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Lesson · Grade 7 · Science

Density: why some things float

  • Grade 7
  • Science
  • 1 min 8 s
  • British Columbia
  • BC7-SC-14

An ice cube dropped into a tank of water floats with just its top above the surface, while an iron cube of the same size sinks to the bottom. Comparing one cubic centimetre each of ice (0.92 g), water (1 g) and iron (7.9 g) shows that things less dense than water float and things more dense sink.

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In one sentence

The question this lesson answers

Why do some things float in water, while others sink?

Drop in a cube of ice, and it floats with just its top above the water.

Transcript

What the lesson says

Why do some things float in water, while others sink?

Drop in a cube of ice, and it floats with just its top above the water.

Drop in an iron cube of the same size, and it sinks to the bottom.

The answer is a property called density.

Density is how much mass is packed into each cubic centimetre.

Let us take one cubic centimetre each of ice, water and iron.

One cubic centimetre of water has a mass of 1 gram.

The same amount of ice has a mass of only 0.92 grams.

So ice is less dense than water.

The same amount of iron has a mass of about 7.9 grams.

So iron is more dense than water.

Anything less dense than water floats in it.

Anything more dense than water sinks in it.

So a thing floats if it is less dense than water, and sinks if it is more dense.

Curriculum

Where this fits

British Columbia, Grade 7

Science, Chemistry: elements and compounds are pure substancesBC7-SC-14

Previous: Pure substances and mixturesNext: What Makes Each Pure Substance Different

The outcomes and strands follow the British Columbia curriculum.

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