Identification of Ions and Gases

Section 1: Identification of Ions

Ions are atoms or groups of atoms that have gained or lost electrons, giving them a positive or negative charge. They are produced during chemical reactions.
An anion test is used to identify negatively charged ions. These tests often involve adding dilute nitric acid and then silver nitrate or barium chloride solution.
A cation test identifies positively charged ions. This usually involves adding sodium hydroxide and observing the resulting precipitate.
Flame tests can also help identify certain metal ions. Lithium turns a flame red, sodium gives a yellow flame, while calcium gives a brick red flame, and copper burns with a green/blue flame.
The sulphate ion can be detected by adding barium chloride solution to the test solution. If a white precipitate forms, the substance contains sulphate ions.
Chlorides react with silver nitrate to form a white precipitate, indicating the presence of chloride ions.
A deep blue solution or precipitate upon addition of sodium hydroxide solution indicates presence of copper ions.

Section 2: Identification of Gases

Oxygen can be identified using a glowing splint. If it relights, oxygen is present.
Hydrogen can be tested by bringing a lit splint near the gas. If a ‘pop’ sound is heard, hydrogen is present.
Carbon dioxide turns lime water (calcium hydroxide solution) cloudy upon bubbling, indicating its presence.
When a glowing splint is inserted into a test tube containing nitrogen gas and extinguishes without a ‘pop’, it indicates the presence of nitrogen.
Chlorine bleaches damp litmus paper, turning it from blue to white.
Ammonia turns moist red litmus paper blue and has a distinctive pungent smell, indicating its presence.
Sulphur dioxide can turn acidified potassium dichromate(VI) paper from orange to green. Additionally, like carbon dioxide, sulphur dioxide also turns lime water cloudy but slower compared to carbon dioxide.