At One Place

Can you mix them?

22 combinations with what actually happens, rather than a blanket rule about never mixing things. 5 of them produce a toxic gas, several are simply pointless, and some of the ones people are most nervous about are entirely fine.

If the two have already been mixed: leave the room, open windows and doors from outside if you can reach them without breathing the air, and ring your national poison line. Do not go back in to neutralise it — adding a third chemical to a reaction in progress is the intuitive move and the wrong one, and most exposures happen to the person who returns to clear it up.

The ones that produce a gas

Combination What it makes
Bleach and ammonia No. This is the classic household poisoning and it produces chloramine gas.
Bleach and vinegar No. Any acid plus bleach releases chlorine gas.
Bleach and toilet cleaner No. Most toilet cleaners are acidic, so this is the bleach-and-acid reaction.
Bleach and rubbing alcohol No. It produces chloroform and other chlorinated compounds.
One drain cleaner and a different drain cleaner No. Drain cleaners come in strongly acidic and strongly caustic types, and combining them is violent.

The pattern behind almost all of them is bleach. Sodium hypochlorite is stable only while it is alkaline, so anything acidic releases chlorine and anything containing ammonia releases chloramine — and the products that contain acid or ammonia frequently do not say so on the front of the bottle. Glass cleaners often contain ammonia; toilet and limescale cleaners are usually acidic. Nobody mixes these deliberately. They use one product, then another, on the same surface.

The famous one that does nothing

Baking soda and vinegar is the most shared cleaning tip on the internet and it produces salt water. An acid and a base neutralise; the fizz is carbon dioxide leaving, and what stays behind is dilute sodium acetate. Bicarbonate cleans as a mild abrasive and a base, vinegar as an acid on limescale, and combining them destroys both properties. The fizz is doing all the persuading.

Everything

Combination Verdict Why
Bleach and ammonia Toxic gas No. This is the classic household poisoning and it produces chloramine gas.
Bleach and vinegar Toxic gas No. Any acid plus bleach releases chlorine gas.
Baking soda and vinegar Pointless Safe, dramatic, and it leaves you with salt water.
Bleach and toilet cleaner Toxic gas No. Most toilet cleaners are acidic, so this is the bleach-and-acid reaction.
Bleach and rubbing alcohol Toxic gas No. It produces chloroform and other chlorinated compounds.
One engine coolant and a different coolant type Do not No — not across chemistries. The additives react and can gel.
Hydrogen peroxide and vinegar Do not No — mixed in a bottle it forms peracetic acid, which is corrosive.
One drain cleaner and a different drain cleaner Toxic gas No. Drain cleaners come in strongly acidic and strongly caustic types, and combining them is violent.
DOT 3 brake fluid and dot 4 brake fluid Fine Yes. Both are glycol-based and fully miscible.
One engine oil and a different brand or grade Fine Yes. Any two oils meeting the same specification mix safely.
One battery brand and a different brand or age Do not No — mixing brands, ages or chemistries in one device causes leaks.
Water-based paint and oil-based paint underneath Do not Not directly. It will look fine and peel off in sheets later.
Bleach and hydrogen peroxide Do not No — it is exothermic and destroys both, and can spray hot liquid.
Regular petrol and premium petrol Fine Yes. They blend, and the tank ends up somewhere between the two octane ratings.
Vinegar and marble or limestone Do not No. Acid etches calcium carbonate permanently.
Superglue and baking soda Fine Yes, and it is a genuinely useful trick — but it gets hot.
DOT 5 brake fluid and dot 4 brake fluid Do not No. DOT 5 is silicone and is not compatible with any glycol fluid.
Bleach and laundry detergent Do not Usually safe but wasteful — and unsafe if the detergent contains ammonia.
Vinegar and castile soap Pointless Safe, and it curdles into a white mess that is worse than either.
Expanding foam and water Fine Yes — polyurethane foam needs moisture to cure, and misting the gap helps.
Vinegar and ammonia Pointless Not dangerous, and they cancel each other out.
PLA filament and abs filament Do not They will not bond to each other, and they want different temperatures.

Cleaning products

Vehicle fluids

Around the house

Paint, glue and materials

Common questions

What happens if you mix bleach and ammonia?
It produces chloramine vapours, which attack the eyes, throat and lungs. Heavy exposure in an enclosed space can cause fluid to accumulate in the lungs hours after the person feels better. Leave the room, ventilate from outside, and ring the poison line.
Why is bleach and vinegar dangerous?
Bleach is stable only while it is alkaline. Any acid — vinegar, lemon juice, or a limescale or toilet cleaner — releases chlorine gas from it. Most accidental exposures come from using an acidic bathroom cleaner in a bowl that already has bleach in it.
Does baking soda and vinegar actually clean anything?
Barely. They neutralise each other to sodium acetate and water — salt water. Each is a good cleaner alone, bicarbonate as a mild abrasive base and vinegar as an acid for limescale, and combining them destroys both properties.
Can you mix different brands of engine oil?
Yes. Oils meeting a common specification are required to be miscible, because topping up from a petrol station shelf is exactly the situation the standards were written for. Coolant is the opposite case — mixing chemistries can gel and block a radiator.