Chemistry · 6 Chemical reactions

6.1 Physical and chemical changes

Before you start

The three states of matter and the names of the changes between them.
That a compound is made of elements chemically joined, while a mixture is not.
How to read a simple chemical equation such as $2 H_{2} + O_{2} \overset{}{\to} 2 H_{2} O$ .

Syllabus objectives covered (1)

Identify physical and chemical changes, and describe the differences between them

Part 1 of 3

The question that decides it

One question separates every physical change from every chemical one: has a new substance been made? If yes, it is a chemical change. If no, it is physical.
Melting ice looks dramatic, but the water molecules are unchanged — the same $H_{2} O$ , differently arranged. No new substance, so it is physical.
Burning magnesium looks similar in energy terms, but the product is magnesium oxide. Different substance, different properties, different formula.
$2 Mg + O_{2} \overset{}{\to} 2 MgO$
Everything else in this topic — reversibility, mass, energy, colour — follows from that one difference. Learn the question, and you can derive the rest rather than memorising a list.

Part 2 of 3

The differences that follow

Reversibility. Physical changes are usually easy to reverse: freeze the water back, evaporate the solvent and the salt returns. Chemical changes are usually hard to reverse — you cannot un-burn magnesium by cooling it.
The word is usually, and it matters. Some chemical reactions are reversible (you will meet the $\overset{}{⇌}$ symbol in 6.3), so reversibility is a strong clue, never a proof.
Mass. In a physical change the mass is obviously unchanged — nothing has been added or taken away. In a chemical change the mass of the system is also unchanged, but the mass of what is left in the beaker often is not, because a gas escapes or joins in.
Burning magnesium in air gains mass, because oxygen from the air is now part of the product. Heating a carbonate loses mass, because carbon dioxide escapes. Neither breaks conservation of mass — you are just not weighing everything.
Energy. Chemical changes usually involve a noticeable energy transfer, because bonds are broken and made. Physical changes involve energy too — melting needs energy — so this is a weaker clue than it looks.
Observations. A colour change, a gas given off, a precipitate forming, or a temperature change with no heating are all signs of a chemical reaction. Any one of them is worth stating as evidence.

Part 3 of 3

Cases that catch people out

Dissolving salt in water is physical. The salt is still sodium chloride — evaporate the water and you get it back unchanged.
Boiling a kettle is physical, even with steam pouring out. Steam is still water.
Rusting is chemical: iron plus oxygen plus water gives hydrated iron(III) oxide, a genuinely different substance that will not turn back into iron on a dry day.
Heating copper(II) sulfate crystals is chemical, even though it looks like drying. Blue hydrated crystals become white anhydrous powder — a colour change and a new substance. You will meet it again in 6.3 as a reversible reaction.
Cutting, grinding or dissolving never make a new substance, whatever the size of the pieces. Always physical.
When a question is borderline, argue from the definition rather than the appearance. Say what the starting substance is, what the finishing substance is, and whether they are the same.

Worked examples

Example 1. A student heats blue copper(II) sulfate crystals in a test tube. The crystals turn white and droplets of liquid collect at the cool end of the tube. State whether this is a physical or a chemical change, and justify your answer.

Show the working

Apply the test: is a new substance formed? Start by naming what you began with — blue hydrated copper(II) sulfate, $CuSO_{4} \cdot 5 H_{2} O$ .
Identify the products: white anhydrous copper(II) sulfate, $CuSO_{4}$ , and water. Two substances where there was one.
The white solid has a different formula and a different colour from the blue one, so it is a different substance. That settles it.
Do not be misled by the fact that adding water turns it blue again. Reversibility is a clue, not the definition — this is a reversible chemical reaction.

Answer: A chemical change. The blue hydrated copper(II) sulfate becomes white anhydrous copper(II) sulfate plus water, which is a new substance with a different formula and colour.

Example 2. A piece of magnesium ribbon of mass $0.24$ g is burned in air. The white ash produced has a mass of $0.40$ g. Explain why the mass has increased, and state what type of change has occurred.

