What are the properties of matter?
Matter is anything that has mass and takes up space: air, water, a stone, you.
A property is something we can observe or measure, like colour, hardness, mass or smell.
- Some properties depend on how much you have: mass, volume, length. Two spoons of sugar have more mass than one.
- Characteristic properties do not depend on the amount. One spoon or a whole bag of pure sugar has the same density and melts at the same temperature. These help us identify a substance.
The main characteristic properties are density, melting point, boiling point and solubility.
Density of solids, liquids and gases
Density tells how much mass is packed into each unit of volume.
density = mass ÷ volume, written ρ = m / V. Units: g/cm³ or kg/m³ (1 g/cm³ = 1000 kg/m³).
- Water: 1 g/cm³. Ice: 0.92 g/cm³ (so ice floats). Iron: 7.9 g/cm³. Air: about 0.0012 g/cm³.
- An object floats in a liquid if it is less dense than the liquid, and sinks if it is denser.
Why solids and liquids are dense and gases are not
In solids and liquids the particles are touching. In a gas they are far apart, with mostly empty space between them. So the same mass of gas takes up about 1000 times more volume, and its density is much lower.
Measuring density
Find the mass on a balance. For a box shape, volume = length × width × height. For an odd shape, lower it into a measuring cylinder of water: the rise in water level is its volume (displacement).
Melting point and boiling point
The melting point is the temperature at which a solid turns to liquid. The boiling point is the temperature at which a liquid boils into gas everywhere inside it.
- Pure water: melts at 0 °C and boils at 100 °C (at normal air pressure).
- While melting or boiling, the temperature stays the same even though heat goes in. The energy is used to pull the particles apart.
- A pure substance melts at one sharp temperature. A mixture melts over a range. Salty water freezes below 0 °C and boils above 100 °C.
So scientists test purity by checking the melting point.
Solubility
When salt disappears in water, it has dissolved. Salt is the solute, water is the solvent, and the mix is a solution.
Solubility is the largest mass of a solute that can dissolve in 100 g of solvent at a given temperature. For table salt in water at 20 °C it is about 36 g per 100 g of water.
- When no more can dissolve, the solution is saturated; extra solid sits at the bottom.
- For most solids, solubility rises when the water is hotter. Sugar dissolves much more in hot tea than in cold water.
- Different substances have very different solubilities, so solubility helps identify them. Chalk hardly dissolves at all.
Physical changes, conservation of mass, diffusion
A physical change changes the form or state of a substance, but no new substance is made. Melting, freezing, boiling, condensing and dissolving are physical changes. They are usually easy to reverse.
A chemical change makes a new substance. Signs: gas bubbles, colour change, heat or light given out, a new smell. Burning paper, rusting iron and cooking an egg are chemical changes. They are hard to reverse.
Conservation of mass
In a change of state, the particles only move further apart or closer together. None are made or lost. So mass stays the same: 100 g of ice gives 100 g of water. If steam escapes from an open pan, the pan gets lighter only because particles left it.
Diffusion and Brownian motion
Particles in liquids and gases move all the time. Diffusion is the spreading of particles from where there are many to where there are few, like the smell of food spreading through a house. Brownian motion is the random jiggling of tiny specks, like smoke or pollen, because invisible moving particles keep hitting them. This was strong evidence that particles exist.
Key formulas and definitions
- density ρ = mass ÷ volume (ρ = m / V)
- mass = density × volume
- volume = mass ÷ density
- 1 g/cm³ = 1000 kg/m³
- Solubility = mass of solute that saturates 100 g of solvent at a set temperature
Worked examples
1. A block has a mass of 54 g and a volume of 20 cm³. Find its density. Will it float in water?
Step 1: ρ = m / V = 54 ÷ 20 = 2.7 g/cm³. Step 2: 2.7 > 1 (water), so it sinks. (This is the density of aluminium.)
2. A box of wood is 5 cm × 4 cm × 2 cm and has mass 24 g. Find its density.
Step 1: V = 5 × 4 × 2 = 40 cm³. Step 2: ρ = 24 ÷ 40 = 0.6 g/cm³. Step 3: Less than 1, so it floats.
3. A stone is lowered into a measuring cylinder. The water rises from 50 cm³ to 65 cm³. Its mass is 39 g. Find its density.
Step 1: Volume = 65 − 50 = 15 cm³. Step 2: ρ = 39 ÷ 15 = 2.6 g/cm³.
4. Iron has density 7.9 g/cm³. What is the mass of 30 cm³ of iron?
Step 1: m = ρ × V. Step 2: = 7.9 × 30 = 237 g.
5. Solubility of salt is 36 g per 100 g water. How much salt can dissolve in 250 g of water?
Step 1: 250 g is 2.5 times 100 g. Step 2: 36 × 2.5 = 90 g.
6. 150 g of ice in a sealed bag melts. What is the mass of water? What if the bag was open and 5 g evaporated?
Step 1: Melting is a physical change; mass is conserved, so 150 g of water. Step 2: If 5 g of particles escape as vapour, the bag holds 145 g. Mass was not destroyed; it left the bag.
Common mistakes
- Thinking a big piece of iron is denser than a small piece. Density does not depend on size; both are 7.9 g/cm³.
- Thinking heavy things always sink. A huge log floats because its density is less than water's.
- Saying dissolving is a chemical change. Salt can be got back by evaporating the water, and no new substance forms.
- Thinking mass is lost when ice melts or water boils in a closed container. The particles are all still there.