Floating and Sinking

For this investigation we started off with some big bowls of water and tested various objects to see if they floated or sank.

We tried some polystyrene, which floated, and a stone, which sank. The stone was much heavier than the polystyrene - did it sink because it was heavy? We tested a golf ball and a squash ball, which were about the same size - the heavier golf ball sank but the lighter squash ball floated. Then we tested two toy cars. Although they looked very similar, one car floated and the other one sank. When we looked closely we saw that the car that sank was metal but the one that floated was plastic. So it did seem that heavier things were more likely to sink.

Next we experimented with a block of wood and a 1p coin.

We weighed them on the scales first - the block of wood weighed 130g and the 1p piece weighed 4g.

Syrup, water and oil density column

The coin sank straight away, even though it weighed very little. We were a bit surprised to find that the wood floated, even though it was much heavier.

our floating experiment


The block of wood was heavy, but it was also big. The coin was light, but small. Whether something will float or not depends on how big it is as well as how much it weighs.

How heavy an object is for its size is called its density. Scientists define density as how much mass there is per unit volume.

If something weighs MORE than the SAME VOLUME OF WATER does, it can basically push the water out of the way. We say it is more dense than water, and so it will sink in water.

If something weighs LESS than the SAME VOLUME OF WATER does, it is less dense than water, so it will float in water.

FLOAT (less dense than water) SINK (more dense than water)
squash ball
plastic toy car
golf ball
metal bolt
metal toy car


Making a density column with liquids

Next we looked at some liquids. We poured different liquids into a 100ml beaker, and weighed them.

100ml of oil weighed 90g.  100ml of water weighed 100g.  100ml of golden syrup weighed 146g.

Syrup, water and oil density column

We poured all the different liquids into a tall jar - half of the syrup, then half of the water, then half of the oil. The syrup stayed at the bottom below the water and the oil floated on the top.

Then we poured the rest of the liquids in a bit at a time and watched carefully. We saw drops of syrup falling straight through the oil and water layers to the bottom, whereas drops of oil floated up to the top.

At the end our column had three really nice clear layers - the most dense at the bottom and the least dense at the top.




How to make a tomato float...

If you drop a cherry tomato into a cup of water, it will sink. We know because we all tried it!

Tomatoes are more dense than tap water, so they sink to the bottom.

But when we stirred a couple of teaspoons of salt into our water, and put the tomatoes back in, they floated!

floating tomatoes
floating tomatoes


Although tomatoes are denser than fresh water, the salt water is even more dense than the tomatoes, so they float in it. Dissolving salt in water increases its density - a cup of salty water is heavier than a cup of fresh water of the same volume.

This is why people float so well in the Dead Sea, one of the saltiest seas on Earth. There is so much salt dissolved in the Dead Sea that you can lie back in it and read a book!
> Read more about the Dead Sea


More science to try at home

You can make your own density column in a tall glass or jar. The bottom layer could be golden syrup, honey, maple syrup or treacle as they are all more dense than water. Use water for the middle layer, and oil for the top layer. Any sort of cooking oil should float on top of water.

Once you have made your column, drop a few small objects in - try a grape or cherry tomato, a wooden bead, a cork or piece of polystyrene, a coin or small metal screw, a marble? Can you get different things to float at different heights in your column?

Make an egg float:

The salt water/cherry tomato experiment should also work with eggs (that's uncooked eggs in their shells!)
Eggs will usually sink in fresh water, but if you stir enough salt into the water you should be able to get an egg to float. Try it.

(Science note: if an egg floats in fresh water, it means it is old or has gone bad!  Why?  Egg shells are slightly porous, and as eggs age, they gradually lose mass and the size of the air pocket inside increases. The density of the egg slowly decreases until it is less than the density of water.)

Does an orange float?

Put an orange in a bowl of water. Does it float or sink? Now peel the orange, and try again. Can you explain what happens?