The five changes of state

1. A change of state happens when a substance moves between the solid, liquid and gas states, for example when ice turns into liquid water.
2. In every change of state the particles stay exactly the same particles, and only their arrangement and movement change.
3. Melting turns a solid into a liquid, as when a knob of butter becomes a golden liquid in a hot frying pan.
4. Freezing is the reverse of melting and turns a liquid into a solid, as when water in an ice-cube tray becomes ice in the freezer.
5. Evaporation turns a liquid into a gas, as when a puddle on the pavement slowly disappears on a warm afternoon.
6. Condensation is the reverse of evaporation and turns a gas into a liquid, as when water vapour from a hot shower forms droplets on a cold bathroom mirror.
7. Sublimation turns a solid straight into a gas without the substance ever passing through the liquid state.
8. Solid carbon dioxide, known as dry ice, sublimes at $-78{}^{\circ }C$, so a lump of it shrinks away to nothing without ever leaving a puddle.
9. Melting and freezing are opposite processes, and so are evaporation and condensation, so each of these changes can be undone by its partner.

Conservation of mass

1. Mass is a measure of the amount of matter in an object, and it depends on how many particles the object contains and on the mass of each particle.
2. A change of state does not create any new particles, and it does not destroy any of the particles already there.
3. Each particle also keeps exactly the same mass, because a water molecule in ice is identical to a water molecule in liquid water or in steam.
4. The number of particles and the mass of each particle are both unchanged, so the total mass of the substance stays the same.
5. This is conservation of mass, and it holds for melting, freezing, evaporation, condensation and sublimation alike.
6. For example, $200g$ of ice that melts completely gives exactly $200g$ of liquid water.
7. If that water is frozen again, the result is still $200g$ of ice.
8. What can change is the volume, because the spacing between the particles changes when the state changes.
9. Water is an unusual case, since ice takes up slightly more space than the liquid it formed from, which is why a full glass bottle of water can crack in a freezer even though its mass has not changed.

Closed and open systems

1. A system is the object or group of objects you choose to study, such as a flask and the substance inside it.
2. In a closed system, no matter can get in or out, so a balance shows the total mass staying the same while a change of state happens inside.
3. A sealed plastic bag containing ice cubes is a closed system, because the meltwater stays inside the bag.
4. In an open system, matter can cross the boundary of the system, so the mass measured on a balance can go down or up.
5. An uncovered dish of water left on a windowsill loses mass over a few days, because water particles evaporate and drift away into the air of the room.
6. That mass has not been destroyed, since every one of those particles still exists as water vapour in the surrounding air.
7. If the air in the room were included in the system, the total mass would be found to stay constant.
8. A cold can of drink taken out of the fridge gains a little mass in the same way, because water vapour from the air condenses into droplets on its outside surface.
9. That extra mass has come from the air around the can rather than being created.
10. So whenever a measured mass seems to change during a change of state, the explanation is that particles have entered or left the part being weighed.