Introduction for Reactions: Study Guide (Chemistry 4.1)

What changes and what stays the same

Unit 4 is about chemical reactions. Before you write equations for them, you need to recognise when a reaction has happened at all. Every change in matter falls into one of two groups.

• A physical change alters the properties of a substance, such as its state, shape or how it is mixed with other things, but not its composition. Ice melting is still H₂O. Sugar dissolved in tea is still sugar.
• A chemical change (a chemical reaction) turns the starting substances into new substances, usually with a different composition. When iron rusts, iron and oxygen become an iron oxide, a new compound with new properties.

The key word is composition: which atoms are present and how they are joined. In a physical change, every particle you started with is still there, in the same form. In a chemical change, the atoms are all still there too, but they are joined in new combinations.

Physical changes

Two families of physical change come up again and again.

Changes of phase. Melting, freezing, boiling, condensing, sublimation (solid straight to gas) and deposition (gas straight to solid). The particles move closer together or further apart, but each particle is unchanged. Iodine crystals turning into a purple vapour are still I₂ molecules.

Making or separating mixtures. Dissolving sugar in water, mixing two liquids, or stirring sand into water makes a mixture. Filtering, evaporating, distilling and chromatography separate one. None of these forms a new substance, so the components can be recovered with their original properties.

A particle view

Figure 1 shows the difference at the particle level. In panel A, water boils: the molecules move far apart, but every molecule is still one oxygen atom bonded to two hydrogen atoms. In panel B, hydrogen burns in oxygen: the H–H and O=O bonds break, and new O–H bonds form. The number of each kind of atom is the same before and after; only the way they are joined has changed.

Evidence of a chemical change

You cannot see atoms rearranging. What you can see are the effects of a new substance forming. Four observations are common evidence.

Evidence	What you see	Example of a reaction
Heat or light produced	The mixture warms up, glows or burns	Magnesium ribbon burns with a bright white light
A gas forms	Bubbles or fizzing in a liquid, or a smell	Chalk (calcium carbonate) fizzes in dilute acid as carbon dioxide forms
A precipitate forms	Two clear solutions mix and a solid appears	Barium chloride and sodium sulfate solutions give a white solid, barium sulfate
Colour change	A new colour that is not just dilution	A copper wire in silver nitrate solution: the solution turns blue and silvery crystals grow

Each of these tells you that something happened. A new substance usually has different properties from the old ones: a different colour, a different solubility, a different state at room temperature. That is why these observations point towards a reaction.

Evidence is a clue, not proof

Every sign in the table can also happen without a reaction. That is the main thing examiners test in this topic.

Observation	Physical change that shows it
Temperature change	Dissolving solid sodium hydroxide in water makes the water hot; dissolving ammonium nitrate makes it cold
Bubbles	Boiling water; opening a bottle of fizzy water (dissolved carbon dioxide leaves the solution)
Solid appears	Cooling a saturated solution until crystals form; freezing
Colour change	Diluting a coloured solution makes it paler; heating some solids changes their colour reversibly

The reverse is also true: some reactions give no obvious sign. When dilute hydrochloric acid is added to sodium hydroxide solution, both solutions and the product are colourless. Only a thermometer shows that a reaction has happened.

So use the observation as a starting point, then ask the deciding question in Figure 2: has a new substance with a different composition formed?

Good tests for a new substance include:

• Identify the product. Carbon dioxide turns limewater cloudy. Hydrogen burns with a squeaky pop. A precipitate can be filtered off and shown to be a different compound from either starting solid.
• Try to get the starting material back by a physical method. If evaporating the water returns the original solid, with the same mass and melting point, no new substance was formed.
• Compare properties. A product with a different melting point, solubility or colour from every starting substance is a new substance.

Writing an evidence-based answer

Questions on this topic often ask you to “justify” or “support a claim”. A strong answer has three parts, and each one earns credit on its own.

1. Claim. State the classification plainly: “This is a chemical change.”
2. Evidence. Quote the specific observation or data: “A white solid formed when the two colourless solutions were mixed.”
3. Reasoning. Link the evidence to composition: “The solid is a substance that was not present in either solution, so atoms or ions have combined in a new way.”

The third part is the one students leave out. “Bubbles formed, so it is chemical” is a claim plus evidence, but no reasoning. Compare it with: “Bubbles formed and the gas turned limewater cloudy, so the gas is carbon dioxide; neither starting substance was carbon dioxide, so a new substance formed.”

When you are asked to predict the result of an experiment, work the other way round. Start from what you know will form (a gas, an insoluble solid, a coloured ion, energy released), then say what each product will look like in the apparatus described: bubbles in a liquid, cloudiness in a clear solution, a temperature rise on a thermometer, or a falling reading on a balance if a gas can escape. Be specific to the set-up: “the solution turns cloudy” is better than “a reaction happens”.

When the evidence is weak, say so. “The temperature rose, which suggests a reaction, but dissolving can also release heat, so a further test is needed” is a better answer than a confident claim built on one observation.

