An acid-base reaction happens when an acid mixes with a chemical base. This reaction is a daily occurrence in automotive maintenance. The most common example is neutralizing a spilled car battery. Sulfuric acid is neutralized by applying a base like baking soda, turning the corrosive acid into harmless salt water and carbon dioxide gas.
This reaction is also built into engine oil chemistry. Combustion creates acidic gases that leak into the engine oil. To counter this, oil makers blend in basic additives like calcium carbonate. These bases react with the acids, neutralizing them before they can pit bearings or ruin rubber seals.
In cooling systems, acid-base reactions are used to flush out scale and rust. A mild acid flush dissolves mineral buildup inside the radiator. Once the scale is gone, a basic neutralizing solution is run through the system to stop the acid action before the new coolant is added.