GCSE Combined Science Chemistry 2 — Questions and Answers
Question 1: What is the test for carbon dioxide gas?
- It relights a glowing splint
- It turns limewater milky (cloudy) (Correct answer)
- It makes a squeaky pop with a burning splint
- It bleaches damp litmus paper
Correct answer: It turns limewater milky (cloudy)
Carbon dioxide turns limewater milky (cloudy). The COâ‚‚ reacts with calcium hydroxide (limewater) to form insoluble calcium carbonate, which appears as a white precipitate making the solution look cloudy.
Question 2: In electrolysis of a molten ionic compound, which electrode do positive ions migrate to?
- The anode (positive electrode)
- The cathode (negative electrode) (Correct answer)
- Both electrodes equally
- Neither electrode
Correct answer: The cathode (negative electrode)
Positive ions (cations) are attracted to the cathode (negative electrode) because opposite charges attract. At the cathode, metal ions gain electrons (reduction) and are deposited as metal atoms.
Question 3: What is the purpose of a catalyst in a chemical reaction?
- To increase the amount of product formed
- To speed up the reaction by lowering the activation energy (Correct answer)
- To make the reaction release more energy
- To change the products of the reaction
Correct answer: To speed up the reaction by lowering the activation energy
A catalyst speeds up a chemical reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is not used up in the reaction and can be recovered chemically unchanged at the end.
Question 4: What is the formula for calculating the number of moles of a substance?
- Moles = mass × Mr
- Moles = mass ÷ Mr (Correct answer)
- Moles = Mr ÷ mass
- Moles = mass + Mr
Correct answer: Moles = mass ÷ Mr
Moles = mass (in grams) ÷ relative formula mass (Mr). For example, 36g of water (Mr = 18) = 36 ÷ 18 = 2 moles. This is one of the most important calculations in GCSE Chemistry.
Question 5: Which process is used to separate crude oil into its fractions?
- Filtration
- Crystallisation
- Fractional distillation (Correct answer)
- Chromatography
Correct answer: Fractional distillation
Fractional distillation separates crude oil into fractions based on their different boiling points. The crude oil is heated and vapours rise up a fractionating column. Shorter hydrocarbons with lower boiling points rise higher before condensing.
Question 6: An atom has 11 protons, 12 neutrons, and 11 electrons. What is its mass number?
- 11
- 12
- 23 (Correct answer)
- 34
Correct answer: 23
Mass number = protons + neutrons = 11 + 12 = 23. This atom is sodium-23 (Na). The mass number does not include electrons because their mass is negligible compared to protons and neutrons.
GCSE Combined Science Chemistry 2
What is the test for carbon dioxide gas?