GCSE Combined Science Chemistry 2

Question 1

What is the test for carbon dioxide gas?

Accepted Answer

It turns limewater milky (cloudy)

Answer

Carbon dioxide turns limewater milky (cloudy). The CO₂ 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?

Accepted Answer

The cathode (negative electrode)

Answer

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?

Accepted Answer

To speed up the reaction by lowering the activation energy

Answer

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?

Accepted Answer

Moles = mass ÷ Mr

Answer

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?

Accepted Answer

Fractional distillation

Answer

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.

GCSE Revision Practice Test

GCSE Revision Practice Test

GCSE Combined Science Chemistry 2