Science Experiment Teaches Kids About Density

Color Water Test Reveals Differences in Water Density

May 29, 2009 Michael Jung

Use this fun science experiment in classrooms or at parties to teach children the basic principles of liquid density.

Learning about density – or the relationship of an object’s mass to its volume – can be fascinating for kids, especially when they get to see the laws of density in action. Now with this cool science project, parents and teachers can show kids how the density of different kinds of liquid affect the way they relate to other objects and each other.

Begin this science experiment by asking kids why objects that are the same size (like a rock and a sponge) can take up the same amount of space (or volume) but have different weights. Explain through discussion that a rock has more mass than a sponge – in other words, the molecules in the rock are more closely packed than the sponge’s molecules, making the rock denser than the sponge. As a result, objects with greater density are heavier than objects of the same size that have less density.

Now explain that liquids like water and soda also have different densities – and that the denser a liquid is, the easier it is to float on that liquid. Likewise, the less dense a liquid is, the easier it is to sink in that liquid.

Sound complicated? Then let’s see how liquid density works first hand!

Educational Materials

3 six liter transparent beakers (glass jars or clear plastic cups can be used too)
3 golf balls
A container of table salt
A gallon of fresh water (have more on hand if you want to do this experiment multiple times – tap water should be fine)
Red and blue food coloring (other colors can be substituted if you like)

Color Water Science Experiment

Fill one of the beakers halfway with fresh water and color it with the blue food coloring. Let students predict whether or not the golf balls will float in fresh water, and then let them test their predictions by dropping the golf balls in the water. Since this beaker is full of fresh water, the denser golf balls will sink to the bottom.

Now fill the second beaker with the same amount of water as the first beaker and color it with blue food coloring – but this time pour salt into the color water. Let students predict if the golf balls will float in salt water – then let them test their predictions by dropping the golf balls in the water. Since salt makes this water denser, the golf balls will float to the top.

Note: Depending on how much water you use, it may take more salt to increase the water density to the point where the golf balls can float. If you're short on time, you may want to prepare the water before doing the experiment. If you have time, however, the kids will enjoy experimenting to see how much salt they need to pour into the water to get the golf balls to float.

Let the kids try and understand why the golf balls floated in salt water but sank in fresh water. Explain that as they added more salt to the blue water, they added more mass to the blue water. As its mass increased, the density of the water increased, allowing the golf balls to float. The fresh water, however, did not have this extra mass, making the denser golf balls sink in it.

Now fill the third beaker with 1.5 liters of fresh water and color it with red food coloring. While the kids are watching the golf balls floating in the blue salt water, slowly pour the red water onto the blue water.

Note: Depending on the amount of salt you mixed into the salt water, you may need more or less than 1.5 liters of fresh water (or roughly half the amount of salt water you used). Experiment to see what your ideal mixture is.

Because the density of the red water is less than the density of the blue water, it will form a separate red layer over the blue water. And since the golf balls are less dense than the blue water but denser than the red water, they will hang suspended between the layers of fresh and salt water.

Optional: Ask the kids to predict what will happen if they stir the two layers of water in the beaker together, then allow them to test their predictions.

Once the salt from the blue water mixes with the red water, the overall density of water will decrease, making the golf balls sink. (And of course the water will turn purple).

Discussion Questions

Once kids have a grasp of the basic principles of liquid density, the teacher can see how well they can apply that knowledge with the following questions.

Would it be more or less difficult for you to float in a freshwater swimming pool or a salt water swimming pool?
Most pools are full of other swimming pool chemicals – like chlorine. Would you sink faster or slower in chlorine swimming pools or a freshwater tank? Why or why not?

Liquid Density:

As the density of a liquid increases, its molecules are more tightly packed, allowing objects of less relative density to float in them more easily than in liquids that are less dense. As a result, people and other objects can float more easily in chlorinated swimming pools than they can in fresh water tanks.

Find more cool science experiments with density at Fun Party Trick Also A Cool Science Experiment and Coke Sinks and Diet Coke Floats.

And read an interview with science writer and award-winning children's author Conrad Storad at Author Conrad Storad Excites Kids About Science.

Source:

Corman, Jessica and Hilairy Hartnett. Ocean Adventures. Tempe: Changing Hands Bookstore, 2009.

The copyright of the article Science Experiment Teaches Kids About Density in Kids Activities is owned by Michael Jung. Permission to republish Science Experiment Teaches Kids About Density in print or online must be granted by the author in writing.

Color Water Density Experiment	Golf Balls Sink in Fresh Water...	...And Float in Denser Salt Water
Golf Balls Caught Between Different Densities	Hilairy Hartnett Demonstrates Density Experiment