Worked example 1: classifying observations

Question. A student records five changes. Classify each as physical or chemical and give the evidence.

Change	Observation
(a)	Wax melts in a warm room
(b)	A piece of zinc is added to dilute sulfuric acid; bubbles form and the zinc gets smaller
(c)	A blue copper(II) sulfate solution is diluted with water; it becomes pale blue
(d)	Pale green iron(II) hydroxide left in air slowly turns orange-brown
(e)	Salty water is distilled; pure water collects and a white solid is left behind

Answer.

1. (a) Physical. Melting is a change of phase. The liquid wax has the same composition as the solid and sets again on cooling.
2. (b) Chemical. The zinc is used up and a gas (hydrogen) forms. Neither zinc nor the acid is a gas, so the gas is a new substance.
3. (c) Physical. The colour fades because the same blue ions are spread through more water. No new colour appears; this is a change in the mixture, not a reaction.
4. (d) Chemical. A new colour appears that is not just dilution. The iron(II) compound reacts with oxygen to form a new iron(III) compound.
5. (e) Physical. Distillation separates a mixture. The water and the salt both have their original composition.

Check. In (b) and (d) a starting substance disappears or a different substance appears. In (a), (c) and (e) every substance is still present.

Worked example 2: predicting the results of an experiment

Question. An open conical flask holds excess dilute hydrochloric acid. The flask and a 0.243 g piece of magnesium ribbon, resting on the pan beside it, sit on a balance that reads 152.367 g in total. The student drops the ribbon into the acid. The reaction is Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Predict what the student will observe, and the final balance reading.

1. Gas. One product is hydrogen gas, so bubbles form on the magnesium.
2. Solid disappears. Magnesium is used up, because the acid is in excess. The ribbon gets smaller and vanishes.
3. Temperature. The flask gets warm. This reaction releases energy as heat (you will meet this in Unit 6).
4. Mass. The open flask lets the hydrogen escape, so the reading falls by the mass of hydrogen produced.
   • n(Mg) = 0.243 g ÷ 24.31 g mol⁻¹ = 0.0100 mol.
   • The equation gives 1 mol H₂ for every 1 mol Mg, so n(H₂) = 0.0100 mol.
   • m(H₂) = 0.0100 mol × 2.016 g mol⁻¹ = 0.020 g.
   • Final reading ≈ 152.367 − 0.020 = 152.347 g.

Interpretation. The mass seems to “disappear”, but atoms are not destroyed. If the student fits a balloon over the neck of the flask to trap the hydrogen, the reading stays the same (apart from a small buoyancy effect of the inflated balloon). A loss of mass from an open container is itself evidence that a gas has formed.

Worked example 3: designing a test

Question. A student dissolves 5.00 g of an unknown white solid X in 50.0 mL of water. The temperature falls from 22.0 °C to 16.4 °C. The student claims that “a chemical reaction happened, because the temperature changed”. Plan a test of this claim and say what result would show the change was physical.

1. Weigh the evidence. A temperature change of −5.6 °C is a sign of a possible reaction, but dissolving alone can absorb or release energy. The observation does not decide the question.
2. Plan the test. Ask whether X is still present. Evaporate the water slowly at room temperature, then dry and weigh the solid that is left. Measure its melting point and compare it with a sample of the original X.
3. Physical result. The student recovers 4.97 g of white solid (99.4% of the original mass; a little is lost on the glassware), and its melting point matches that of X. The same substance came back, so dissolving X was a physical change.
4. Chemical result. If the recovered solid had a different mass that cannot be explained by losses, a different melting point, or if a gas had been given off, a new substance would have formed.

Why this works. The test targets composition, which is what separates the two kinds of change. The temperature reading on its own could not.

Common misconceptions

• “Any temperature change means a reaction.” Dissolving and changes of state also take in or give out energy.
• “Bubbles mean a gas has been made.” Boiling and dissolved gas escaping give bubbles too. Ask whether the gas is a new substance.
• “A colour change is always chemical.” Dilution changes how strong a colour looks. A new colour is better evidence than a paler one.
• “No visible change means no reaction.” Many reactions, such as strong acid with strong base, look like nothing is happening. Measure the temperature.
• “In a chemical change, atoms are created or destroyed.” Atoms are only rearranged. Mass is conserved in a closed container; an open one loses mass only because a gas escapes.
• “Physical changes are always easy to reverse.” Many are (melting, dissolving), but breaking glass is physical and you cannot easily undo it. Composition, not reversibility, is the test.

Where this leads

This topic follows Topic 3.13, Beer-Lambert Law, where colour was measured rather than just observed. Next, Topic 4.2, Net Ionic Equations, shows how to write balanced equations for the changes you have learned to recognise. Topic 4.4, Physical and Chemical Changes, then explains physical and chemical changes in terms of the bonds and intermolecular forces involved. Try the practice questions now, then use the revision notes and the checklist to consolidate.