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A ship contains lots of air, allowing it to float.A ship contains lots of air, allowing it to float.Density is the amount of matter in a certain volume. The more matter within a certain volume, the denser the substance or object and the greater its mass. Some objects are denser than others: their atoms are more tightly packed together than others, or more massive, or both. Metals such as iron are dense materials. But if the metal is made into a boat’s hull, its shape contains lots of air, which has a very low density. The overall density of the iron-plus-air is less than the density of water, and so a boat will float.

Floating and sinking

Things floating or swimmingThings floating or swimmingDensity is important because it determines whether things float or sink. A lump of iron is more dense than water and so it sinks. Air has a very low density, so anything that contains lots of air, such as plants and wood, floats in water. Our bodies, which contain both air and water, just about float, but a rubber ring makes it easier. Boats and buoys contain a lot of air. Fish let gases in or out of their swim bladders, according to whether they want to rise or fall in the water.

Gold is nearly 20 times the density of water, but it is not the densest element. That is osmium.

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