Grade 5 Science · Properties of and changes in matter

The Science of Baking: Changes in Matter

Explore why melting butter can be undone but a baked cake cannot — and learn to tell a physical change from a chemical one.

Section 1 · Physical changes

Melting butter: a change you can undo

By the end of this section I can describe a physical change in matter and explain why melting is reversible — so I can recognise changes of state around the kitchen and the natural world.

Success criteria 0 / 4 ticked

Tick each one when you can do it without help.

1A What is a physical change?

Matter is everything around you that has mass and takes up volume — including the butter, flour, and eggs in a recipe. A physical change alters the state, volume, or form of a substance without making a new substance.

Key idea

physical changestatereversible

In a physical change the substance is the same stuff before and after — only its form looks different. The particles are the same; they are just arranged or moving differently.

Melting butter on the stove

Solid butter (holds its shape) particles are locked in place
Add thermal energy → butter melts particles gain energy and slide past each other
Result: liquid butter — still butter no new substance formed
Cool it again → solid butter returns the change is reversible

Check yourself

Melting ice into water makes a new substance. Is this statement describing a physical change or a chemical change? Type physical or chemical.

Decide whether water and ice are the same substance in different states.
Ice and liquid water are both H₂O — the same substance, just a different state. So melting is a physical change, and the statement is about a physical change.

A pot of melted chocolate is put back in the fridge and turns solid again. Is this change reversible or irreversible?

Ask: can the chocolate return to its original form?
The chocolate changed back to a solid, so the original state returned. The change is reversible.

1B Changes of state and thermal energy

A change of state happens when matter absorbs or releases thermal energy. Heating gives particles more energy so they move freely; cooling removes energy so they lock together.

Solid → Liquid → Gas

meltingevaporatingfreezingcondensing

Each of these is a reversible change of state — the substance stays the same, only the state changes.

Interactive · heat and cool the butter

5

Butter melts at about 32 °C. Drag the temperature up and down and watch the particles. Notice that cooling reverses the change.

Check yourself

When solid butter absorbs ________, its particles gain energy and it melts. What provides this energy?

Think about the kind of energy transferred from a warm stove.
The warm stove transfers thermal energy to the butter, giving the particles enough energy to move past each other and melt.

Section 2 · Chemical changes

Baking a cake: a change you cannot undo

By the end of this section I can describe a chemical change and tell whether a change is reversible or irreversible — so I can reason about the changes I see in cooking and beyond.

Success criteria 0 / 4 ticked

Tick each one when you can do it without help.

2A What is a chemical change?

A chemical change results in the formation of one or more different substances. The starting materials are changed into new substances with new properties — and you usually cannot get the originals back.

Signs a chemical change has occurred

colour changegas produced (bubbles)light or heat given offnew smellsolid forms from liquids

Not every sign appears every time, but seeing one or more is strong evidence that a new substance has formed.

Baking a cake

Mix flour, sugar, eggs, butter, baking powder raw ingredients
Add thermal energy in the oven baking powder releases gas; proteins cook; sugars brown
Result: a cake — a new substance colour changed, new smell, fluffy texture
Cool it down → it stays a cake the change is irreversible

Check yourself

When bread dough bakes, it turns brown and gives off a new smell, forming a different substance. Is this a physical or chemical change?

A new substance with a new smell and colour has formed.
A colour change and a new smell are signs that a new substance formed, so this is a chemical change.

Once a cake is baked, can you get the raw flour, sugar, and eggs back? Is this change reversible or irreversible?

Decide whether the original ingredients still exist as separate substances.
The ingredients have reacted to form a new substance (the cake), so the originals cannot be recovered. The change is irreversible.

2B Sorting reversible and irreversible changes

Some changes of state — like melting — are reversible. Many chemical changes — like baking, burning, or cooking an egg — are irreversible. Practise telling them apart.

Interactive · sort each change

Tap a change, then tap a column to place it. Tap a placed card to return it to the pool.

Reversible
Irreversible

Check yourself

Burning a log in a campfire produces ash and smoke. Is this change reversible or irreversible?

Consider whether the ash and smoke can become a log again.
Burning forms new substances (ash, smoke, gases) that cannot be turned back into wood. The change is irreversible.

Frying an egg turns a runny liquid into a firm solid with a new texture and smell. Is this a physical or chemical change?

A new texture and smell suggest a new substance has formed.
The cooked egg is a different substance from the raw egg, with new properties. This is a chemical change (and it is irreversible).

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