Show the working

Decide the type of change first. Magnesium burns to give magnesium oxide, a new substance, so this is chemical.
Now account for the mass. The magnesium has combined with oxygen from the air.
$2 Mg + O_{2} \overset{}{\to} 2 MgO$
The oxygen is invisible before the reaction but is part of the solid product afterwards, so the solid you weigh at the end contains atoms that were not on the balance at the start.
The increase, $0.40 - 0.24 = 0.16$ g, is the mass of oxygen that reacted. Total mass is still conserved once you count the air.

Answer: A chemical change. The mass increased by $0.16$ g because oxygen from the air combined with the magnesium to form magnesium oxide; total mass is still conserved.

Where people go wrong

Mistake: Treating 'hard to reverse' as the definition of a chemical change.

Instead: The definition is whether a new substance is formed. Reversibility is only a clue.

Some chemical reactions are readily reversible, so a reversibility argument alone can give the wrong answer.

Mistake: Saying a chemical change 'breaks conservation of mass' when the mass in the beaker changes.

Instead: Mass is always conserved. A gas has either escaped from, or joined, the system you weighed.

You weighed the container, not the whole system. Naming the gas is what earns the mark.

Mistake: Calling dissolving a chemical change because the solid 'disappears'.

Instead: Dissolving is physical — the solute is still there and can be recovered by evaporation.

Disappearing from view is not the same as ceasing to exist. Nothing new has been made.

Mistake: Answering 'chemical' with no reason given.

Instead: Name the starting substance, name the product, and say they are different.

The command word is usually Identify plus Describe or Explain, and the reasoning carries most of the marks.

Check yourself

State the single test that distinguishes a physical change from a chemical change.
Answer
Whether a new substance is formed. If it is, the change is chemical.
Classify each of these: (i) crushing limestone, (ii) heating limestone until it gives off carbon dioxide, (iii) condensing steam.
Answer
(i) physical, (ii) chemical, (iii) physical.
Give three observations that suggest a chemical reaction has taken place.
Answer
Any three of: a colour change, a gas given off (effervescence), a precipitate forming, a temperature change without heating.
A sealed flask containing a reaction mixture has the same mass before and after the reaction, but the same reaction in an open beaker loses mass. Explain.
Answer
A gas is produced. In the sealed flask it cannot escape, so the mass is unchanged; in the open beaker it escapes, so the measured mass falls. Mass is conserved in both.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student carries out four processes: A melting candle wax B burning candle wax C dissolving sugar in water D adding dilute hydrochloric acid to marble chips

(a) Identify which two processes are chemical changes. 1 mark No calculator

Mark scheme

B1 B and D, both required

(b) Describe two differences between a physical change and a chemical change. 2 marks No calculator

Mark scheme

B1 a chemical change forms a new substance, a physical change does not, ORA
B1 any one from: a chemical change is usually difficult to reverse / usually involves a larger energy change, ORA

(c) In process D the mass of the beaker and its contents decreases. Explain why. 2 marks No calculator

Mark scheme

B1 carbon dioxide gas is produced
B1 the gas escapes from the open beaker, so the measured mass falls; mass is still conserved overall

Question 2. Iron wool is left in a dish of damp air for a week. It gains mass and turns orange-brown.

(a) State the type of change taking place, and give one piece of evidence from the description. 2 marks No calculator

Mark scheme

B1 chemical (change)
B1 any one from: a colour change occurs / a new substance (rust / hydrated iron(III) oxide) is formed / it cannot easily be reversed

(b) Suggest why the iron wool gains mass. 1 mark No calculator

Mark scheme

B1 oxygen (and water) from the air combines with the iron and becomes part of the product

Words to know

Physical change: A change in which no new substance is formed, such as melting or dissolving.
Chemical change: A change in which one or more new substances are formed, usually difficult to reverse.
Anhydrous: Containing no water of crystallisation, e.g. white anhydrous copper(II) sulfate.
Hydrated: Containing water of crystallisation, e.g. blue hydrated copper(II) sulfate.

All Chemistry lessons Next: 6.2 Rate of reaction