This topic sets out what counts as matter: anything that has mass and occupies volume. It then shows how scientists manage the enormous variety of materials by classifying them according to physical properties. A physical property is a characteristic that can be observed or measured without changing the substance's identity.
It matters because later topics, including density, the kinetic particle model, and elements and compounds, assume you can answer these questions precisely. Density is the property school examiners test most heavily, because it combines a calculation, careful unit handling and a prediction of whether an object floats or sinks.
What these notes cover
- •What counts as matter
- •Why we classify matter
- •Physical vs chemical properties
- •Melting point, boiling point and purity
- •Electrical and thermal conductivity
- •Strength, hardness and flexibility
- •Choosing a material for a job
- •Density and its units
- •The floating and sinking rule
- •Density as an intensive property
- •Measuring density in the laboratory
- •Common misconceptions and exam technique
Key definitions
- Matter
- Anything that has mass and occupies volume (takes up space). Both conditions must hold at the same time. Light, heat and sound are forms of energy, not matter.
- Physical property
- A characteristic of a substance that can be observed or measured without changing the substance's identity. Examples are melting point, density, hardness and conductivity.
- Chemical property
- A description of how a substance changes into a new substance, for example flammability or rusting. It cannot be observed without the original substance being used up or transformed.
- Thermal conductivity
- A measure of how easily heat can pass through a material. Metals are good thermal conductors. Plastics, wood, glass, air and styrofoam are poor thermal conductors.
- Density
- How much mass is packed into each unit of volume of a material. It is an intensive property: every sample of one material has the same density, whatever its size or shape.
Key formulas
Density
ρ = m ÷ V
ρ (the Greek letter rho, not p) is density, m is mass and V is volume. Units are g/cm³ (laboratory) or kg/m³ (SI).
Rearranged density formula
m = ρ × V and V = m ÷ ρ
Use these to find mass or volume when density is known. Check that the units cancel: g/cm³ × cm³ leaves g.
Density unit conversion
1 g/cm³ = 1000 kg/m³
Multiply by 1000 to change g/cm³ into kg/m³. The density of water is approximately 1.0 g/cm³, which is 1000 kg/m³.
Volume of a cuboid (regular solid)
V = l × b × h
l is length, b is breadth and h is height, measured with a metre rule or a vernier caliper.
Volume by displacement (irregular solid)
V = V₂ − V₁
V₁ is the initial measuring cylinder reading and V₂ is the reading with the solid fully immersed. The solid must sink and must not dissolve in or react with water.
Density of a liquid (mass difference)
ρ = (m₂ − m₁) ÷ V
m₁ is the mass of the empty measuring cylinder, m₂ is the mass with the liquid inside, and V is the liquid volume read from the cylinder.
Worked example
A rectangular metal block measures 4.0 cm × 2.5 cm × 2.0 cm and has a mass of 54 g. (a) Find the volume of the block. (b) Find the density of the block and suggest what metal it could be. (c) Will the block sink or float in water?
- 1For a regular solid, calculate the volume from its dimensions: V = length × breadth × height = 4.0 cm × 2.5 cm × 2.0 cm = 20.0 cm³.
- 2Write the density formula: ρ = m ÷ V.
- 3Substitute with units: ρ = 54 g ÷ 20.0 cm³ = 2.7 g/cm³.
- 4This matches the density of aluminium (2.7 g/cm³), so the block could be made of aluminium.
- 5Since the density of the block (2.7 g/cm³) is greater than the density of water (1.0 g/cm³), the block sinks in water.
Watch out: The mark for part (c) is for the comparison sentence. Quote both densities and compare them; never judge sinking by how heavy the block is.
Common mistakes
Heavy objects sink; light objects float
Wrong: Deciding whether an object sinks or floats from how heavy it is.
Right: Sinking and floating are decided by density. Compare the object's density with the fluid's density. A steel ship floats while a 5 g steel coin sinks.
Cutting an object in half halves its density
Wrong: Assuming a smaller piece of a material has a smaller density.
Right: Cutting halves both the mass and the volume, so ρ = m ÷ V is unchanged. Objects cut from the same sheet of steel all have the same density.
Units on density don't matter
Wrong: Treating 1 g/cm³ and 1 kg/m³ as interchangeable, or writing 'density of water = 1' with no unit.
Right: The two units differ by a factor of 1000: 1 g/cm³ = 1000 kg/m³. Always quote a unit; water is about 1.0 g/cm³, which is 1000 kg/m³.
Questions students ask
Is air matter?
Yes. Air has mass: an inflated ball weighs more than the same ball after the air is released. Air also occupies volume: air trapped in an inverted beaker stops water rising into it. Air passes both tests, so it is matter even though it is invisible.
What is the difference between a physical property and a chemical property?
A physical property, such as density or melting point, can be observed or measured without changing the substance's identity. A chemical property, such as flammability or rusting, describes how a substance turns into a new substance. To tell them apart, ask: after observing it, do I still have the same substance?
How do you know if an object will float or sink in water?
An object sinks if its density is greater than the density of the fluid and floats if its density is less. Water's density is about 1.0 g/cm³, so a glass paperweight of density 2.6 g/cm³ sinks. In an exam answer, always state both densities in the comparison.
How do you measure the density of an irregular solid?
Use the displacement method. Weigh the solid, record the measuring cylinder reading V₁, then fully immerse the solid on a thread and record V₂. Its volume is V₂ − V₁ and its density is m ÷ V. The method only works if the solid sinks and does not dissolve in or react with water.
In the full PDF
- •Further worked examples: showing that air is matter, sorting physical from chemical properties, choosing a frying-pan handle material and ordering hardness from a scratch test
- •More density calculations, including the same object in different fluids and liquid density by mass difference
- •The mass-against-volume graph for steel samples and the hazard warning symbol diagrams
- •The full material-choice table linking each use to its key property and why it suits the use
- •The remaining misconceptions: mass vs density, hollow objects and single-property material choices
- •Exam technique checklist, laboratory procedures with parallax-error guidance, and the end-of-section summaries