Particles and states of matter

Ten times

HintSpacing is a length, and a volume depends on length in three directions at once — so undo the growth accordingly.

The whyParticles in a liquid are already touching; in a gas they are separated by empty space. If the volume grows a thousandfold, each direction grows by the cube root of a thousand, which is 10 — so gas particles sit roughly ten diameters apart. That spacing is exactly why a gas can be squashed into a smaller space and a liquid essentially cannot.

They are held further apart than in the liquid

HintFloating means less mass squeezed into the same space, so the locked pattern must be more open.

The whyWater is the great exception. As it freezes, its particles lock into an open six-sided pattern that leaves gaps, so a given mass of ice takes up about 9% more room than the same mass of liquid. Less mass in the same volume means a lower density, so ice floats — and a pond freezes from the top down, which is why fish survive the winter beneath it.

Melting point

HintIce reaches it at 0 °C, and the reading holds steady there until the last of it has gone.

The whyFor a pure substance it is a fixed value, so it can be used to identify the substance or to check its purity — an impure sample softens over a range of temperatures instead. Heating gives the particles enough energy to break out of their fixed pattern, so the solid structure gives way and it flows as a liquid.

It stays constant

HintEnergy is being given out as the structure locks together, so the reading holds still.

The whyDuring freezing the energy released as particles lock into place keeps the temperature fixed until all the liquid has solidified.

Boiling

HintIt is the one that makes bubbles rise from the bottom of the pan.

The whyEvaporation happens only at the surface and at any temperature; boiling happens all through the liquid at its fixed boiling point.

The surface

HintOnly the topmost particles, in contact with the air, can escape.

The whyAt the surface, the fastest particles can escape into the air. This can happen at any temperature, which is why a puddle dries up without boiling.

Diffusion

HintIt is how the smell of baking travels across a room with no fan.

The whyParticles in liquids and gases move randomly, so over time they spread out evenly from where they are concentrated — no stirring required.

Particles colliding with the walls

HintBillions of tiny moving bits keep bumping the sides.

The whyEach collision gives a tiny push. Countless particles hitting the walls every second add up to the steady outward pressure of the gas.

They separate and spread out evenly among the water particles

HintThe sugar is still there after it dissolves — so where could it be hiding out of sight?

The whyWhen sugar dissolves, its particles break away from one another and mix evenly all through the water — far too small to see, so the tea looks clear even though every particle of sugar is still in the cup. Very small particles can slip into the spaces between water particles, but sugar particles are large, so a strong sugar solution takes up noticeably more room than the water did on its own.

same

HintNo particles are lost, so nothing is added to or taken from the reading on the balance.

The whyDissolving does not destroy any particles — they are all still present, just spread out — so the mass of the solution equals the salt plus the water.

It is made of only one type of particle

HintNothing else is mixed in — a single kind throughout.

The whyIn everyday speech 'pure' can mean 'natural', but in chemistry it strictly means one substance only — one kind of particle, with nothing else present.

Changing state

HintThe energy is busy rearranging particles, not making them hotter.

The whyDuring melting or boiling the energy supplied breaks the forces between particles rather than raising the temperature, so the graph levels off.