Chemistry 5070/11 — May/June 2017

Walkthrough

The question gives four observations and asks which gas matches all four.

• Colourless: ammonia, hydrogen and oxygen are colourless, but chlorine is pale green/yellowish, so B is immediately unlikely.
• No effect on red or blue litmus: ammonia is alkaline and would turn red litmus blue, so A cannot be the answer. Chlorine also has an effect on litmus, usually bleaching it, so B is ruled out too.
• No effect on limewater: this test is usually used to detect carbon dioxide, which is not one of the options, so this statement does not help identify the gas. It simply confirms the gas is not carbon dioxide.
• Flammable: ammonia is not commonly used as a fuel, chlorine is not flammable, and oxygen does not burn. Hydrogen, however, is a flammable gas. It also gives the other three observations.

So only hydrogen fits all four properties.

Key Takeaways

• Hydrogen is a colourless, odourless gas that is neutral to litmus and has no effect on limewater, but it burns.
• Oxygen is not flammable; it supports combustion.
• Ammonia is an alkaline gas and turns red litmus paper blue.
• Chlorine is a greenish, bleach litmus paper.

Common Mistakes

• Choosing oxygen because the candidate knows oxygen is common and supports combustion. However, the question says the gas itself is flammable, which oxygen is not.
• Choosing ammonia because it is a common gas, without remembering that ammonia is alkaline and would affect litmus.
• Forgetting that chlorine has a colour and so fails the "colourless" property.

Things to Be Careful About

• Read each property carefully and check every option against all four statements.
• Keep the gas tests clear: limewater tests for carbon dioxide, glowing splint tests for oxygen, and litmus tests for acidic or alkaline gases.
• A gas that has "no effect" is usually a neutral gas such as hydrogen.