Grade by Grade
C. Matter and Energy · Properties of Liquids and Solids

Float or Sink?

C2.6 A1.2 A1.3 A1.4 A3.3About 40 minutes
Learning goals
  • I can predict and test whether objects float or sink.
  • I can sort objects and materials into floaters and sinkers.
  • I can explain that the material and the shape of an object both matter.
  • I can design and build a boat that floats and carries a load.

Words to know

float
To stay at the top of a liquid.
sink
To go down to the bottom of a liquid.
buoyancy
The push of water upward on an object. It helps things float.
predict
To make a smart guess about what will happen.
classify
To sort things into groups by a property.
load
The weight something carries, like coins in a toy boat.

Bath time puzzle

Your rubber duck bobs on top of the bath water. But when you drop a metal spoon, it goes clunk on the bottom of the tub.

Why do some things float and some things sink? Let's be scientists and find out!

A yellow rubber duck floating in a bathtub of water, with a metal spoon lying on the bottom.
The rubber duck floats on top of the water, while the metal spoon has sunk to the bottom. AI-generated image

Float or sink

  • An object floats if it stays at the top of the water.
  • An object sinks if it goes down to the bottom.

When you put something in water, the water pushes up on it. This push is called buoyancy. If the push is strong enough, the object floats. If not, it sinks.

wood: floatsboat: floatsrock: sinkscoin
Wood and a boat float at the top of the water. A rock and a coin sink to the bottom.
Usually floatsUsually sinks
corkcoin
wooden blockrock
plastic bath toymetal spoon
leafmarble
applekey
ice cubemetal paper clip
Classifying objects as floaters and sinkers

Big does not always mean sink

Many people think big things sink and small things float. But a huge log floats down a river, and a tiny pebble sinks! Size is not the main clue.

Two things matter a lot:

  1. The material. Wood and cork usually float. Rock, glass marbles and most metal objects sink when they are solid lumps.
  2. The shape. A ball of modelling clay sinks... but the same clay shaped into a boat with thin sides floats! A boat shape is wide and hollow. It pushes away lots of water, so the water pushes up on it more. That is how big steel ships float.

Birchbark canoes

For a very long time, First Nations peoples, including the Anishinaabe, have built birchbark canoes. Builders shape the bark of the birch tree over a light frame of wood such as cedar, sew it with tree roots and seal the seams with tree gum. The canoes are light, strong and float very well, so they can carry people and loads across lakes and rivers. Some builders still make birchbark canoes today and teach young people how.

Worked example

Let's classify: Predict, then test

We will test 6 objects in a sink of water: a cork, a coin, a wooden block, a key, an apple and a plastic lid.

  1. Predict: Before testing, sort them into two piles: "I think it will float" and "I think it will sink".
  2. Test: Gently place each object on the water, one at a time. Do not drop or throw them.
  3. Observe: The cork, wooden block, apple and plastic lid stay at the top. The coin and key go to the bottom.
  4. Record: Make a chart with columns: object, prediction, result.

Floaters: cork, wooden block, apple, plastic lid. Sinkers: coin, key. Compare your results with your predictions.

Worked example

Let's test shape: The clay challenge

Two pieces of modelling clay (or play dough) are the same size. One is rolled into a ball. One is flattened and shaped into a boat with thin sides. Which floats?

  1. Same material, same amount: Both pieces are the same amount of clay. Only the shape is different.
  2. Test the ball: Place the ball gently in the water. It sinks to the bottom.
  3. Test the boat: Press the clay flat, then pinch up thin sides to make a wide boat. Place it gently on the water. It floats! (If it sinks, make it wider with thinner sides and try again.)
  4. Think: The boat shape is wide and holds air inside. It pushes away more water, so the water pushes up on it more.

The boat floats and the tight ball sinks. Shape matters!

clay ballsinks to the bottomsame clay, boat shapefloats at the topwater pushes up
The same amount of clay in two shapes. The ball sinks. The wide, hollow boat pushes away more water, so the water pushes up on it enough to float.
Hands-on activity

Float or sink sort, and the boat challenge

You need: a sink, tub or large bowl of water, a towel, small objects from home (cork, coin, crayon, spoon, plastic toy, bottle cap, leaf, key, sponge), aluminum foil, and coins or marbles for a load. A grown-up must be with you near water.

Part 1: Classify

  1. Predict which objects will float and which will sink. Sort them into two piles on the towel.
  2. Test each one gently. Record the results in a chart.
  3. Were any predictions wrong? Which ones surprised you?

Part 2: Engineer a boat

  1. Use a piece of foil about the size of a sheet of paper. Design and build a boat.
  2. Float your boat. Add coins one at a time, gently, in the middle.
  3. Count how many coins it holds before it sinks.
  4. Improve the design (wider? taller sides?) and test again. Did it hold more?
  5. Share your best design and the number of coins it held.

Safety: Always have a grown-up with you near water. Wipe up spills so no one slips.

Just for fun

Why did the rubber duck get the best grades in bath class?

It always stayed on top of its work!

Your turn

1

Does a cork float or sink?

2

Does a metal key float or sink?

3

What does buoyancy mean?

4

Name two things that float.

5

A log is very big. Does it float or sink?

6

A ball of clay sinks. How can you make the same clay float?

More practice: warm-up

1

Does a rock float or sink?

2

Does a plastic bath duck float or sink?

3

Does an ice cube float or sink in water?

4

Does a coin float or sink?

5

What should you do before you test: predict or guess wildly?

More practice: on your own

1

Ben says, "Big things sink and small things float." Give an example that shows he is wrong.

2

Why are canoes and boats shaped to be wide and hollow?

3

Sort these materials into floaters and sinkers: cork, glass marble, wood, metal paper clip.

4

What did First Nations builders use to make birchbark canoes?

5

In the clay challenge, why must both pieces of clay be the same size?

Challenge

1

A steel ship weighs more than a house, but it floats. A steel nail sinks. Explain.

2

Your foil boat sinks after 8 coins. Give two ways to improve the design.

3

A toy boat floats. Then a wave splashes water inside it, and it fills up and sinks. Why?

Check your understanding

Quiz

1.

Which object sinks?

2.

The upward push of water on an object is called...

3.

A ball of clay sinks. What will make the same clay float?

4.

Which statement is true?

5.

Birchbark canoes are built by...

6.

To classify objects as floaters and sinkers, you should...

For parents and teachers

Big idea: Solid objects and materials can be classified by whether they float or sink (buoyancy, C2.6). Both the material and the shape matter, not just size. Boats and canoes float because of their wide, hollow shape.

Indigenous perspectives (A3.3): Birchbark canoes are a living technology of First Nations peoples, including the Anishinaabe. Present them as knowledge and craft that continue today, not only as history.

How to help: Bath time is perfect: predict, test and talk. When a prediction is wrong, say, "Great! Now we learned something." In the boat challenge, focus on improving the design.

Common difficulties:

  • "Heavy things sink, light things float." Partly true for lumps of material, but shape can change the result (clay ball vs. clay boat).
  • Dropping objects in. Splashing can push things under; place them gently.
  • Things that sink after a while (a sponge, paper). They soak up water. This leads into the next lesson.

Safety (A1.4): Always supervise children near water, even a sink or a bowl.