Science notes, unit 7

Density

Textbook Science Year 3 (DLP), pages 95–104

What this chapter covers

You can sort objects into those that float and those that sink (7.1.1)
You can say that objects less dense than water float and objects more dense than water sink (7.1.2)
You can show that adding salt or sugar makes water more dense (7.1.3)
You can compare how dense different liquids are by looking at their layers (7.1.3)
You can explain how density helps us in daily life, such as life jackets, anchors and fish farm floats (7.1.4)

A girl points at a yellow toy duck floating in an aquarium while her father watches — "Father, my toy duck floats!" Why doesn't the toy duck sink?

Picture: Science Year 3 (DLP) textbook, p. 95.

7.1Float and sink

Drop a stone into a river. Plop! It goes down. Now drop a dry twig. It stays on top. Some things float and some things sink.

An object floats when it stays on the surface (the top) of the water.
An object sinks when it goes down under the water.

Children at a river in a kampung. A stone thrown into the water sinks. A branch floats. — At the river near the kampung: the stone sinks, but the branch floats.

Picture: Science Year 3 (DLP) textbook, p. 96.

Let's test some objects

In the textbook, pupils put six objects into a small aquarium full of water. Then they looked carefully. Did each object float or sink? Here is what they found.

Object	Float or sink?
Coin	Sinks
Ping-pong ball	Floats
Marble	Sinks
Soap	Sinks
Cork	Floats
Sponge	Floats

We can classify them. Classify means to sort things into groups.

Objects that float	Objects that sink
Ping-pong ball, cork, sponge	Coin, marble, soap

Look at these two photos. The big, heavy logs float. The small, light rubber bands sink! So floating is not just about being big or small.

Many long logs floating on a river — **Logs**
Float

Rubber bands lying at the bottom of a cup of water — **Logs**
Float

Photos: Science Year 3 (DLP) textbook, p. 99.

Did you know? A coconut can float on the sea for a very long time. Some coconuts float to another island and grow into a new coconut tree there.

Ali drops a key and a plastic bottle cap into a basin of water. Which one floats? Which one sinks?

Show answerHide answer

The plastic bottle cap floats. The key sinks.

7.2Density

So why do some things float and others sink? The answer is density.

Density tells us how tightly packed the stuff in an object is. An object that is more dense has its stuff packed tightly, like a solid block. An object that is less dense has its stuff spread out, often with lots of air inside.

Look at the sponge and the soap. They are about the same size.

The sponge has lots of tiny holes full of air. We call them air spaces.
The soap has no air spaces. It is solid all the way through.

Close-up of a sponge full of holes and a bar of soap with no holes — The sponge has a lot of air space. The soap has no air space.

In a tank of water, the sponge floats at the top and the soap lies at the bottom — The sponge floats. The soap sinks.

Pictures: Science Year 3 (DLP) textbook, p. 98.

Floating and sinking depend on the density of the object compared with the density of water.

Objects that are less dense than water will float.
Objects that are more dense than water will sink.

	Float	Sink
Where is the object?	On the surface of the water	Under the water
Density	Less dense than water	More dense than water
Examples	Cork, sponge, log, apple, feather	Coin, soap, marble, stone, paper clip

"Heavy things sink and light things float" is not always true. A big log is heavy, but it floats because it is less dense than water. A tiny paper clip is light, but it sinks because it is more dense than water.

Did you know? Ice is less dense than water. That is why the ice cubes in your teh ais float at the top of the glass!

Fill in the blanks: An object that is ______ dense than water will sink. An object that is ______ dense than water will float.

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more; less

7.3Making water more dense

A grape is more dense than water. So a grape sinks in plain water. Can we make it float without touching it? Yes! We can change the water.

Adding salt

Stir some salt into the water. The salt dissolves. Dissolve means it mixes into the water until we cannot see it any more. Now the water is more dense. The grape that sank before now floats!

