Changes of State and Conservation of Mass

When ice melts to water, or water boils away to steam, the substance changes from one state to another — but it is still the same substance, made of the same particles. These changes of state are physical changes: they can be reversed, and no new substance is made. That single idea has an important consequence — because no particles are created or destroyed, the mass stays the same throughout. This lesson, part of Topic P1 (Matter) of OCR Gateway Science A, names the changes of state and their directions, explains why they are physical and why mass is conserved, introduces the idea of internal energy, and previews the heating curve you will meet in detail in the next two lessons.

By the end of this lesson you should be able to name the changes of state and their directions, explain why a change of state is a physical change in which mass is conserved, describe internal energy qualitatively, and interpret a simple heating curve.

---

Naming the Changes of State

There are six changes of state to learn, each with a name and a direction between two of the three states. The diagram below shows them as a cycle between solid, liquid and gas.

graph LR
  S[Solid] -->|melting| L[Liquid]
  L -->|freezing| S
  L -->|boiling / evaporating| G[Gas]
  G -->|condensing| L
  S -->|sublimation| G
  G -->|deposition| S

The changes are:

Change of state	Direction	What happens
Melting	solid → liquid	a solid is heated until it turns to liquid (at its melting point)
Freezing	liquid → solid	a liquid is cooled until it turns to solid (at its freezing point)
Boiling / evaporating	liquid → gas	a liquid turns to gas (boiling happens throughout at the boiling point; evaporation happens at the surface, below boiling point)
Condensing	gas → liquid	a gas is cooled until it turns to liquid
Sublimation	solid → gas	a solid turns straight to gas without melting (e.g. solid carbon dioxide)
Deposition	gas → solid	a gas turns straight to solid without becoming liquid (e.g. frost forming)

Melting and freezing happen at the same temperature for a given substance — the melting point (for ice/water this is $0\,°\text{C}$0°C). Boiling and condensing happen at the boiling point (for water this is $100\,°\text{C}$100°C at normal atmospheric pressure).

Exam Tip: Learn the changes as three pairs of opposites: melting/freezing, boiling/condensing, sublimation/deposition. Knowing that freezing is just melting in reverse, at the same temperature, saves you learning two separate facts.

---

Boiling Versus Evaporating

It is worth being clear about the difference between boiling and evaporating, since both take a liquid to a gas:

• Boiling happens at the boiling point, throughout the whole liquid, with bubbles of gas forming inside it. It is rapid.
• Evaporation happens only at the surface of the liquid, at temperatures below the boiling point. It is slower. Evaporation is why a puddle dries up on a warm day and why wet washing dries on a line without being boiled.

In both cases the fastest-moving particles at the surface (in evaporation) or throughout (in boiling) gain enough energy to break away from the liquid and become gas.

---

Why a Change of State Is a Physical Change

A change of state is a physical change, not a chemical one. This means:

• No new substance is made. Ice, liquid water and steam are all the same substance — water — made of the same particles ($\text{H}_2\text{O}$H2​O molecules). Only the arrangement, spacing and motion of those particles change, not the particles themselves.
• It is reversible. You can freeze water back into ice and melt it again as many times as you like; the change runs both ways.
• No chemical bonds within the molecules are broken. When water boils, the molecules move apart from each other, but each $\text{H}_2\text{O}$H2​O molecule stays intact — the hydrogen and oxygen do not separate.

This contrasts with a chemical change (such as burning), where new substances are made and the change usually cannot be simply reversed.

Exam Tip: The key phrase is "no new substance is made" and "the change is reversible". A change of state rearranges the same particles; it does not turn them into anything new.

---

Conservation of Mass in a Change of State

Because a change of state simply rearranges the same particles — none are created and none are destroyed — the mass does not change. If you seal $100 \text{ g}$100 g of ice in a container and let it melt, you still have exactly $100 \text{ g}$100 g of liquid water; boil that water in a sealed container and you still have $100 \text{ g}$100 g of steam. This is the conservation of mass: the total mass before a change of state equals the total mass after it.

The only reason a mass reading sometimes appears to fall during boiling or evaporation is that the gas produced escapes into the air from an open container and the balance no longer "sees" it. If the gas were trapped and weighed too, no mass would be lost at all. The particles — and so the mass — are still all there.

Worked Example — Conservation of mass on melting

A sealed flask contains $250 \text{ g}$250 g of ice. The flask is left in a warm room until all the ice has melted. What is the mass of the water now in the flask, and why?

Step 1 — recall the principle: melting is a physical change that only rearranges the particles; no particles are created or destroyed.

Step 2 — apply conservation of mass: because the flask is sealed, nothing can enter or leave, so the mass is unchanged.

Step 3 — state the answer: the mass of water is still $250 \text{ g}$250 g.

Answer: $250 \text{ g}$250 g — the same $\text{H}_2\text{O}$H2​O particles are present, just arranged as a liquid instead of a solid, so the mass is conserved.

Exam Tip: If a question says a beaker of water loses mass as it is heated in the open, the reason is that water vapour (gas) escapes — not that mass has been destroyed. In a sealed container the mass would not change.

---

Internal Energy

To understand why a substance changes state when you heat it, you need the idea of internal energy. The internal energy of a substance is the total energy stored in its particles. It comes in two parts:

• the kinetic energy of the particles — the energy they have because they are moving (vibrating, sliding or flying about); and
• the potential energy of the particles — energy stored because of the positions of the particles and the forces between them.

When you heat a substance, you transfer energy to its particles and increase its internal energy. That extra energy does one of two things:

• it makes the particles move faster, which raises the temperature (more kinetic energy); or
• it breaks the forces between particles during a change of state, which increases the potential energy without raising the temperature.