Teacher guide: Year 7 Chemistry

The idea in plain words

Everything is made of particles, far too small to see. The particle model has four key ideas:

1. All matter is made of particles.
2. There is empty space between them (nothing at all, not even air).
3. They are always moving.
4. They attract each other.

Solids, liquids and gases are the same particles arranged and moving differently:

Why it matters

Properties come from these three features: a gas squashes because its particles are far apart; honey flows slowly because its particles attract strongly; a gas is much less dense because far fewer particles fit in each cm³. Every separation technique in the next unit is explained the same way.

Where the picture breaks

• Particles are drawn as circles, but real particles are not little balls and are much further apart in a gas than any diagram shows.
• Particles never change size. Students who say "particles expand when heated" are describing the spaces, not the particles.
• Ice is an exception: it is less dense than liquid water because its particles lock into an open pattern.

Common misconceptions

• Matter is continuous, or particles float in something (air, water).
• Particles in a solid don't move.
• Gases rise because they are light.
• Particles squash, shrink or grow.

Energy and changes of state; the model's limits (nodes 4 and 5)

Melting and boiling take energy in; freezing and condensing give it out. While a substance changes state its temperature stays constant: the energy separates the particles (the flat parts of a heating curve). The particle model explains states, diffusion and pressure but not colour, reactions or electricity, because its particles have no parts.

Investigate in class

Optional ideas if you run a practical for this unit: pick one, adapt it to your equipment, or use your own.

• Does the temperature of water change how fast a drop of food colouring spreads? Change: water temperature (cold, room, warm). Measure: time until the colour looks even. Keep the same: volume of water, size of drop, no stirring. (Warm tap water is enough; a phone video from above makes timing fair.)
• Does the temperature rise while ice is still melting? Change: time (read every 30 seconds). Measure: temperature of crushed ice in a beaker, stirred. Keep the same: amount of ice, the room. (Expect a flat stretch near 0 °C until the ice is gone: the heating curve's plateau.)
• Does air squash more than water? Change: what fills a sealed syringe (air, water, sand). Measure: how far the plunger moves under a firm push. Keep the same: syringe, starting volume, push. (No needles; a finger or cap seals the tip.)

Safety and ethics: warm water only, never boiling; take care with glass thermometers.

The idea in plain words

A pure substance contains only one kind of particle (distilled water, copper, sugar). A mixture contains two or more kinds (salt water, air, milk). You cannot tell by looking: salt water looks like one substance but is a mixture.

A mixture can be separated because its parts differ in a physical property:

Separating does not change the particles: you get back the same substances.

Why it matters

Water treatment, recycling, mining and food processing all rely on these property differences. Choosing a technique means naming the difference first.

Where the picture breaks

• "Pure" on a food label means nothing added, not one kind of particle.
• Here "one kind of particle" covers both elements and compounds (water is one kind of particle). Atoms and compounds come in Year 8.
• Salt dissolves as charged particles; the tutorials draw it as one kind of particle.

Common misconceptions

• Clear or even-looking means pure.
• A dissolved substance has disappeared.
• Distillation is the same as evaporation.
• Dyes separate by weight in chromatography.

Investigate in class

Optional ideas if you run a practical for this unit: pick one, adapt it to your equipment, or use your own.

• Does the mass of salt you dissolve change the mass you get back by evaporating? Change: salt dissolved in 50 mL of water (2, 4, 6 g). Measure: mass of solid left. Keep the same: volume of water, dish, drying method. (Expect slightly less back; a sunny windowsill avoids heating.)
• Does salt water instead of water change how far each dye in a marker travels? Change: the solvent. Measure: distance of each coloured band from the start line. Keep the same: pen, paper strip, how long it runs. (Water-based black felt pens split into several colours.)
• Does the filter change how fast muddy water gets through? Change: filter (paper towel, coffee filter, filter paper). Measure: volume through in 2 minutes. Keep the same: mud mixture, volume poured, funnel.

Safety and ethics: hot evaporating dishes can spit and burn: safety glasses, let them cool.