Density: how much matter fits — Chemistry, 7–10 years
Some objects are heavy for their size and others are light for their size. Density helps compare how much matter is packed into the same amount of space, which explains why some things float and others sink.
Packed into space
Density tells us how much matter is packed into a certain amount of space. A small metal cube can be heavier than a much larger sponge because its matter is packed more tightly. Density is not simply how big or heavy something is; it compares the two.
Why compare size and mass?
A scale alone cannot tell us what a material is like, because a large piece usually weighs more than a small piece. Scientists needed a fair comparison between different-sized samples. Density gives that comparison, so materials can be identified and objects can be designed to float, sink or balance.
A mystery block
A block has a mass of 200 grams and takes up 50 cubic centimetres. Divide mass by volume: 200 ÷ 50 = 4 grams per cubic centimetre. If a reference table says a material with this density is a certain metal, the measurement gives us a strong clue about the block.
Heavy does not mean dense
It is reasonable to call a large object dense when it feels heavy, because our hands notice its total mass. But a large object can weigh more simply because it contains more material. To compare density fairly, we must consider equal volumes or calculate mass divided by volume.
Boats and balloons
A steel ship can float because its hollow shape includes a lot of air, making the ship’s average density lower than water. A solid steel ball sinks. Hot-air balloons rise when the air inside is less dense than the cooler air outside. Shape and temperature can change the average density of an object or mixture.
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