The taste of the food is due to the presence of acids and bases in them.

Acids

• Acids are defined as the substances which produce hydrogen ions or Hydronium ions (H₃O+) in water. For Example, Sulphuric Acid, Hydrochloric, Nitric Acid, Acetic Acid etc.
• They taste sour.
• Acids turn blue litmus to red. This is used as a confirmation test for the presence of acid.
• When acids react with metals, gases are evolved.

Reactions with Acids

1. Reaction of Acid with Metal

Acid + Metal → Salt + Hydrogen gas

Mg + H₂SO₄ → H₂ + MgSO₄

2. Reaction of Acid with Carbonates

Na₂CO₃ (s) + 2 HCl (aq) → 2NaCl (aq) + H₂O(l) + CO₂(g)

3. Reaction of Acid with Bicarbonates

NaHCO₃ (s) + HCl (aq) → NaCl(aq) + H₂O (l) + CO₂ (g)

Similarity between Acids and Bases

• Both acids and bases react with water. They produce ions in water
• Both acids and bases act as electrolytes, so are good conductors of electricity.
• Both of them change the colour of the litmus paper.

Classification of Acids

Acids are classified as Organic Acids and Mineral Acids. Acids which are derived from plants and animals, they are known as Organic Acids. 

Example, Citric Acid from fruit. 

Mineral acids are inorganic acids such as Sulphuric Acid. They are dangerous to be used, so need more precautions.

Acids are also classified as Strong Acids or Weak Acids. Strong acid is an acid that completely dissociates into ions in aqueous solutions. For Example, Sulphuric Acid, Hydrochloric Acid.

Weak acid is the one which does not dissociate completely into ions in aqueous solutions. For Example, Acetic Acid.

Acids can also be as Dilute Acid and Concentrated Acids. The one which has low concentration of acids in aqueous solution, they are known as Dilute Acids whereas the one which has high concentration of acids in aqueous solution, are known as Concentrated Acids.

It is advisable to add acid to water and not vice versa because a large amount of heat is released if water is added to acid. This released heat is large enough to cause harm.

Acids can also be classified based on the number of hydrogen ions. Monoprotic acid is the one which gives one mole of hydrogen ions per mole of acid, such as HCl. Diprotic Acid is the one which produces two mol of hydrogen ions per mole of acid. For Example, H₂SO₄.

Bases

• Bases are the one which produces hydroxide ions in aqueous solutions. Bases which are water soluble are known as Alkalis.
• They turn red litmus to blue.
• They have a bitter taste.
• They also produced carbon-dioxide when reacted with carbonates.
• They also evolved hydrogen gas when bases react with metals.

Reactions of Bases

1. Reaction with Metals

Base reacts with metals and produces hydrogen gas.

2NaOH + Zn → Na₂ → Na₂ZnO₂ + H₂

2. Reaction with Acids

Base reacts with acids to form salts. For Example,

KOH + HCl → KCl + H₂O

3. Reaction with Non-metallic Oxides

Base reacts with non-metallic oxides to form salt and water.

2NaOH + CO₂ → CO₂­ → Na₂CO₃ + H₂O

Classification of Bases

Bases are classified as Strong Base and Weak Base. Strong base is the one which dissociates completely into its ions in aqueous solution. For Example, NaOH.

Weak base is the one which does not dissociate completely into its ions in aqueous solutions. For Example, Ammonium Hydroxide, NH4OH

Bases are also classified as Dilute Base and Concentrated Base. The solution which has low concentration of base in aqueous solution is defined as Dilute Base whereas the one which has high concentration of base in aqueous solution is known as Concentrated Base.

Strength of Acid or Base Solutions

The dissociation constant of weak acid or weak base can be represented as-

Suppose HA is weak acid, then dissociation constant is represented as-

Strength of an acid or base can be determined using a pH scale. It is a scale to measure the hydrogen ion concentration in a solution. The p stands for ‘potenz’, it is a German word which means power.

• If pH is equal to 7, means the solution is neutral.
• If pH is greater then 7 means alkaline solution.
• If pH is less then 7 means the solution is acidic.

Fig.1. pH scale

Importance of pH

• Human body works at a pH of about 7.4.
• Stomach has a pH of about 2 due to the presence of hydrochloric acid in it. It is needed for the activation of pepsin protein required for protein digestion.
• When we eat food containing sugar, then the bacteria present in our mouth break down the sugar to form acids. This acid lowers the pH in the mouth. Tooth decay starts when the pH of acid formed in the mouth falls below 5.5. This is because then the acid becomes strong enough to attack the enamel of our teeth and corrode it. This sets in tooth decay. The best way to prevent tooth decay is to clean the mouth thoroughly after eating food.
• Many animals and plants protect themselves from enemies by injecting painful and irritating acids and bases into their skin.
  • When a honey bee stings a person, it injects an acidic liquid into the skin. Rubbing with a mild base like baking soda solution on the stung area of the skin gives relief.
  • When a wasp stings, it injects an alkaline liquid into the skin. Then rubbing with a mild acid like vinegar on the stung area of the skin gives relief.
• Soil pH and plant growth: Most of the plants grow best when the pH of the soil is close to 7. If the soil is too acidic or basic, the plants grow badly or do not grow at all. The soil pH is also affected by the use of chemical fertilisers in the field. Chemicals can be added to soil to adjust its pH and make it suitable for growing plants. If the soil is too acidic then it is treated with materials like quicklime or slaked lime. If the soil is too alkaline then alkalinity can be reduced by adding decaying organic matter.

Salts

When acid and base neutralise, salts are formed. Strong acid and strong base combine to form neutral salt.