Left: salt being added to a beaker with a grape at the bottom. Right: the grape floats in the salty water. — Before: the grape sinks. After adding salt: the grape floats.

Picture: Science Year 3 (DLP) textbook, p. 100.

Adding sugar

Sugar works too. In the textbook, pupils carved a fish shape from a carrot. The carrot fish sank in plain water. They added sugar one spoonful at a time and stirred. After enough sugar, the fish floated!

A carrot fish at the bottom of a beaker of water — **Plain water**
The carrot fish sinks

The carrot fish floating in very sweet water — **Plain water**
The carrot fish sinks

Photos: Science Year 3 (DLP) textbook, p. 100.

Water becomes more dense when we dissolve salt or sugar in it.

The cutter is sharp. Always ask an adult to help when you carve the carrot.

Did you know? The Dead Sea is so salty that people float on top easily, like lying on a bed. They can even read a book while floating!

Layers of liquids

Liquids have density too. Pour coloured water, cooking oil and glycerine slowly into one bottle. They do not mix. They make layers, like a kuih lapis!

A bottle with a yellow oil layer on top, purple water in the middle and clear glycerine at the bottom — **The textbook bottle**
Three layers

Diagram: cooking oil on top, coloured water in the middle, glycerine at the bottom — **The textbook bottle**
Three layers

Photo: Science Year 3 (DLP) textbook, p. 101.

The most dense liquid goes to the bottom. Here it is glycerine.
The least dense liquid stays at the top. Here it is cooking oil.
Cooking oil floats on water because oil is less dense than water.

When Mak fries ikan, a drop of cooking oil falls into a bowl of water. Where will the oil be? Why?

Show answerHide answer

The oil floats on the surface of the water, because oil is less dense than water.

7.4Density in daily life

People use what they know about density to stay safe and to work.

Ship and anchor

A ship and an anchor are both made of iron. But the ship floats and the anchor sinks. Why?

The ship is hollow. It has lots of air inside. So the whole ship is less dense than water. It floats.
The anchor is solid iron. It is more dense than water. It sinks to the sea bed and holds the boat in place.

A boat floats on the sea while its iron anchor lies on the sea bed — The boat floats. Its iron anchor sinks to the sea bed.

Picture: Science Year 3 (DLP) textbook, p. 102.

Life jacket and fish farm

A boy swimming in a pool wearing a yellow life jacket — **Life jacket**
It is less dense than water, so it floats. It keeps us from drowning.

Fish farm cages held up by blue floating drums — **Life jacket**
It is less dense than water, so it floats. It keeps us from drowning.

Photos: Science Year 3 (DLP) textbook, p. 102.

A buoy (say "boy") is a float, often a closed drum filled with air. It floats on water and holds things up.

Think like a scientist

Adik's orange floats in a beaker of water. How can we make it sink?

An orange floating in a beaker of water — The orange floats.

Peel the skin off. Orange skin has lots of tiny air spaces, like a sponge, so it acts like a little life jacket. Without the skin, the orange is more dense than water, so it sinks.

Picture: Science Year 3 (DLP) textbook, p. 104 (HOTS question and answer).

Did you know? A submarine can float or sink when it wants to. It lets water into big tanks to sink. It blows air in to push the water out and rise up again.

Why must we wear a life jacket when we ride a boat to Pulau Redang?

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A life jacket is less dense than water, so it floats. If we fall into the sea, it keeps us on the surface so we do not drown.

Summary

Idea	Remember
Float	The object stays on the surface of the water. It is less dense than water.
Sink	The object goes under the water. It is more dense than water.
Air spaces	Things with lots of air inside (sponge, cork, ship) are less dense, so they float.
Making water more dense	Dissolve salt or sugar in it. A grape or a carrot fish that sank can now float.
Layers of liquids	The most dense liquid is at the bottom. Oil floats on water because oil is less dense.
Uses	Life jackets, iron anchors and buoys at fish farms.