These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Data: C 12.01, O 16.00 g mol⁻¹.
Question 1 (multiple choice · foundation)
Which of the following is a chemical change?
- (A) Dry ice (solid carbon dioxide) turns directly into a gas
- (B) Sugar dissolves in a cup of water
- (C) A slice of bread turns brown and crisp in a toaster
- (D) Salt is recovered by evaporating sea water
Answer and explanation
Answer: (C). Toasting forms new brown compounds with a different taste and smell from the starting bread. Cooling the toast does not turn it back into bread.
- (A) is sublimation, a change of phase. The gas is still CO₂.
- (B) makes a mixture. Evaporating the water returns the sugar.
- (D) separates a mixture. The salt was in the sea water all along.
Question 2 (multiple choice · core)
A student mixes two colourless solutions in a beaker. Which observation is the strongest evidence that a chemical change has occurred?
- (A) The mixture is colourless
- (B) A few small bubbles appear on the walls of the beaker as it stands in a warm room
- (C) A yellow solid forms and settles to the bottom
- (D) The temperature of the mixture rises by 0.2 °C
Answer and explanation
Answer: (C). A solid appearing when two solutions are mixed is a precipitate: an insoluble substance that was not present in either solution. It is also a new colour.
- (A) is no evidence either way. Many reactions, and many mixings with no reaction, give a colourless mixture.
- (B) describes dissolved air coming out of solution as the liquid warms, a physical change.
- (D) is a very small temperature change. Mixing or diluting solutions can cause a change this size with no reaction, so it is much weaker evidence than a new solid.
Question 3 (multiple choice · core)
Four particle diagrams are described below. A and B are different elements. Which one shows a chemical change?
- (A) Before: 6 AB molecules packed closely. After: the same 6 AB molecules spread far apart.
- (B) Before: 4 A₂ molecules in the left half of a box and 4 B₂ molecules in the right half. After: the same 8 molecules evenly mixed.
- (C) Before: 4 A₂ molecules and 2 B₂ molecules. After: 4 A₂B molecules.
- (D) Before: 6 A atoms in a regular pattern. After: 6 A atoms in an irregular arrangement, still touching.
Answer and explanation
Answer: (C). The A–A and B–B bonds break and new A–B bonds form, making a new substance, A₂B. Atoms are conserved: 8 A and 4 B before; 4 × 2 = 8 A and 4 × 1 = 4 B after.
- (A) is a liquid boiling (or a gas expanding). The molecules are unchanged.
- (B) is mixing two gases. No new molecules form.
- (D) is a solid melting. The atoms are the same; only their arrangement changes.
Question 4 (constructed response · foundation)
Classify each change as physical or chemical. Give one piece of evidence and link it to whether a new substance forms.
(a) A puddle of water disappears on a sunny day. (b) An antacid tablet dropped into water fizzes; the gas turns limewater cloudy. (c) Green nickel(II) chloride solution is mixed with colourless sodium hydroxide solution and a pale green solid forms. (d) Ethanol and water are mixed; the mixture warms slightly and stays clear and colourless.
Worked solution and suggested Marlbridge rubric
(a) Physical. Evaporation is a change of phase; the water vapour is still H₂O. (b) Chemical. The gas is carbon dioxide (limewater test), which was not present as a substance in the tablet or the water, so a new substance has formed. (c) Chemical. A precipitate forms: a solid that was not present in either solution (nickel(II) hydroxide). The colour is similar, so the solid, not the colour, is the evidence. (d) Physical. Mixing two liquids makes a solution. The small temperature rise is not enough on its own: no new substance is detected, and the ethanol can be separated by distillation.
| Point | What earns it |
|---|---|
| 1 | (a) physical, with a phase-change reason |
| 1 | (b) chemical, linking the identified gas to a new substance |
| 1 | (c) chemical, using the precipitate as the evidence |
| 1 | (d) physical, explaining why the temperature change does not decide it |
No point for a classification without a reason.
Question 5 (constructed response · core)
A student mixes 25.0 mL of 1.0 mol L⁻¹ hydrochloric acid with 25.0 mL of 1.0 mol L⁻¹ sodium hydroxide solution. Both solutions are colourless and start at 21.2 °C. The products are sodium chloride and water.
