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Eggcellent Density

Floating and sinking are due to density. If something is heavy for its size, it has a high density. A pebble is heavy for its size compared to a popped popcorn kernel. If an object is light for its size it has a low density. Imagine a big bowl of popcorn compared to a big bowl of pebbles. The heavier bowl (pebbles) has higher density.

Why do some things float, while others sink? You might expect heavier objects to sink and lighter ones to float, but sometimes the opposite is true. The relative densities of an object and the liquid it is placed in determine whether that object will sink or float. An object that has a higher density than the liquid it's in will sink. An object that has a lower density than the liquid it's in will float.

You can really see relative densities at work when you look at a heavy object floating and a lighter one sinking. For example, imagine putting a small piece of clay and a large, heavy wax candle in a tub of water. Even though it's smaller and lighter, the piece of clay has a higher density (it's heavier for its size) than water and therefore sinks. Even though it's larger and heavier, wax has a lower density (it's lighter for its size) than water, and it floats.

Sinking and floating applies to liquids too. In this activity, we compare the density of salty water and fresh water. Salty water is more dense (heavier for its size) than fresh water. As long as the two kinds of water don't mix, fresh water floats on top of salty water.

Objectives

  • Compare the density of salty and fresh water.

  • Compare the density of an egg with that of salty and fresh water.

Materials

  • Per group of students:
    1 egg
    3 tbsp salt
    a measuring cup
    3 large clear cups
    water
    a spoon
    food colouring

Key Questions

  • Which is more dense, the egg or the fresh water?
  • Which is more dense, the egg or the salty water?
  • Which is more dense, the salty water or the fresh water?

What To Do

Preparation
Check ahead of time that the eggs you plan on using in this activity SINK in fresh water. If they float, they’re not fresh, and this activity won’t work.

Explore

  1. Prepare salty water by mixing 3 tbsp of salt into 2 cups (500 ml) of water.
  2. Fill one cup with fresh water.
  3. Gently lower the egg into the water. Does it float or sink?
  4. Fill the second cup with salty water.
  5. Gently lower the egg into the salty water. Does it float or sink?
  6. Add a couple of drops of food colouring into the fresh water to make it a bright colour.
  7. Pour salty water into the third cup until it’s about half full.
  8. VERY CAREFULLY pour fresh water on top of the salty water. To do this, pour the fresh water onto the back of a spoon that’s just touching the surface of the salty water. Be careful not to disturb the salty water.
  9. Observe the fresh and salty water. Which one is on top?
  10. Gently lower the egg into the cup with the fresh and salty water. Be careful not to stir it!
  11. Where is the egg floating?

Extensions

  • Start with a cup of fresh water and add salt a teaspoon at a time. How much salt does it take to float the egg?
  • Test other (waterproof) objects. Do they float, sink or "flink" (something that doesn’t float to the top or sink to the bottom)?