Particle theory & states of matter

Year 7 answers

Fluency

(i) All matter is made of tiny particles. (ii) Particles are in constant motion. (iii) Particles have forces of attraction between them. (iv) Particles have spaces between them. (v) Higher temperature means faster motion.
Solid: particles touching, arranged regularly, vibrating on the spot. Liquid: particles touching, disordered, sliding past. Gas: particles far apart, moving fast and randomly.
(a) Melting. (b) Condensation. (c) Sublimation.
Density is mass per unit volume. Formula: $\rho = m/V$ .
$\rho = 120/40 = 3.0$ g/cm $^3$ .
Float — it is less dense than water.
Diffusion is the spreading of particles from high to low concentration. Example: the smell of perfume spreading through a room.
Cooling slows the gas particles, reducing collisions with the wall. Outside pressure pushes the balloon inward until the pressures balance.
Expansion joints allow the bridge materials to expand in heat and contract in cold without cracking.
Evaporation.

Reasoning

Gases have large spaces between particles that can be squeezed closer. Liquid particles are already touching, so there is almost no space left to compress.
During melting, the heat energy goes into breaking the forces holding the solid lattice, rearranging particles rather than speeding them up. Speed — and therefore temperature — stays constant.
Dye particles collide randomly with water particles, gradually spreading from the crowded area into less crowded water until evenly distributed.
Heated air particles move faster and spread apart, so hot air has lower density than the cooler air around it. Denser cool air sinks beneath, pushing hot air up.
Atoms do not change size. Heating makes them vibrate more, increasing the average spacing between them, which makes the whole object larger.
Gas particles move fast with large spaces and negligible attractive forces, so they spread to fill any container. Liquid particles still attract each other, so they stay together at the bottom while taking the container’s shape.

Reasoning

Volume = $3^3 = 27$ cm $^3$ . Density = $216/27 = 8.0$ g/cm $^3$ . Close to iron ( $7.9$ g/cm $^3$ ), not aluminium.
Density = $500/625 = 0.8$ g/cm $^3$ . Less dense than water, so it floats.
The car’s interior heats the trapped air. Particles move faster and hit the bottle walls harder and more often, raising pressure. If the pressure exceeds the bottle’s strength, it bursts.
Warming $100$ g of water by $10$ °C transfers energy into faster motion. Melting $100$ g of ice transfers energy into breaking all the solid-lattice bonds. Melting requires considerably more energy because bond-breaking is “expensive” — this is why ice is so effective for cooling drinks.

Reasoning

Liquid water particles have weak but flexible bonds. When freezing, water molecules form a hexagonal crystal with more empty space than in the liquid — so ice is less dense. Consequence: lakes freeze from the top down, insulating water below and allowing fish to survive winter.
A sealed pressure cooker traps steam, raising the gas pressure above the water. Higher external pressure makes it harder for water particles to escape as vapour, so water must reach a higher temperature before it boils — around $120$ °C. Hotter water cooks food faster.
Hydrogen particles have smaller mass than helium, so at the same temperature they move faster (same kinetic energy, lower mass means higher speed). The hydrogen balloon has lower density than helium and rises faster. (Hydrogen is also flammable — one reason helium is used in practice.)
At depth, high water pressure dissolves more nitrogen from the breathing air into the blood. Surfacing drops the pressure rapidly; dissolved nitrogen comes out of solution as bubbles in blood vessels, blocking flow. Slow ascents allow nitrogen to be exhaled gradually.