An impure hydrocarbon fuel is burned in the apparatus in Figure 7 - Edexcel - GCSE Chemistry - Question 9 - 2020 - Paper 1

The colourless liquid that forms in tube A is likely to be water, produced from the combustion of hydrogen atoms present in the hydrocarbon fuel. The combustion reaction often yields water vapor, which then condenses into liquid form when cooled in the tube.

The indicator in tube B indicates the presence of an acidic gas, most likely carbon dioxide. This gas is a product of burning hydrocarbons, and when dissolved in the indicator solution, it forms carbonic acid, which would turn the indicator a different colour, signifying acidity.