Let us first discuss what acids are-
Acids are compounds that can donate a proton, that is, a hydrogen ion on dissociation.
They contain at least one hydrogen (for example- hydrochloric acid HCl). Some might contain more than one, for example- sulphuric acid (H2SO4) contains 2 hydrogen atoms, phosphoric acid (H3PO4) contains three, etc.
Acids can be regarded as strong acids, that are highly corrosive in nature (example- sulphuric acid) or as weak acids that are not so corrosive (example- acetic acid).
Acids are also present in our diet, surprised?? Yes, acids are also present in the food that we eat, for example- lemon, it contains ascorbic acid, whereas folic acid is present in green leafy vegetables, beans and peas. These are necessary for our growth and development and might lead to deficiency diseases if not taken in adequate amounts.
Acid turns moist blue litmus paper red.
Acids are sour in taste.
However, any acid requires water to act as an acid.
The reason behind this lies in the definition of the acid itself. As said, acid is a hydrogen containing compound that gives a proton in solution. Thus, for an acid to act like an acid, that is, to give its proton it must be present in water, so that it can dissociate and produce protons (H+ ions)
HCl ↔ H+ + Cl-
For example, HCl, hydrogen chloride, is a greenish yellow gas with a pungent smell. It will not act like an acid until mixed with water, that is, it will not dissociate into its ions, H+ and Cl- until present in an aquatic medium.
>Also, dry HCl will not turn blue litmus paper red if the paper has not been moistened. It will not act as an acid until it gets an aqueous medium. But it will readily turn moist blue litmus paper red.
The strength of an acid lies in the number of hydrogen ions or protons it was deliver or dissociate into. It is known as the pH of the solution which is actually the inverse logarithm of concentration of hydrogen ions.
pH = -log [H+]
when the pH lies between 0-6, the solution is said to be acidic whereas when it lies between 8-14, it is said to be basic. pH 7 is considered to be neutral. The pH of water is 7.
Thus, to conclude with, an acid shows acidic behaviour in presence of water, where it can dissociate and deliver Hydrogen ions or protons. The acidic strength, behaviour, conditions all depend upon the amount and number of protons an acid can donate.