Acids Acids are substances that release hydrogen when they are dissolved in water. Acid is also a common word in everyday use, but it has a precise definition in chemistry. Acids are also the opposite of bases. Acids are usually mentioned with bases, because they are sort of similar. Acids have a sour taste and can sting the skin.
People use acid too, they make ink and cleaning substances to shine cars.
A type of acid is citric acid. It is found in some fruits, like lemons. That is why they taste sour.