Density, States of Matter and Changes of State
What is Density?
Density is a measure of how much mass is packed into a given volume. Every material has its own characteristic density, which helps us identify different substances and predict whether they will float or sink.
Density is calculated using the equation: \(\text{density} = \frac{\text{mass}}{\text{volume}}\) or \(\rho = \frac{m}{V}\)
The unit for density is kilograms per cubic metre (kg/m³), though you might also see grams per cubic centimetre (g/cm³). To convert between these units, remember that 1 g/cm³ = 1000 kg/m³.
A block of wood has a mass of 800 g and a volume of 1000 cm³. Its density is:
$$\rho = \frac{800\text{ g}}{1000\text{ cm}^3} = 0.8\text{ g/cm}^3$$
Since this is less than the density of water (1.0 g/cm³), the wood will float.
The Three States of Matter
Matter exists in three main states: solid, liquid, and gas. The differences between these states depend on how the particles are arranged and how they move.
In solids, particles vibrate in fixed positions. In liquids, particles can slide past each other. In gases, particles move freely and rapidly in all directions.
Solids
In a solid, particles are packed closely together in a regular pattern. They are held in place by strong forces of attraction and can only vibrate about their fixed positions. This gives solids their definite shape and volume.
Liquids
In a liquid, particles are still close together but not in a regular arrangement. The forces between particles are weaker than in solids, allowing particles to move past each other. This means liquids have a definite volume but take the shape of their container.
Gases
In a gas, particles are far apart and move randomly at high speeds. The forces between particles are very weak. Gases have no definite shape or volume and will expand to fill any container.
Changes of State
When we heat or cool a substance, we can change its state. These changes are physical changes — the substance itself doesn’t change, only the arrangement and movement of its particles.
There are six possible changes of state:
- Melting: solid → liquid (heating)
- Freezing: liquid → solid (cooling)
- Evaporation/Boiling: liquid → gas (heating)
- Condensation: gas → liquid (cooling)
- Sublimation: solid → gas (heating)
- Deposition: gas → solid (cooling)
Temperature During Changes of State
During a change of state, the temperature remains constant even though we continue to supply energy. This is because the energy is being used to break or form bonds between particles rather than increase their kinetic energy.
The temperature at which a solid melts is called its melting point. The temperature at which a liquid boils is called its boiling point. These are characteristic properties of each substance.
Density Changes During State Changes
When a substance changes state, its density usually changes too. This happens because the particles become more or less tightly packed together, changing the volume while the mass stays the same.
Generally, solids are denser than liquids, which are denser than gases. However, there are important exceptions — ice is less dense than liquid water, which is why ice floats.
Water is unusual because its solid form (ice) is less dense than its liquid form. When water freezes, the molecules arrange themselves in a hexagonal crystal structure that takes up more space than the liquid arrangement.
Ice has a density of about 0.92 g/cm³ while liquid water has a density of 1.00 g/cm³. This 8% difference is enough to make ice float, which is crucial for life on Earth as it insulates water beneath from freezing completely.
Practical Applications
Understanding density and states of matter has many practical applications. Engineers use density to choose materials for different purposes — aircraft need low-density materials to reduce weight, while ship anchors need high-density materials to sink effectively.
In everyday life, we use changes of state constantly: cooking involves melting butter and boiling water, while refrigeration relies on condensation and evaporation cycles.
Exam Tip
When calculating density, always check your units carefully. Make sure mass is in kg and volume is in m³ for the standard SI unit of kg/m³. Remember that during changes of state, temperature stays constant — this is a common exam question.