Limescale and hard water marks need an acid. Grease and food need an alkali. Germs need a disinfectant left on for the time the label states. Working out which one you have turns most jobs into short ones.
- Mineral deposits are limescale, hard-water marks, soap scum and the white crust round a tap. These need acid: white vinegar for the mild version, a proper descaler for the serious one.
- Grease and organic soil are cooking fat, body oils and food residue. These need alkali: washing-up liquid, bicarbonate of soda, or a dedicated degreaser.
- Germs are a different problem from dirt, and a surface can be spotless and still contaminated. These need a disinfectant, left wet for the contact time printed on the bottle, which is usually far longer than people leave it.
Most of the time people reach for whatever is under the sink and scrub harder. Thirty seconds spent deciding which of the three you are looking at saves the afternoon.
Acid and alkali cancel each other out. Mixing vinegar and bicarbonate of soda produces a satisfying fizz, some carbon dioxide and a weak salt solution that is worse at both jobs than either would have been alone.
Dirt is held on by a chemical bond of some kind, and each type of bond has an opposite that breaks it. You are not scrubbing the dirt off, you are dissolving the thing holding it on and then rinsing.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.