Measurements: Density

These two cubes are made of different metals. One is aluminium and the other one is copper. They are both the same size. Do you think they also weigh the same? Let’s check!

Copper is heavier than aluminium! Copper and aluminium cubes are made of different atoms which are different sizes, have different masses and are arranged in different ways. Atoms of copper have a higher atomic mass, so they are heavier than atoms of aluminium. At the same time, they are smaller, so more of them fit in the same space. If we take a piece of copper and a piece of aluminium that weigh the same, the piece of aluminium will be bigger.

There is a relationship between how much different materials and substances weigh and how much space they take. It describes how much MASS fits in a given unit of VOLUME. It is called DENSITY. To find out what the density of a substance is, we need to know its mass and volume. Let’s take a dice.

We can measure its mass using a scale. To express measurement values in physics, we use the international system of units - so called SI units. The SI unit of mass is a kilogram. We can measure volume in different ways. For some objects, like this dice, we can calculate the volume easily from their shape.

The volume of the dice is equal to its length multiplied by its width and its height. We can also measure the volume by submerging the object in a FLUID, like water. When you drop an object into water, the level will rise. The amount by which the water rises is equal to the volume of the object. This method uses the DISPLACEMENT OF FLUID to measure the volume of a solid object.

The SI unit of volume is a cubic metre. So the SI unit for density is kilograms per cubic metre. Another common unit used to express density is grams per cubic centimetre. 1 gram per cubic centimeter is equal to one thousand kilograms per cubic metre. Now, what do you think this steel bridge and this steel bolt have in common?

They are made of the same material, so the bridge has the same density as the bolts used to build it! In the same conditions, density remains the same for the same material, no matter its size. Density is a CHARACTERISTIC PROPERTY of a substance. However, the density can change, if we change conditions such as temperature or pressure. If we increase the pressure, the particles are pressed together.

They occupy less space but their mass does not change, so the density increases. If we increase the temperature, the particles move faster and move further apart. The volume increases but their mass stays the same. The density decreases. This is why hot air balloons can fly!

In liquids or gases, substances with lower densities float, and those with higher densities sink. If we heat up the air inside the balloon’s envelope, its density becomes lower than the air outside. The hot air floats up and lifts the balloon thanks to the difference in density!