Floating and Sinking Notes & Questions | Class 5 Chemistry | Drishti OS

Floating and Sinking

What you'll learn

Some materials float on water (wood, dry leaves, plastic, ice, cork); others sink (iron, stone, glass marbles, coins).
Whether an object floats or sinks depends on how heavy it is compared to the same amount (volume) of water — this is linked to the idea of density.
Shape also matters: a flat sheet of aluminium foil can float, but the same aluminium rolled into a tight ball sinks.
To connect with simple sink-or-float classroom experiments using a tub of water.

Key concepts

Level 1 — Floating and sinking basics

Verbal: An object floats if it is light for its size compared to water; it sinks if it is heavy for its size compared to water.

Symbolic: Lighter-for-size than water → floats. Heavier-for-size than water → sinks.

Object	Floats or sinks?	Everyday reason
Wooden log	Floats	Wood is light for its size
Iron nail	Sinks	Iron is heavy for its size
Ice cube	Floats	Ice is slightly lighter than water
Stone	Sinks	Stone is heavy for its size
Cork	Floats	Cork is very light and full of tiny air pockets

NCERT link: Simple sink-float sorting activity with a bucket of water and classroom objects.

Level 2 — Why shape and material both matter

Verbal: The same material can float or sink depending on its shape — a flat, spread-out shape can push more water aside and trap air, helping it float; a tightly balled-up shape of the same material sinks.

Real-life: A steel boat floats because its hollow, spread-out shape displaces (pushes aside) a large amount of water and traps air inside, even though a solid lump of the same steel would sink.

Shape example	Floats or sinks?	Reason
Flat aluminium foil sheet	Floats	Spreads out, traps air underneath
Same foil rolled into a tight ball	Sinks	Loses spread shape, no trapped air
Hollow steel boat	Floats	Shape displaces lots of water and holds air
Solid steel block	Sinks	No hollow space or trapped air

Worked example

Why does a heavy steel ship float, but a small steel nail sinks?

Step 1 — The ship has a hollow shape that spreads its weight over a large area and traps air inside.
Step 2 — This makes the ship overall light for the huge amount of water space it takes up.
Step 3 — A solid nail has no hollow shape and is heavy for its small size.
Answer: Shape lets the ship float despite being made of the same heavy material (steel) that sinks as a solid nail.

Will a dry wooden log float or sink in a pond?

Step 1 — Wood is light for its size compared to water.
Step 2 — Wood also often has tiny air-filled pores.
Answer: A dry wooden log floats.

Common mistakes

Mistake	Why it happens	Fix
Heavy objects always sink	Ignoring shape and spread	A heavy hollow ship still floats due to shape
All metals sink	Forgetting shape effects	A flat foil sheet of metal can float
Big objects always sink	Confusing size with density	A big log of wood floats; a tiny dense pebble sinks
Floating means an object has no weight	Misunderstanding floating	Floating objects still have weight; they are just light for their size compared to water

Quick check

Name two objects that float and two that sink in water.
Why does a steel ship float while a steel nail sinks?
Does ice float or sink in water?
Stretch: If you flatten a lump of clay into a boat shape, will it float better than a solid ball of the same clay? Why?

Revision tip: Test five household objects in a tub of water and record whether each floats or sinks.

Open the Practice tab for graded questions on Floating and Sinking.

Floating and Sinking