• Acids is defined as the one which produces hydrogen ions in water. For Example, Sulphuric Acid, Hydrochloric Acid etc.
  • They give sour taste.
  • Acids turn blue litmus to red. This is used as confirmation test for the presence of acid.
  • When acids react with metals, gases are evolved.
  • The acids present in plants and animals are called organic acids. Eg: acetic acid, citric acid, lactic acid, tartaric acid, oxalic acid etc.
  • The acids prepared from the minerals of the earth are called mineral acids. Eg: hydrochloric acid, sulphuric acid and nitric acid.
  • 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.

  • Concentrated acids: A concentrated acid is one which contains the minimum possible amount of water in it.
  • Dilute acid: A dilute acid is one which contains much more water in it.
  • The dilution of concentrated acid should always be done by adding concentrated acid to water and not by adding water to concentrated acid. Because:
  • When conc. acid is added to water, then the heat is evolved gradually and easily absorbed by the large amount of water.
  • If, however, water is added to conc. acid, then large amount of heat is evolved at once. This heat changes some water to steam explosively which can splash the acid on our face or clothes and cause acid burns. Even the glass container may break due to excessive heating.
  • Common thing in all acid is that they produce hydrogen ions (H+ ions) when dissolved in water. For example: NaOH(s) →   H+(aq)   +    Cl
  • H2SO4(aq) →   H+(aq)      +    SO42-
  • HNO3(aq) →   H+(aq)    +    NO3

Reactions of Acids

  1. 1. Reaction of Acid with Metal

Acid + Metal → Salt + Hydrogen gas

Mg + HSO4 → H2 + Mg SO4

Metal in the above reactions displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt.

Take about 5 mL of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it. You will observe formation of gas bubbles on the surface of zinc granules.

Pass the gas being evolved through the soap solution. Take a burning candle near a gas filled bubble. The gas present in the soap burns with a pop sound. Only hydrogen gas burns with a pop sound.

  1. Reaction of Acid with Carbonates and Bicarbonates

Metal carbonate + Acid → salt + carbon dioxide + water

Na2 CO(s) + 2 HCl (aq) → 2NaCl (aq) + H2O(l) + CO2(g)

Metal bicarbonate + Acid → salt + carbon dioxide + water

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

On passing the carbon dioxide gas evolved through lime water,

Ca(OH)2 + CO2 → CaCO3 + H2O

(Lime water)            (White precipitate)

On passing excess carbon dioxide the following reaction takes place:

CaCO3 + CO2 + H2O → Ca(HCO3)2

                                 Calcium hydrogen carbonate

                                       (Soluble in water)

  • Take two test tubes, label them as A and B. Take about 0.5 g of sodium carbonate (Na2CO3) in test tube A and about 0.5 g of sodium hydrogen carbonate (NaHCO3) in test tube B.
  • Add about 2 mL of dilute HCl to both the test tubes. We will observe that brisk effervescence of a gas is produced.
  • Pass the gas produced in each case through lime water (calcium hydroxide solution). The lime water turns milky.
  • Keep on passing CO2 gas through the lime milky water for some more time. The lime water becomes clear again. This shows that the white ppt. of calcium carbonate dissolves on passing excess of CO2
  1. Reaction of Acid with base

The effect of a base is nullified by an acid and vice-versa. The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.

Acid + base → salt + water

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

  1. Reaction of Acid with metal oxide

The general reaction between a metal oxide and an acid can be written as –

Metal oxide + acid → salt + water

CuO + 2HCl → CuCl2 + H2O

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.


Similarity between Acids and Bases

  • Both acids and base react with water. They produce ions in water
  • Both acids and bases acts as electrolytes, so are good conductors of electricity.
  • Both of them changes the colour of the litmus paper.
  • Both of them are corrosive in nature.


Indicators for testing acids and bases

An indicator is a dye that changes colour when put into an acid or base.

Natural indicator – litmus, turmeric, red cabbage

  • Turmeric turns red in basic solution.
  • Red cabbage changes it colour from red to green in basic solution.
  • Acid turns blue litmus to red
  • Base turns red litmus to blue.

Synthetic indicator – methyl orange, phenolphthalein.

  • Methyl orange gives red colour in acid solution.
  • Methyl orange gives yellow colour in basic solution.
  • Phenolphthalein is colourless in acid solution.
  • Phenolphthalein is gives pink colour in basic solution.

Olfactory indicator- the term olfactory means relating to the smell. Those substance whose smell changes in acidic or basic solutions are called olfactory indicators. Eg: onion, vanilla extract.

  • Onion and vanilla smell vanish on reaction with acid but remains odour in presence of basic medium.

Universal indicator- universal indicator is a mixture of many different indicators which gives different colours at different pH values of the entire pH scale. Since pH depends on hydrogen ion conc., so we can say that universal indicator shows different colours at different conc. of hydrogen ion.

