Science Experiments for Kids: Buoyancy

Science can be an exciting and fun subject for kids, especially when the learning includes great experiments where magic happens using only scientific principles. In this article, I’ll detail a few exciting experiments that are simple, practically free, fun, and teach your child all about the wonders of buoyancy.

1. Archimedes’ Discovery:
This is a great experiment to start out with, because it shows the principle discovered by Archimedes way back in the 3rd century BC. The story goes that Archimedes was given a job by the king – to determine whether the king was being cheated by a goldsmith who was suspected of providing the king with less than pure gold for a crown. While thinking about how to determine the purity of the gold, Archimedes lowered himself into a bath, noting the water displacement, and realized that a specific amount of water would be displaced in relation to an item’s weight. At this point, Archimedes was so excited about his discovery that he ran from the bath into the public streets, yelling “Eureka!”

What you’ll need:
Large glass container
Block of wood
A scale
Water

Step One: Fill the glass container to the brim with water. Weight the container and write down your results.
Step Two: Carefully place the piece of wood into the water. Some of the water will spill out of the container as the wood displaces the water.
Step Three: Weight the container a second time, and note the results. The two recorded weights should be the same.

Why does it work?
The water that spills out of the glass container weighs the same amount as the piece of wood. This is because an object placed into water will lose as much weight as the weight of the displaced liquid. This is known as buoyancy, and is why you feel so much lighter in a pool than on dry land!

2. Water Mystery:

What you’ll need:
A glass
A small, empty matchbox (only the bottom, open box portion will be used)
A large coin (quarter or half-dollar)
Water
Crayon

Step One: Fill the glass with water. Place the coin into the empty matchbox.
Step Two: Carefully float the matchbox in the water, and mark the height of the water on the side of the glass.
Step Three: Now take the coin out of the matchbox (leaving the matchbox in the glass) and lower the coin directly into the water. Does the water level rise as expected? You may be surprised to see that the water level actually falls!

Why does it work?
The coin is multiple times heavier than the water, and when it is placed into the box, the box also becomes multiple times heavier than the water. Since the box has increased in volume (due to it holding the coin), more water is displaced than when the coin and matchbox are in the water separately.

3. Egg Suspension:

What you’ll need:
A glass container (wide enough to hold an egg)
Salt
Spoon
Water
1 egg

Step One: Fill the glass container about halfway and add a lot of salt. Stir well until you have cloudy, very salty water.
Step Two: Slowly add more water until the container is nearly full. Make sure to slowly pour the fresh water into the container slowly, over the back of a spoon, so that the salt water and fresh water do not mix.
Step Three: Carefully lower an egg into the water and be amazed as it floats magically in the middle of the container.

Why does it work?
The egg is heavier than the fresh tap water, but lighter than the salt water. Because of this, the egg sinks down through the fresh water, but floats on the salt water.

4. Bead Diving:

What you’ll need:
A match with the end broken off
1 Plastic bead with a hole sized for the matchstick
1 Bottle (we used to use old milk bottles with plastic caps, and it always worked perfectly!)
Water

Step One: Fill the bottle with water. Leave a bit of air space at the top.
Step Two: Stick the matchstick about halfway into the hole of the bead. Break off pieces of the matchstick until the bead floats perfectly on the surface of the water.
Step Three: Now cover the bottle opening with your hand or a plastic cap and watch as the bead dives deeper into the bottle! This is very cool to watch, especially if you use a few different colored beads.

Why does it work?
Plastic is a little bit heavier than water, but not much. The wooden matchstick and air give the plastic bead most of its buoyancy, allowing it to float on the surface of the water. When the top of the bottle is covered, the air pressure compresses the air in the bead. Once the air in the bead is compressed, the bead loses its buoyancy, and sinks deeper into the bottle. Experiment with different lids/caps, and see which ones give you the best results.