Acid strength refers to how completely an acid splits into ions when mixed with water. Strong acids, like the sulfuric acid in car batteries, split completely. This high reactivity is what allows the battery to produce high starting currents, but it also makes the fluid extremely corrosive to skin, clothing, and metal brackets.
In contrast, weak acids only split partially. The organic acids that form in engine oil over time are weak acids. While they react slower than battery acid, they can still etch bearing surfaces and harden rubber seals if the oil is not changed regularly. This is why oil contains acid-neutralizing additives.
Testing acid strength in engine coolant is a common maintenance check. As coolant ages, its corrosion inhibitors break down, and the fluid becomes acidic. Checking the pH level tells the mechanic if the coolant has lost its ability to protect the aluminum radiator and cylinder head.