4.3Chapter 4: Chemical Reactions 1

Investigating a chemical reaction — Alhussan Chemistry - Stage 7 Revision Notes & Topical Questions

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4.3 Investigating a chemical reaction — Practical Observations — Alhussan International Academy — Chemistry (Stages 7–9) notes: Plan and carry out an investigation into a chemical reaction, record observations and use them and the conservation of mass to draw a conclusion.Chemica…

Physical change
Evidence of a chemical reaction
Hypothesis
Evidence needed
Prediction
Explanation

Section 1 of 10Chemical change versus physical change

Chemical change versus physical change

Chemical reaction

Chemical reaction new substances form with different properties from the starting materials.

Physical change

Physical change no new substance forms, for example melting or dissolving.

Evidence of a chemical reaction

Evidence of a chemical reaction a colour change, bubbles of gas (effervescence), a precipitate forming, a temperature change, light or a flame,
- or a new smell.
None on its own is absolute proof, because boiling also makes bubbles, but several together are good evidence.

Section 2 of 10Hypothesis and prediction

Observation

Observation big pieces of iron do not burn, but thin strands called iron wool burn well.

Hypothesis

Hypothesis if iron wool is heated in air, the iron will react with oxygen to make a compound, iron oxide.
It is based on scientific knowledge that magnesium burns by reacting with oxygen.

Evidence needed

Evidence needed if the mass of the product is greater than the mass of the iron, it shows that iron reacts with oxygen from the air.

Prediction

Prediction iron atoms and oxygen atoms will join, so the mass of iron oxide will be greater than the mass of iron at the start.
A ruler balanced with iron wool on one end should tilt down at the iron end.

Section 3 of 10Planning the method

Apparatus

Apparatus a metal ruler balances the iron wool, and a hot flame heats it.
A wooden ruler might catch fire, so a metal ruler is used.

Safety

Safety wear gloves because iron wool can cut skin, wear eye protection, and keep spare iron wool in a jar with a lid because it catches fire easily.

Heating

Heating heat the iron wool for one minute.

Section 4 of 10Results and conclusion

1Result

Result the iron wool end of the ruler tilted down, so the prediction was correct.

2Explanation

Explanation iron atoms joined with oxygen atoms from the air to make the compound iron oxide, which had a greater mass than the iron.

3Improving the investigation

Improving the investigation take measurements.
Mass of iron before heating was 4.1 g and mass of iron oxide after heating was 5.6 g.
Total mass of reactants equals total mass of products, so mass of oxygen = 5.6 g − 4.1 g = 1.5 g.

4Steps of an enquiry

Steps of an enquiry make a hypothesis and prediction, choose equipment, work safely, make a conclusion and suggest improvements.

Section 5 of 10Making observations

Describe what you see, hear, smell (only if safe) and feel, using precise words.

Write that the blue solution turned colourless, a grey solid appeared and the test tube became warm.

Observations are facts, while conclusions are explanations.

The observation is that gas bubbled off, and the conclusion is that a gas was produced.

Smell

Smell waft vapours towards you instead of smelling chemicals directly.

Temperature

Temperature measure with a thermometer, since exothermic reactions raise it and endothermic reactions lower it.
Use a results table with headings and units.

Section 6 of 10Typical practicals

Magnesium with acid

Magnesium with acid magnesium ribbon in dilute hydrochloric acid bubbles, the ribbon shrinks and disappears and the tube gets warm.
A lighted splint at the mouth gives a squeaky pop, showing hydrogen.
magnesium + hydrochloric acid → magnesium chloride + hydrogen.

Iron with copper sulfate

Iron with copper sulfate iron nails become coated in brown solid and the blue colour fades.
iron + copper sulfate → iron sulfate + copper, which is a displacement reaction.

Section 7 of 10Writing a conclusion and evaluating

Writing a conclusion and evaluating

Three parts

Three parts say what happened (observation), say what it shows, and explain why (atoms rearranged).
- (a chemical reaction because a new substance formed)

Errors

Errors impure chemicals, imprecise measuring cylinders or reading the thermometer too early.

Improvements

Improvements repeat to check reliability and suggest ways to reduce errors.

Section 8 of 10Planning a fair test

Example

Example to investigate how the amount of acid changes the temperature rise, the independent variable is the volume of acid.

Dependent variable

Dependent variable the temperature change.

Control variables

Control variables the mass of magnesium, the starting temperature and the container used.

Reporting

Reporting include units, label every column of the results table and say how many times each measurement is repeated.

Section 9 of 10Key terms

Term	Meaning
Hypothesis	A possible explanation based on evidence that can be tested further.
Prediction	A statement of what will happen in a test.
Observation	A fact recorded using the senses or instruments.
Conclusion	An explanation based on the observations.
Effervescence	Bubbles of gas in a liquid.
Iron oxide	The compound formed when iron reacts with oxygen.

Section 10 of 10Exam technique

Treating an observation and a conclusion as the same.

Saying that dissolving is always a chemical reaction.

Not recording a temperature change.

Smelling chemicals directly instead of wafting.

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Last updated: 18 October 2026