Density

2026 Syllabus Objectives

Core:

  1. Define density as mass per unit volume; recall and use the equation ρ = m/V
  2. Describe how to determine the density of a liquid, of a regularly shaped solid and of an irregularly shaped solid which sinks in a liquid (volume by displacement), including appropriate calculations
  3. Determine whether an object floats based on density data

Supplement: 4. Determine whether one liquid will float on another liquid based on density data given that the liquids do not mix

What is Density?

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Density is the mass per unit volume. In simpler terms, it tells us how much matter (mass) is packed into a certain amount of space (volume).

Think of it this way: if you have two blocks of the same size (same volume), but one is made of iron and one is made of plastic, the iron block feels heavier. This is because iron is denser than plastic – it has more mass packed into the same space.

The Density Formula

Density is represented by the Greek letter ρ (pronounced "rho").

Formula: ρ=mV\rho = \frac{m}{V}

Where:

  • ρ = density (measured in kg/m³)
  • m = mass (measured in kg)
  • V = volume (measured in m³)

SI Unit: The standard unit for density is kg/m³ (kilograms per cubic metre).

Key Point: Density is a scalar quantity, which means it only has a size (magnitude) and no direction.

How to Find the Density of a Liquid

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To find the density of a liquid, follow these steps:

Step 1: Find the Mass of the Liquid

  1. Place an empty beaker on an electronic balance and note its mass. Let's call this m₁ (mass of empty beaker).

  2. Pour the liquid into the beaker.

  3. Place the beaker with the liquid on the electronic balance and note the new mass. Let's call this m₂ (mass of beaker + liquid).

  4. Calculate the mass of just the liquid: mL=m2−m1m_L = m_2 - m_1

Step 2: Find the Volume of the Liquid

Look at the markings (graduations) on the beaker to read the volume of the liquid. Let's call this V.

Step 3: Calculate the Density

Use the formula: ρ=mLV\rho = \frac{m_L}{V}

Example: If the empty beaker has a mass of 50 g, the beaker with liquid has a mass of 150 g, and the volume of liquid is 100 cm³:

  • Mass of liquid = 150 g – 50 g = 100 g
  • Density = 100 g ÷ 100 cm³ = 1 g/cm³

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