What's the chemical reaction that happens when you mix bleach with vinegar?

When you mix bleach with vinegar, an acid-base reaction occurs that releases chlorine gas, and that's what makes the mixture dangerous. Bleach (sodium hypochlorite) is a base, and vinegar (acetic acid) is an acid. When the two come into contact, the acid breaks down the hypochlorite molecule and releases chlorine gas - that strong, toxic smell you notice when you do this by accident.

The simplified reaction is basically this: the acetic acid in vinegar reacts with the hypochlorite, producing hypochlorous acid, which is unstable and decomposes rapidly into water, chlorine gas, and other substances. This chlorine gas is extremely irritating to your lungs and mucous membranes - even in small amounts it causes coughing, burning eyes, and difficulty breathing. If you inhale a lot of it, it can get really serious.

That's why you should never mix these two products, not even in a small amount "just to test it." If you need to disinfect something, use bleach alone or vinegar alone, never both. And if you have to use different products in sequence, rinse the surface well with water between one and the other. It's not worth risking your health for a cleaning job that can be done safely another way.