Particles and states of matter, Year 7: more practice

They are far apart, so there is lots of empty space to squeeze into

提示Think about the gaps between neighbouring particles.

为什么Gas particles are spread out with large gaps between them, so pushing the plunger just moves them closer. Liquid particles are already touching, so there is almost no gap left to close.

The salt particles are dissolved and small enough to pass through the holes in the paper

提示Compare the size of the gaps in the filter with the size of a single grain of the solid once it has broken up.

为什么Filtering only holds back solid pieces too big to fit through the paper. Dissolved salt has been broken up into particles far smaller than the holes, so it runs straight through with the water.

They gain enough energy to break out of their fixed positions and slide past each other

提示Nothing about the pieces changes; the pattern is what changes.

为什么In a solid the particles vibrate in fixed places. Heating gives them more energy until the pattern gives way and they can move around, which is what makes it a liquid.

Water vapour (water in its gas state)

提示They form inside the liquid itself, not from anything added to it.

为什么At the boiling point, water particles throughout the liquid have enough energy to become gas. They collect as bubbles of vapour that rise to the surface and escape.

The smell in air, because gas particles are far apart and move faster

提示Compare how freely a gas and a liquid let things travel through them.

为什么Gas particles have lots of space and little in their way, so they spread quickly. Liquid particles are packed together and keep bumping into each other, so diffusion is much slower.

It increases

提示Think about how the energy of the particles changes when they are heated.

为什么Heated particles move faster, so they hit the walls harder and more often. More force on the walls from the same space means higher pressure.

The dissolved particles are too small to block or scatter the light

提示Think about what has happened to the salt crystals.

为什么Dissolving breaks the salt into particles far too small to see, spread evenly through the water. A solution may be coloured, but it is always see-through, unlike a cloudy suspension.

It contains many different substances mixed together

提示Think about everything that is found in the water in juice.

为什么Juice holds water, sugars, acids and flavour chemicals. In everyday language pure can mean nothing added, but in chemistry it means one type of particle only.

250

提示Nothing has been added or taken away during dissolving.

为什么50 + 200 = 250 g. The sugar is still there in the solution, so the total mass is the sum of the two masses.

The salt particles are all still there, just spread out among the water particles

提示Out of sight does not mean gone.

为什么Dissolving only separates the particles and mixes them in. Because every particle is still in the beaker, the total mass is exactly what it was before.