We can find the Acid or Base with the help of reaction, whether a substance is acidic or basic by change in colour. There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators.
The metal in the above reactions with hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt. Thus, the reaction of a metal with an acid can be summarised as −
Acid + Metal → Salt + Hydrogen gas
Limestone, chalk and marble are different forms of calcium carbonate.
All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water.
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
The effect of a base is nullified by an acid and vice-versa. The reaction taking place is written as −
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as −
Base + Acid → Salt + Water
The colour of the solution becomes blue-green and the copper oxide dissolves. The blue-green colour of the solution is due to the formation of copper(II) chloride in the reaction. The general reaction between a metal oxide and an acid can be written as −
Metal oxide + Acid → Salt + Water
The metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides.
Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. Since this is similar to the reaction between a base and an acid, we can conclude that nonmetallic oxides are acidic in nature.
Hydrogen seems to be common to all acids. Let us perform an Activity to investigate whether all compounds containing hydrogen are acidic. The bulb will start glowing in the case of acids.
Acids contain H+ ion as cation and anion such as Cl− in HCl, NO-3 in HNO3, SO-4 in H2SO4, CH3COO− in CH3COOH. Since the cation present in acids is H+, this suggests that acids produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties.
The hydrogen ions in HCl are produced in the presence of water. The separation of H+ ion from HCl molecules cannot occur in the absence of water.
HCl + H2O → H3O+ + Cl-
Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+(aq) or hydronium ion (H3O+).
H+ + H2O → H3O+
Bases generate hydroxide (OH−) ions in water. Bases which are soluble in water are called alkalis.
Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH−) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
Acid-base indicators can be used to distinguish between an acid and a base. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution.
A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power.
On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH− ion concentration in the solution.
Generally paper impregnated with the universal indicator is used for measuring pH.
Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change. When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers becomes difficult.
Plants require a specific pH range for their healthy growth.
It is very interesting to note that our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach.
Salts of a strong acid and a strong base are neutral with pH value of 7.
On the other hand, salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7.
The salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that you use in food.
These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal.
Copper sulphate crystals which seem to be dry contain water of crystallisation. When we heat the crystals, this water is removed and the salt turns white.
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is Cu SO4 . 5H2O.