each glass as “#1,” “#2,” “#3,” and “#4.”

2. Fill the measuring cup with water and stir in enough sugar so that a grape will float at the surface of the water. If some sugar remains undissolved, allow it to fall to the bottom of the cup.

3. Fill Glass #1 full of water.

4. Place one grape into Glass #1 and observe what happens to it.

5. Fill Glass #2 with the sugar water solution you already prepared.

Fun Facts

Did you know both cement and steel can float? Due to the Archimedes Principle, which says that objects are supported by a force equal to the weight of the water they displace, even heavy ships, if they are designed right, can float!

6. Place one grape into the sugar water solution. You should see it float at the surface.

7. Now fill Glass #3 half-full of the sugar water solution.

8. Slowly and carefully fill the rest of Glass #3 with plain water, taking care NOT to mix the heavier sugar water below it. You might want to place a spoon just inside the glass and pour the water so it falls onto the spoon before it hits the sugar water. This step may take a few tries to master, but when you are done, you should find that you can't tell the difference between the two liquids in Glass #3.

9. Place a grape gently into Glass #3 and observe what it does.

WHAT'S HAPPENING

The grape is denser than the water, so it sinks immediately to the bottom of the glass. The sugar water solution contains more matter in the same glass, so it is denser than the plain water. It is also denser than the grape, so the grape floats on top. The third glass is your “trick” glass. You know what is in it, but your unsuspecting audience does not.

The grape sinks through the water, as it is denser than the water, but it floats on the surface of the sugar water solution since it is less dense than the solution. If you prepare the solution far enough in advance, it will be almost impossible to detect the separation between the plain water and the sugar water.

FOLLOW-UP

With the final glass (#4), experiment to see if you can come up with a new sugar water solution that, when fully mixed, will cause the grape to float in the middle just like in Glass #3.

KIDS' LAB LESSONS

QUESTION Can you make a liquid float?

EXPERIMENT OVERVIEW You will be pouring liquids with different densities into the same container and producing a layered solution. Using colors, you will show how additional liquids poured into your solution find their way to the “right” layer. You will also “clean” some of the colored water and be invited to consider other possible color patterns.

SCIENCE CONCEPT Ice cubes float in water because ice is less dense than water. In the same way, an oil spill tends to ride on the surface of the water because oil is also less dense than water. However, solid objects, or even thicker liquids, will sink in water because they are denser than the water.

To compare the densities of two or more materials, you can set up a liquid test container with layers of liquids, each with different density. By noticing how the materials separate from one another, scientists can identify unknown materials. This makes it easier to clean up pollution in lakes and streams.

MATERIALS

• Red and blue food coloring
• Measuring cup
• 1 cup corn syrup
• Clear glass bottle — empty 24–32-ounce glass bottles work well
• 1 cup vegetable oil
• ½ cup water
• ½ cup liquid bleach (Be very careful with bleach. Always ask for an adult's help before using it.)

PROCEDURE

1. Mix red food coloring into a measuring cup filled with the corn syrup. Pour the syrup into the bottle.

2. Pour the oil into the bottle on top of the corn syrup. Do these liquids mix?

3. Mix blue food coloring into a measuring cup filled with the water. Pour the water into the bottle on top of the oil. Give it several minutes to settle. Where does the water go when you pour it into the bottle? Can you explain why it does this?

4. At this point, you should have three distinct layers in your bottle. The bottom layer will be red; the middle, a thinner layer of blue; and the top layer will be clear.

5. Now pour the bleach into the mixture and observe what happens to the blue water. Again, allow several minutes for the mixture to settle.

QUESTIONS FOR THE SCIENTIST

• What happened to the blue water?_______________________

• Can you describe where the bleach ended up?__________________

• Why didn't the bleach mix with the syrup?_______________________

CONCLUSION The first three liquids all have different densities and exist in layers in the bottle. When you added the bleach, it sank through the oil, because it is denser than oil. It is not as dense as corn syrup, so it did not sink through the bottom layer. This put it at the same layer as the blue water and it happens to mix easily with water. So the bleach and water were mixed and the bleach turned the blue water clear again!

FOLLOW-UP It would be fun to make a mixture that has red, white, and blue bands, in that order. This would mean dyeing the oil (the top layer) either red or blue. Why isn't this possible in the setup you are using?_______________________