What are acids? Definition:

As acids (English: Acid) refers to chemical compounds with a pH of less than seven. Acids act as so-called proton donorsthat is, they are hydrogen ions (H+) can be transferred to their reactants. The counterpart to acids are bases, also known as alkalis.
Acids are very corrosive depending on the concentration. Therefore, never work without suitable safety goggles. If necessary, protective clothing and protective gloves can be useful. If the acid comes into contact with your hands, wash with water immediately. The instructions of the experimenter / teacher must be followed.
When dealing with acidic liquids, an important donkey bridge has established itself: 'first the water, then the acid, otherwise the monstrous' happens. Water must never - eg. for dilution - to be added to an acid. The suddenly released hydration energy causes the acid to evaporate and spray. That's why you always add acids to the water. The subsequent reaction is no less exothermic, but the released energy distributes evenly to the surrounding amount of water.
Incidentally, the phrase is also in the English language: Always do things as you oughta, add the acid to the water!

Properties of acids:

Acids are corrosive.
Acids can dissolve base metals and organic matter.
Sour taste (do not try it!).
Acids can be liquid, gaseous or solid.
Bases / alkalis neutralize acids.
Dilution with water produces heat (exothermic reaction).
The universal indicator turns red.
Acids have a pH of approx. 0 - 6.5
Acids are electrically conductive.

Examples of acids:

formic acid (CH2O2): Natural occurrence e.g. in the stinging nettles, defensive secretions of some ant species
cyanide (HCN): synonym for hydrogen cyanide; contained in the kernels of many stonefruit; highly toxic
citric acid (C6H8O7): Part of many detergents; high levels of citric acid in fruit fruits
acetic acid (C2H4O2): characteristic, pungent odor; Acidifier in the food industry
carbonic acid (H2CO3): Reaction product of carbon dioxide (CO2) and water (H2O)
nitric acid (HNO3): used to extract gold from other metals
hydrochloric acid (HCl): component of stomach acid
sulfuric acid (H2SO4): Component of battery acid; central importance in the emergence of acid rain