Which one of the following is NOT correctly matched? Substance : Use

Silver bromide is a light-sensitive salt. On exposure to light it breaks down into metallic silver and bromine, which is the principle behind photographic film and photographic paper. It has no established role in artificial photosynthesis, so pairing it with that use is the mismatch. Artificial photosynthesis research relies on photocatalysts such as titanium dioxide and on water-splitting or CO₂-reduction catalysts.

The other three pairs hold up:

• Silver iodide has a hexagonal crystal structure close to that of ice, so it is dispersed into clouds to seed precipitation. This is cloud seeding, or artificial rain.
• Sodium bicarbonate is a mild alkali. Its alkalinising action has long been used in remedies for rheumatic and arthritic pain, so this pairing is accepted in the key, though it is a weaker link than the others.
• Milk of magnesia is a suspension of magnesium hydroxide that neutralises excess stomach acid, which makes it an antacid.

The core concept is the chemistry of silver halides and their light-sensitive and crystal-structure properties. In Mains, the photocatalysis theme links to green hydrogen and clean energy.

Trap: aspirants often judge the pairs by how familiar they sound and move on. Check each use against the actual chemistry of the substance instead.