Particles of Matter: Quick Revision

A brief note on Particles of Matter. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Matter has mass and takes up space. Light, sound and heat are not matter.
• Matter is made of particles, far too small to see. A few crystals can colour a bucket of water.
• There are spaces between the particles: sugar dissolves with no rise in the level.
• The particles are always moving, faster when hotter. Their spreading by themselves is diffusion: fastest in gases.
• The particles attract each other: strongly in solids, weakly in gases.
• Density is mass divided by volume. A thing floats on a liquid that is denser than itself.

The three states at a glance

A worked example

A plastic block has a mass of $290\ \text{g}$ and a volume of $125\ \text{cm}^3$. Does it float on water? Then $36\ \text{mL}$ of air, of density $1.2\ \text{kg/m}^3$, is squeezed into $9\ \text{mL}$. What is its new density?

1. The block. $\rho = \dfrac{290\ \text{g}}{125\ \text{cm}^3} = 2.32\ \text{g/cm}^3 = 2320\ \text{kg/m}^3$. That is more than $1000\ \text{kg/m}^3$, so it sinks.
2. The air. The mass stays the same and the volume is $\dfrac{36\ \text{mL}}{9\ \text{mL}} = 4$ times smaller, so $\rho = 1.2\ \text{kg/m}^3 \times 4 = 4.8\ \text{kg/m}^3$.

Practice with new numbers

A program to try

The program finds the mass of one litre of four liquids and says whether each floats on water.

Mistakes to avoid

• Calling air nothing. Air has mass and takes up space.
• Saying particles grow when heated. They move faster and farther apart; their size stays the same.
• Dividing volume by mass. Density is mass on top.
• Saying the particles of a solid do not move. They vibrate in place.

Check yourself

1. Which of these are matter: a puff of smoke, a sunbeam, a cloud, a sound?

2. Why does a smell spread faster in a warm room than in a cold one?

3. A stone has a mass of $0.66\ \text{kg}$ and a volume of $2.4 \times 10^{-4}\ \text{m}^3$. What is its density?

4. Why can a gas be squeezed into a small cylinder, but a liquid hardly at all?