Acetic acid, in the form of vinegar is used in making pickles and tomato ketchup, tartaric acid is used in baking powder, and carbonic acid is used in soda water and fizzy soft drinks.

A concentrated acid contains minimum possible amount of water, whereas a dilute acid contains much more water. The process of diluting a concentrated acid with water is highly exothermic (heat producing). Therefore, dilution should be carried out by slowly adding concentrated acid to water and not by adding water to acid. If water is added to the acid, the large amount of heat produced converts water to  steam which can splash the acid on the body or clothes and cause acid bums.

Acid solutions conduct electricity. Acids react with metals to form hydrogen gas, which burns making a 'pop' sound. Sour foodstuffs, such as curd, lemon juice, etc., should not be kept in metal vessels because the acids present in these foodstuffs can react with the metal to form poisonous compounds which can cause food poisoning.

Acids react with bases (alkalis) to form salt and water. This is known as neutralisation reaction. They also react with metal oxides to form salt and water. Mineral acids are corrosive in nature. They cause severe burns on the skin, make holes in clothes, burn wood, and corrode metal structures and stonework. That is why acids are stored in containers made of glass or ceramic and not in metal containers.

All acids produce hydrogen ions (H⁺ ions) when dissolved in water:

HCl (aq) → H⁺ (aq) + Cl^- (aq)

H₂SO₄(aq) → 2H⁺ (aq) + SO₄^-2` (aq)

Though all acids contain hydrogen, all hydrogen containing compounds are not acids. Acids produce H⁺ (aq) ions only in the presence of water. So an acidic substance will not furnish H⁺ (aq) ions in the absence of water and will, therefore, not show acidic behaviour.

Strong and Weak Acids

An acid which is completely ionized in water to produce a large amount of H⁺ ions is called a strong acid. Examples: HCl, H₂SO₄, HNO₃. These acid have high reactivity and high electrical conductivity. Thus, they are strong electrolytes.

An acid which partially ionized in water to produce a small amount of H⁺ ions is called a weak acid. Examples: CH₃COOH, and H₂CO₃. They have low reactivity and low electrical conductivity. Thus, they are weak electrolytes.

Some common Uses of Acids

• Sulphuric acid is used in the manufacture of fertilizers, paints, dyes, plastics, synthetic fibres, detergents, car batteries, etc.
• Nitric acid is used for making fertilizers, dyes, plastics, and explosives like Trinitro Toluene (TNT).
• Hydrochloric acid is used in dyes, textiles, and leather industry. It is used for removing deposits from inside boilers. It is used for making plastics like Polyvinyl chloride (PVC). It is used in medicines and cosmetics.

Antacids and Acid Inhibitors

Gastric juice is an acidic digestive fluid secreted by glands in the mucous membrane that lines the stomach. It contains hydrochloric acid (HCl). Overeating and emotional factors can cause the stomach to produce too much HCl. This leads to hyperacidity, commonly known as 'acid indigestion' or 'heartburn'.

Two approaches are used to combat the problem of excess stomach acid:

• Removal of excess acid through neutralisation, which involves the use of antacids like Digene and Gelusil [Mg(OH)₂. Al(OH)₃], Milk of Magnesia [Mg(OH)₂], and Turns [CaCO₃]. These are basic substance capable of neutralising the HCl present in gastric juice. Neutralisation involving sodium bicarbonate and calcium carbonate produces carbon dioxide, which causes a person to belch often.
•  Decrease in the production of stomach acid, which involves the use of acid inhibitors like Pepcid, Tagamet, and Zantac. These substances inhibit the production of gastric acid by blocking the action of histamine, a gastric acid secretion regulator.

BASES

Bases are substances which turn red litmus blue. They have a bitter taste and are soapy to touch. They neutralise acids. All metal oxides and metal hydroxides are bases. For example, Na₂O, CaO, NaOH, KOH, Ca(OH)₂, Mg(OH)₂, Ba(OH)₂ are bases. NH₄OH, Na₂CO₃, CaCO₃, and NaHCO₃ are also considered as bases because they neutralise acids. Water soluble bases are called alkalis. For example: NaOH, KOH, NH₄OH, Mg(OH)₂ and Ca(OH)₂.

All bases produce hydroxide ions (OH^-ions) when dissolved in water. For example,

KOH(aq) → K⁺ (aq) + OH^-(aq)

Mg(OH)₂(aq) → Mg²⁺(aq) + 2OH^-(aq)

Strong and Weak Base

A base which is completely ionized in water produce a large amount of OH ions is called a strong base. Example: NaOH and KOH.

A base which is partially ionized in water to produce a small amount of OH ions is called a weak base. Example: NH₄OH, Mg(OH)₂, Ca(OH)₂.

Some Common Uses of Bases

• Sodium hydroxide is used in the manufacture of  soap, paper, and rayon (synthetic fibre). It is also used in oil refining and making dyes and bleaches.
• Potassium hydroxide is used in the manufacture of shampoos and shaving creams.
• Magnesium hydroxide  is used as an 'antacid'.
• Calcium hydroxide is used in the manufacture of bleaching powder.