- 2" x 2" block of pumice
- 2" x 2" block of granite
- Tiny pebbles the size of your baby fingernail
- Hand towels
- Notebook and pencil
- Are you ready to rock? First, collect your rocks. If you don’t live near a beach, you can find sand at craft stores as well. Pumice stones are common fish tank ornaments, and they’re also used for personal grooming. Granite is common in landscaping gravel. If you’d like to add other rocks and compare them to the pumice and granite, make sure that they’re around the same size so that you can compare them easily.
- Now that you’re rocking, fill a dishpan with water. Keep your hand towels handy in case of spills. Put the dishpan in a waterproof place.
- Hold each of the rocks in your hand. What does each look like? What does each feel like? Write down your observations in your notebook, or get an adult to help.
- Create a hypothesis, your best guess about what is going to happen. Will each rock sink or float? Do you think that rocks can float?
- Now, test your rocks. Start with the big ones and move to the smaller ones. Gently place the piece of granite on top of the water. As you let it go, get a helper to start the stopwatch. What happens? If it sinks, how long does it take to sink to the bottom?
- Do the same thing with the pumice, the tiny pebbles, and the sand. Make sure that you experiment with the sand last.
- As you experiment with each rock or set of rocks, write down whether your rock floats or sinks, the time it takes to sink, and any other observations you may have about the rocks and the water.
The pumice will float for a while and then gradually sink. The granite will sink. Small pebbles will float momentarily and then sink. Sand will float, then move into the water, and gradually drift down to the bottom of the container.
If you’ve ever thrown a rock into a pond, you know that rocks usually sink. So why do some rocks float?
The reason that most rocks sink is because of the law of buoyancy, which is all about how things float or sink. This law is also called Archimedes’ Principle. Archimedes was a scientist who lived in ancient Greece.
Archimedes’ Principle says this: An object will float if it displaces as much water as it weighs. This sounds complicated, but what it means is that everything you put into water pushes the water away from itself. If the amount of water it pushes away is the same as the weight of the object, then the object will float. Light things float, but heavy things can also float if they’re designed to push enough water away. Think about big ships on the ocean!
So what does this have to do with your experiment? Granite and small pebbles generally sink to the bottom right away. When it comes to rocks, pumice is a bit of an oddball. Pumice has hundreds of tiny air bubbles in it. When it was made in a volcano, the volcano’s force pushed air into the rock. This makes pumice very light. It usually floats for a while, but then water gets into it and it starts to sink.
What about sand? If you use dry sand and a small container, the sand might be able to sit on the surface for a short amount of time. This is because of the surface tension of the water. Surface tension means that the water has a tough “skin” of water molecules that allow some things to float on the surface, like leaves. After a while, the sand gets wet and the water usually moves, and the sand will sink to the bottom.
Can you think of anything else that might float on water? Armed with your new knowledge, try to find out why it can!