Jupiter2U School
Chapter 03

Floating & Sinking, Materials & States

Primary
At a glance
Core ideaThings float when they are light for the space they take up; heat shifts solid↔liquid↔gas.
Key termBuoyant force — the upward push water gives to a submerged object.
You can…Predict float-or-sink and name the three states of matter.
Watch outShape matters as much as material — flatten foil into a boat and it floats.
Theory

Why some things float and others sink

Drop a rubber duck in water and it floats. Drop a coin in and it sinks straight to the bottom. Whether something floats or sinks depends on how its weight compares to the water it pushes out of the way — something light for its size floats; something heavy for its size sinks.

All materials exist in different states: solid (keeps its own shape, like ice or a rock), liquid (takes the shape of its container, like water or juice), and gas (spreads out to fill all the space, like air or steam). Water can be all three: ice (solid), water (liquid), and steam (gas) — it's the same substance, just at different temperatures.

Explanation

Why huge steel ships float

Steel is much heavier than water — a solid steel block sinks instantly. So why do giant steel ships float? Because a ship isn't a solid block — it's hollow, shaped like a big bowl full of air. Spread out over a wide, hollow shape, the ship's total weight is still less than the water it pushes aside, so it floats — even though the steel itself would sink alone.

Heating a solid usually turns it to liquid (ice → water), and heating a liquid enough turns it to gas (water → steam). Cooling works in reverse: gas → liquid → solid. The stuff inside is the same; heat just makes it move around more freely.

Key idea. Shape matters as much as material. A ball of foil sinks; the same foil flattened into a wide boat-shape floats — same material, same weight, different shape.
FLOATS: weight = buoyant force weight buoyant force SINKS: weight > buoyant force weight buoyant
A boat floats when its weight equals the buoyant force; a coin sinks because its weight is greater.
Practical

Try it — the float-or-sink tub test

  1. Fill a tub or sink with water.
  2. Gather objects: a coin, a cork, a spoon, a rubber duck, an apple, a small stone, a piece of foil.
  3. Before dropping each one in, guess: will it float or sink?
  4. Drop it in and check. Keep a tally of your guesses versus what really happened.
  5. Take the piece of foil — first scrunch it into a tight ball, then flatten it into a little boat shape. Test both. What changed?
Q&A
Name the three states of water.

Ice (solid), water (liquid), and steam (gas).

Why does a heavy steel ship float?

Its hollow shape spreads the weight out so it's still lighter than the water it pushes aside — even though solid steel alone would sink.

A small stone and a large cork — which one floats?

The cork floats (it's light for its size); the stone sinks (it's heavy for its size).

What happens to ice if you leave it out on a warm table?

It melts into liquid water as heat flows from the warm air into the cold ice.

Concept mind map

How the ideas connect

Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.

Floats on topSinks to bottomHeavy vs lightSolid, liquid, gasWater pushes upBoat shapesFloating & Sinking
Infographic

The key facts, visualised

Float
Stays on top of the water
Sink
Drops to the bottom
Upthrust
Water pushes objects upward
States
Matter can be solid, liquid or gas
Solved examples

Worked problems, step by step

Follow each solution line by line, then try to reproduce it on paper before moving on.

Example 1A small nail sinks but a big steel ship floats. Why?

  1. A ship is hollow and holds air
  2. The wide shape pushes lots of water aside
  3. Water pushes back up hard enough to hold it

Example 2Which floats: a cork or a stone?

  1. Cork is very light for its size
  2. Stone is dense and heavy
Practice problem set

Now you try

Work each one out first, then tap to reveal the worked answer.

1Does a wooden block float or sink?
It floats because wood is light for its size.
2Why does a coin sink in a glass of water?
Metal is dense, so the water cannot hold it up and it sinks.
3Name the three states of matter.
Solid, liquid and gas.
4What shape helps a boat float instead of sink?
A wide, hollow shape that holds air and pushes water aside.
5When water turns to ice, which state does it become?
It becomes a solid.
6Why can you float more easily in the sea than a pool?
Salty sea water pushes up harder, so it is easier to float.