Heavy for Size

• Two objects of the same size can weigh differently — this is the idea of being heavy or light for size.
• If an object is heavier for its size than an equal amount of water, it usually sinks.
• If an object is lighter for its size than an equal amount of water, it usually floats.
• A solid iron ball sinks, but a hollow iron ship floats because its shape spreads the iron over a larger space.
• Shape and trapped air can help even dense materials like iron float when hollowed out.

Level 1 - Core idea

Verbal: Whether an object floats or sinks depends on how heavy it is compared to the same amount of water, and its shape can change this by spreading the material out and trapping air.

Symbolic: heavy for size (denser) than water -> sinks; light for size (less dense) than water -> floats; hollow shape can flip the outcome

Visual: Compare a solid iron ball, which sinks, with a hollow iron bowl of the same iron, which can float because it traps air and spreads its weight.

Level 2 - Going deeper

Notice where you see this idea at home, at school, and in your neighbourhood — connecting the concept to daily life makes it easier to remember and use.

Level 3 - NCERT anchor

NCERT EVS Looking Around 4 — activities comparing solid and hollow objects in water build the idea that shape affects floating, alongside material.

A solid iron ball sinks in water, but a hollow iron bowl of similar iron floats. Why?

Step 1 - The solid ball is heavy for its size — denser than water.
Step 2 - The hollow bowl has the same iron spread over a bigger, hollow shape.
Step 3 - The hollow shape traps air and makes it lighter for its overall size.
Answer: The hollow shape makes the bowl float, while the solid ball sinks.

A huge iron ship carries goods on the sea without sinking. How is this possible if iron is denser than water?

Step 1 - A solid block of iron is denser than water and would sink.
Step 2 - The ship's hull is hollow and holds a large volume of air inside.
Step 3 - This makes the ship, as a whole, light for its very large size.
Answer: The ship's hollow shape lets it float even though iron itself is dense.

• What decides whether an object floats or sinks, along with its material?
• Does a solid iron ball float or sink in water?
• Why can a hollow iron ship float on water?
• Give one example other than a ship where shape helps something float.
• Stretch: If you flatten a lump of clay into a wide, shallow bowl shape, how might that change whether it floats?

Revision tip: Remember: it is not just what an object is made of, but also its shape, that decides floating or sinking.

Open the Practice tab for graded questions on Heavy for Size.