Density

Understanding Density

  • Density is a measure of mass per unit volume of a material or substance.

  • The formula to calculate density is density = mass/volume where mass is measured in kilograms (kg) and volume in cubic meters (m³)

  • In practical terms, density tells us how ‘packed’ the particles in a substance are.

  • A substance with high density will have particles that are tightly packed, while a substance with low density will have particles that are more spaced out.

Investigating Density

  • It’s important to note that you can measure the density of a regular object by measuring its mass and its dimensions and then using the formula density=mass/volume.

  • For irregular shaped objects, you find the volume by displacement. This involves dropping the object into a known volume of water and measuring the amount of water displaced by the object.

Factors That Can Affect Density

  • Temperature: As temperature increases, most substances expand causing the density to decrease. Conversely, when temperature decreases, substances contract, which increases their density.

  • Pressure: If you compress a substance, it becomes denser because you are decreasing its volume.

Density, States and Changes of State

  • The state of a substance (solid, liquid, or gas) will affect its density because it influences how closely its particles are packed.

  • Generally, solids are denser than liquids, and liquids are denser than gases. For example, ice (solid water) is less dense than liquid water, which is why ice floats on water.

  • When a substance changes state, its density will change because the volume changes.

Density and Buoyancy

  • Objects float in liquids and gases if their density is less than the density of the liquid or gas.

  • If an object has a greater density than the liquid or gas, it will sink. This is known as buoyancy principle.

Understanding Relative Density

  • The relative density (or specific gravity) of a substance is the ratio of the density of the substance to the density of a reference substance. It is a dimensionless quantity.

  • Usually, water is used as the reference substance for liquids and solids, and air is used for gases.

  • For example, if a substance has a relative density greater than 1, it means it’s denser than the reference substance.

Remember to practise problems related to density as density is a very practical aspect of Physics and it is likely that you’ll encounter problem-solving questions relating to this topic.