(a) Predict what the student will see by eye when the solutions are mixed. (b) The mixture reaches 28.0 °C. Calculate the temperature change. (c) The student writes: “I saw nothing happen, so there was no reaction.” Evaluate this claim. (d) Suggest one further test that would show a new substance has formed.
Worked solution and suggested Marlbridge rubric
(a) Nothing visible: the mixture stays clear and colourless, with no bubbles and no solid. (b) ΔT = 28.0 − 21.2 = +6.8 °C (a rise). (c) The claim is not supported. A reaction does not have to be visible. The 6.8 °C rise is evidence that energy was released when the solutions were mixed, which suggests a new substance formed. (d) Any one: test the pH of the mixture and show it is close to neutral, unlike either starting solution; or evaporate the water and show that the white solid left is sodium chloride, a substance that was not in either starting solution.
| Point | What earns it |
|---|---|
| 1 | Predicts no visible change |
| 1 | +6.8 °C with the sign or the word “rise” |
| 1 | Rejects the claim, using the temperature data as evidence |
| 1 | A valid test that targets a new substance (pH or recovered NaCl) |
For (c), accept a cautious answer that says the temperature rise strongly suggests a reaction and (d) would confirm it.
Question 6 (constructed response · core)
A student adds a spoonful of grey zinc powder to a blue solution of copper(II) sulfate and stirs. Zinc sulfate solution is colourless. The reaction is Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s).
(a) Predict three observations the student could make. (b) Explain how each observation supports the claim that a chemical change has occurred.
Worked solution and suggested Marlbridge rubric
(a) Any three:
- the blue colour of the solution fades and the solution can become colourless;
- a red-brown (pinkish) solid, copper, appears;
- the grey zinc powder gets less as it is used up;
- the mixture gets warm.
(b) The fading blue shows that copper(II) ions are being used up and replaced by colourless zinc ions. The red-brown solid is a new substance (copper metal) that was not present at the start. Zinc being used up shows a starting substance has changed into something else. Warming shows energy is released as new substances form, though on its own this would be weak evidence.
| Point | What earns it |
|---|---|
| 1 | Colour of solution fades |
| 1 | Red-brown or pink-brown solid forms |
| 1 | A third valid observation (zinc used up, or temperature rise) |
| 1 | Links at least two observations to a new substance forming |
Do not award “the solution turns a different colour” without saying it fades or loses its blue.
Question 7 (constructed response · stretch)
Two identical flasks each hold excess dilute hydrochloric acid and some marble chips (calcium carbonate). The marble chips react, giving carbon dioxide gas. Flask A is left open. Flask B has a balloon fixed over its neck. Each flask is weighed before and after the reaction; ignore any buoyancy effect of the balloon.
| Before (g) | After (g) | |
|---|---|---|
| Flask A (open) | 85.20 | 84.76 |
| Flask B (balloon) | 112.58 | 112.58 |
(a) Calculate the change in mass of Flask A. (b) Calculate the amount, in moles, of carbon dioxide that left Flask A. (c) A student says: “Flask A shows that atoms are destroyed in some reactions.” Evaluate this claim using both sets of data.
Worked solution and suggested Marlbridge rubric
(a) 84.76 − 85.20 = −0.44 g (a loss of 0.44 g). (b) M(CO₂) = 12.01 + 2(16.00) = 44.01 g mol⁻¹; n = 0.44 g ÷ 44.01 g mol⁻¹ = 0.010 mol (2 significant figures, because 0.44 g has 2). (c) The claim is wrong. Flask B, where the gas cannot escape, shows no change in mass, so the total mass of atoms is the same after the reaction. Flask A loses mass only because the carbon dioxide formed by the reaction escapes into the air. The atoms are rearranged into new substances, not destroyed.
| Point | What earns it |
|---|---|
| 1 | −0.44 g (or “loss of 0.44 g”) |
| 1 | 0.010 mol with molar mass shown |
| 1 | Uses Flask B (no change) to show mass is conserved when nothing escapes |
| 1 | Explains Flask A’s loss as gas escaping, not atoms being destroyed |
Accept carried-forward errors from (a) in (b).
How did you do?
- Q1 or Q3 wrong: re-read “What changes and what stays the same” and Figure 1 in the study guide.
- Q2, Q4 or Q5 wrong: review “Evidence is a clue, not proof” and the claim–evidence–reasoning pattern.
- Q6 or Q7 incomplete: compare your answer with Worked example 2 (predicting results and mass changes).
Then tick off the topic checklist.