Sink or Float? Teaching density to toddlers

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Does your child play with bath toys that float in the tub, balls that float in the water table, or even notice that it’s easier to float with a life vest on than with no vest? That gives you a great opportunity to talk about why things sink and float!

The Sink or Float experiment teaches that objects (including a person!) can float in liquid. When the density of the liquid is greater than the density of the object, the object can easily float on the surface of the liquid. This experiment uses an egg to test which is more dense: salt water or fresh water.

This experiment is great to explain so many things in everyday life:

  • Ice floating in a drink
  • Floaties that allow you to float in the pool
  • Bath toys floating in the tub

How to make the Sink or Float science experiment

Supplies you will need

For the Sink or Float experiment, you’ll need:

  • 2 see-through glasses
  • Water
  • Salt
  • 2 eggs
  • A small amount of warm water (to dissolve the salt)
Supplies needed for the Sink or Float experiment

Before you start

It is easy to accidentally dunk the eggs in and create a splash, so it would be helpful to either have a towel nearby or place a towel under the experiment.

Instructions

Here is how to do the Sink or Float experiment with your toddler:

Step 1: Add salt to the warm water

Adding salt to warm water (to help dissolve)

I used about 6 tablespoons, but that is likely too much salt. You could get away with just a few tablespoons, depending on the size of the cup you use for the experiment.

If your egg does not float during the experiment, it means there is not enough salt yet. Add just another tablespoon of salt to the glass to see if it changes the density. That’s a great opportunity to talk to your child about how salty the water has to be in order for the density to become large enough!

Stir the mixture well to ensure as much is dissolved as possible.

Get your preschooler involved: Allow your toddler to measure and pour the tablespoons of salt into the glass of warm water (just keep a watchful eye, in case the water is hot!).

Step 2: Pour water into glasses

Pouring salt water in one glass and fresh water in another

Empty the salt water into one of the cups and fill it the rest of the way with fresh water. Then, fill the second cup with only fresh water.

Be sure to leave some room for the egg to displace the water in your two glasses. Otherwise, you may make a large spill under your experiment.

Step 3: Drop one egg in each glass

Carefully drop the eggs into each glass

Place one egg in each glass (gently!). If the salted water is dense enough, the egg will rise to the top of the glass immediately. If the egg cannot quite make it to the top of the glass, add another tablespoon of salt and see if it begins to float.

Get your preschooler involved: Allow your toddler to drop each egg into the glasses. Talk to them about what they see happening, as the egg should begin floating immediately after dropping it in the glass.

The science behind the Sink or Float experiment

In the Sink or Float science experiment, we are showing how the density of water can change when we add salt to it.

How it works

The density of something is defined as the mass per unit volume of a substance. In our experiment, we are testing the density of salt water versus that of fresh water.

Have you ever seen pictures or videos of people floating on the Dead Sea? If not, check this out:

https://youtu.be/WbWUsvKBlTw

The density of water can change dramatically when you add salt to it. When you have a higher-density liquid, like the saltwater in the Dead Sea, it is much easier to float on it than to float on fresh water.

Buoyancy, or the ability to float, boils down to these key concepts:

  • If an object is more dense than the fluid it’s in, it sinks
  • If an object has equal density as the fluid it’s in, it hovers (so it may not rise to the surface)
  • If an object has less density than the fluid it’s in, it floats

In our experiment, we have a controlled density with the egg that we drop into each glass.

The only difference is that we are using salt water in one glass and fresh water in the other glass.

What does this mean? It means that salt water’s density is greater than the density of fresh water. Because of that, our egg sinks in fresh water because it is denser than the water. Our egg floats in salt water because it is less dense than salt water.

We could conduct this same experiment using sugar water. This is simply because we are adding more mass (grains of sugar or salt) per unit volume to the fresh water.

However, salt is about 25% denser than sugar, so you will need to add more sugar to the glass in order to achieve the same result.

You can use this lesson in density the next time your child is taking a bath: talk about how the bath toys that float are empty inside (no mass), so they are less dense than the bath water. Then, you could try out a toy that is not hollow to compare the two objects and their ability to float.

More physics experiments to try out with your child

FAQ about the Sink or Float experiment

How does salt affect buoyancy?

Buoyancy is the ability to float in a liquid. In order for something to be more buoyant, that object must have less density than the liquid it is in. Salt has more density than plain, fresh water, and therefore allows the object to float much easier than floating in fresh water.

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