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Some Things Float and Some Things Sink · Properties of Matter · Physical Properties · floating

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Some Things Float and Some Things Sink

with Atlas

Atlas the friendly explorer kneels beside a big clear tub of water on a sunny porch, holding a rubber duck in one hand and a smooth pebble in the other, grinning as he prepares to drop them both in and watch what happens.

Identify which objects float on top of water and which objects sink to the bottom.
Compare two objects by describing what each one does when placed in water.
Predict whether a new object will float or sink before testing it.
Explain that floating and sinking are things we can observe and test.

Key terms

float: to stay up on top of the water
sink: to go down to the bottom of the water
predict: to guess what will happen before you test it
test: to try something out to find the real answer
hollow: having an empty space with air inside

Why Some Things Float

An object floats when it is lighter than the same amount of water it pushes out of the way. Things with air trapped inside, like a rubber duck or an empty bottle, are very light for their size, so the water can hold them up. Wood and leaves float for the same reason — they are light compared to water.

Why Some Things Sink

An object sinks when it is heavier than the same amount of water it would push aside. A coin or a pebble is solid all the way through with no air inside, so it is heavier than the water around it. The water cannot hold it up, and the object drops to the bottom of the tub.

Worked examples

You want to know if a metal spoon will float or sink. What should you do?

First make a prediction: a spoon is solid metal with no air inside, so guess it will sink.
Gently place the spoon in the tub of water.
Watch what happens — the spoon drops to the bottom.

Answer: The metal spoon sinks because it is heavier than the water around it.

Hi, I am Atlas! Let us learn something cool about water today. When you put something in water, one of two things can happen. The object might FLOAT — that means it stays up on top of the water. Or the object might SINK — that means it goes down to the bottom. Try this: think about a rubber duck. When you put it in the bath, it bobs on top. It floats! Now think about a little rock. Drop a rock in a puddle and it goes straight down. It sinks. Some things that float: a rubber duck, a leaf, a piece of wood, an empty plastic bottle. Some things that sink: a coin, a pebble, a metal spoon, a marble. Here is a hint: you cannot always tell just by looking. The best way to find out is to TEST it — put the object in water and watch what happens. That is what scientists do!

Activity

Drag each object into the water tub and watch whether it floats or sinks.

Practice

Will a wooden block float or sink when placed in water?

Will a solid metal marble float or sink in a tub?

Common mistakes to avoid

Heavy things always sink and light things always float.A huge wooden log floats while a tiny coin sinks, so size and weight alone do not decide it.
Shiny objects float because they look special.Being shiny has nothing to do with floating — what matters is whether the object is lighter than water.

Check your understanding

You drop a small rock into a tub of water. What happens?

A rubber duck and a coin are placed in water. Which one floats?

What is the best way to find out if an object will float or sink?

Recap

When you put something in water it can float on top or sink to the bottom. Things with air inside tend to float, while solid heavy things tend to sink. The best way to know for sure is to test it.

Reflect

What is one object at home you would like to test in water?