In this activity, you and your child will explore how a hard-boiled egg sinks or floats in water with different amounts of salt.
Before the activity, make a hard-boiled egg. Add the egg to a pot and cover it with water. Bring to a boil over high heat. As soon as the water starts to boil, turn off the heat and put the lid on the pot. Let the egg cook in the hot water for about 10 minutes. Then run cold water over the egg to stop the cooking process. Keep the shell on.
Start by asking your child, “What happens when you drop an object in water?” Talk about how it can sink (go to the bottom of the water) or float (stay at the top of the water). Spend some time talking about what makes an object sink or float. You can say something like, “Whether or not an object sinks or floats in water depends on its density. Density is how much space an object takes up (volume) compared to how much there is of what the object is made of (mass).
“If an object is heavy and small, like a rock, it has a high density. When an object has a higher density than water, it will sink. If it’s light and takes up a lot of space, like a white foam cup, it has a low density. When an object has a lower density than water, it will float. Can you think of any other items with high or low density?” Discuss.
“Let’s explore the density of an egg. You will test whether an egg sinks or floats in regular water and in saltwater. How do you think the salt will affect the egg?” Allow your child to share their prediction and write it in their journal.
Your child should observe how the density of water changes as they add more salt to it. They should notice the egg sinking in the first glass of water and floating at different levels in the glasses that have saltwater. After completing the experiment, ask, “What makes the hard-boiled egg sink or float in the different glasses?” and “So what does that mean?” If your child is not able to make inferences about what causes the egg to sink or float, review the concept of density. The egg sinks in regular water because the egg has a greater density than the water. Adding the salt increases the density of the water, so the egg floats because now the saltwater is denser than the egg.