Particles and states of matter

Ten times

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

为什么Particles 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

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

为什么Water 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

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

为什么For 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

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

为什么During freezing the energy released as particles lock into place keeps the temperature fixed until all the liquid has solidified.

Boiling

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

为什么Evaporation happens only at the surface and at any temperature; boiling happens all through the liquid at its fixed boiling point.

The surface

提示Only the topmost particles, in contact with the air, can escape.

为什么At 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

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

为什么Particles 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

提示Billions of tiny moving bits keep bumping the sides.

为什么Each 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

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

为什么When 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

提示No particles are lost, so nothing is added to or taken from the reading on the balance.

为什么Dissolving 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

提示Nothing else is mixed in — a single kind throughout.

为什么In 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

提示The energy is busy rearranging particles, not making them hotter.

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