NaOH + HCl → NaCl + H₂O

Eq.1. Formation of Neutral Salt

Strong acid and weak base combine to form acidic salt. For Example, Hydrochloric Acid and ammonium hydroxide combine to form ammonium chloride. Other examples, sodium hydrogen carbonate, sodium hydrogen sulphate etc.

HCl + NH₄OH → NH₄Cl + H₂O

Eq.2. Formation of Acidic Salt

Similarly, weak acid and strong base combine to form basic salt. For Example, Acetic Acid and sodium hydroxide combine to form sodium acetate. Other examples are calcium carbonate, potassium cyanide etc.

CH₃COOH + NaOH → CH₃COONa + H₂O

Eq.3. Formation of Basic Salt

The most common salt is table salt or sodium chloride (NaCl).

Indicators

They are the substances that indicate acidic or basic nature of the solution using colour change. For Example, litmus solution, methyl orange, phenolphthalein, methyl red etc. Acids convert blue litmus paper red in colour. Bases turn red litmus blue. Phenolphthalein remains colourless in presence of acids but turns pink in presence of bases.

Some Important Chemical Compounds and their uses

Salt	Preparation	Uses
Common Salt (Sodium Chloride) (NaCl)	1. NaOH + HCl → NaCl + H2O2. From sea water by evaporation3. From underground deposit{Large crystals of common salt found in underground deposits which are brown due to presence of impurities in it. It is mined from underground deposits like coal.}	1. Raw material for making large numbers of useful chemicals in industry. Eg: NaOH (caustic soda), Na2CO3 (washing soda), NaHCO3 (baking soda).2. Preservative in pickle and curing meat and fish.3. To melt ice and clear roads in winters in cold countries.4. Used in the manufacturing of soap.
Caustic Soda (NaOH)(Sodium Hydroxide)	Passing electricity through concentrated solution of NaCl (called ‘brine’)2NaCl (Brine) + 2H2O 2NaOH (Caustic Soda) + Cl2 + H2At anode (+ve electrode): Cl2 is producedAt cathode (-ve electrode): H2 is producedIt is called chloro-alkali process because products formed are chlorine (Chloro) and NaOH (alkali).	Uses of H21. Hydrogenation of oil to get vegetable ghee (margarine)2. To make ammonia for fertilisers3. In fuel for rockets.Uses of Cl21. In water treatment2. To clean water in swimming pools3. To make plastic, e.g. PVC4. To make CFCs, chloroform, dyes etc.Uses of NaOH1. Used in making soap and detergent.2. Used in manufacturing of paper3. De-greasing metals4. Refining oil5. Making dyes and bleachesUses of HCl1. Cleaning steel2. Preparation of chloride, e.g. NH4Cl3. In making medicines and cosmetics4. In making plastics, PVC etc.
Baking Soda (NaHCO3)(Sodium Hydrogen Carbonate)	NaCl + NH3 + H2O + CO2 → NaHCO3 + NH4ClPropertiesAction of Heat:	1. Used as antacid in medicine to remove acidity of the stomach2. Used in making baking powder (Basic soda + tartaric acid)NaHCO3 + H⊕ (from mild acid) → Na⊕ (sodium salt of acid) + CO­2 + H2OThe CO2 produced during the process gets trapped in wet dough and bubbles out slowly to make the cake ‘rise’ so that it becomes soft and spongy.Tartaric acid neutralises it, and so it has a pleasant taste.3. Used in soda-acid fire extinguisher
Washing Soda (Na2CO3.10H2O)(Sodium Carbonate)	Na2CO3 + 10 H2O → Na2CO3.10H2OPreparation of Na2CO3{NaCl + NH3 + H2O + CO2   NaHCO3 + NH4ClNaHCO3 → Na2CO3 + CO2 + H2O}	1. Used in glass, soap and paper industries2. Used in manufacturing of sodium compounds such as Borax3. Cleaning agent for domestic purpose4. Remove permanent hardness of water
Bleaching Powder (CaOCl2)Calcium Oxychloride	Ca(OH)2 + Cl2 → CaOCl2 + H2OSlaked Lime     Calcium OxychloridePropertiesCaOCl2 + H2SO4 → CaSO4 + Cl2 + H2OThe Cl2 produced by action of dilute acid acts as a bleaching agent.	1. For bleaching cotton and linen in textile industry, for bleaching wood pulp in paper factories, for bleaching washed clothes in laundry2. Oxidising agent in chemical industries3. Disinfecting drinking water
Plaster of Paris (P.O.P) (CaSO4.1/2 H2O)(Calcium Sulphate Hemihydrate)	CaSO4.H2O (Plaster of Paris) +3/2 H2O* Heating of gypsum should not be done above 100oC as above that temperature, water of crystallisation will be eliminated and anhydrous CaSO4 will be obtained. This anhydrous CaSO4 is known as Dead Burnt Plaster.* CaSO4.1/2 H2O means that two molecules of CaSO4 share one molecule of water.PropertiesHas the remarkable property of setting into a hard mass on wetting with water, as gypsum is formed.CaSO4.1/2 H2O (P.O.P) + 1/2 H2O → CaSO4.2H2O (Gypsum set as hard mass)Hence, P.O.P should be stored in moisture-proof containers as moisture can cause slow setting of P.O.P by hydrating it.	1. Used in hospital for setting fractured bones in the right position to ensure correct healing.2. Making toys, decorative materials, cheap ornaments, and casts of statues.3. Used as fire-proofing material4. Used in chemistry labs for setting air gaps in apparatus.5. Making smooth surfaces, such as For making ornamental designs on ceilings of houses and other buildings