When acid or base solution is added to the indicator, the indicator produces a new colour. The colour produced is used to find the pH value of acid or base solution by matching the colour with the colours of pH colour chart.

pH colour pH colour pH colour
0 Dark red 5 Orange yellow 10 Navy blue
1 Red 6 Greenish yellow 11 Purple
2 Red 7 Green 12 Dark purple
3 Orange red 8 Greenish blue 13 Violet
4 Orange 9 Blue 14 Violet


  • Bases are the one which produces hydroxide ions in aqueous solutions. Bases which are water soluble they are known as Alkalis.
  • They turn red litmus to blue.
  • They have a bitter taste.
  • Base is a chemical substance which neutralise an acid.
  • They also produced carbon-dioxide when reacted with carbonates.
  • They also evolved hydrogen gas when bases react with metals., but not all metals.
  • Common thing in all Base is that it dissolves in water to produce hydroxide (OH) ions in solution. For example: NaOH(s) →   Na+(aq)   +    OH
  • KOH(s) →   K+(aq)   +    OH
  • Mg(OH)2(s) →   Mg+(aq)   +    OH


Reactions of Bases

  1. Reaction with Metals

Base reacts with metals and produce hydrogen gas.

2NaOH + Zn → Na2ZnO2 + H2

  1. Reaction with Acids

Base reacts with acids to form salts. For Example,

KOH + HCl → KCl + H2O

  1. Reaction with Non-metallic Oxides

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

2NaOH + CO2 → CO → 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 ExampleNaOH.

Weak base is the one which does not dissociate completely into its ions in aqueous solutions. For ExampleAmmonium HydroxideNH4OH

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: pH scale

in pure water concentration of hydrogen ion is equal to concentration of hydroxide ion. Due to this water is neither acidic or basic.

Acidic solution has excess of hydrogen ion.

Basic solution has excess of hydroxide ion.

The pH of a solution is inversely proportional to the concentration of hydrogen ion present in them.

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. If pH is equal to 7, means the solution is neutral.

  • If pH is greater than 7 means alkaline solution.
  • If pH is less than 7 means the solution is acidic.

pH scale

Importance of pH

  • Human body works at a pH of about 7.4.
  • Stomach has a pH of about 2, due to 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 honey bee stings a person, it injects an acidic liquid into the skin. Rubbing with 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.


A salt is a compound formed from an acid by the replacement of the hydrogen in an acid by a metal.

Salts are ionic compounds and like acid and base conduct electricity.

Formation of Neutral Salt

When acid and base neutralize, salts are formed. Strong acid and strong base combines to form neutral salt. (pH = 7)

NaOH + HCl → NaCl + H2O

Formation of Acidic Salt

Strong acid and weak base combine to form acidic salt (pH less than 7). 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

Formation of Basic Salt

Similarly, weak acid and strong base combine to form basic salt (pH more than 7). For Example, Acetic Acid and sodium hydroxide combine to form sodium acetate. Other examples are calcium carbonate, potassium cyanide etc.


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

Some Important Chemical Compounds and their uses


  Preparation Uses
Common Salt (NaCl)

(Sodium Chloride)


1. NaOH + HCl → NaCl + H2O

2. From sea water by evaporation

3. From underground deposit

{Large crystals of common salt found in underground deposit which is brown due to presence of impurities in it. It is mined from underground deposit like coal.}

1. Raw material for making large number 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 manufacturing of soap.

Caustic Soda (NaOH)

(Sodium Hydroxide)

Passing electricity through concentrated solution of NaCl (called ‘brine‘)


2NaOH (Caustic Soda) + Cl2 + H2

At anode (+ve electrode): Cl2 is produced

At cathode (-ve electrode): H2 is produced

It is called chloro-alkali process because products formed are chlorine (Chloro) and NaOH (alkali).

Uses of H2

1. Hydrogenation of oil to get vegetable ghee (margarine)

2. To make methanol, ammonia for fertilizers

3. In fuel for rockets.

4. In the production hydrochloric acid.

Uses of Cl2

1. In water treatment

2. To clean water in swimming pools

3. To make plastic, e.g. PVC

4. To make CFCs, chloroform, dyes etc.

5. in the production of bleaching powder and hydrochloric acid.

Uses of NaOH

1. Used in making soap and detergent.

2. Used in manufacturing of paper

3. De-greasing metals

4. Refining oil

5. Making dyes and bleaches

Uses of HCl

1. Cleaning steel

2. Preparation of chloride, e.g. NH4Cl

3. In making medicines and cosmetics

4. In making plastics, PVC etc.

5. used in textile, dyeing, and tanning industries.

Baking Soda (NaHCO3)

(Sodium Hydrogencarbonate)

Preparation: NaCl + NH3 + H2O + CO2 → NaHCO3 + NH4Cl


·         Sparingly soluble in water

·         Mildly alkaline

·         Action of Heat:

1. Used as antacid in medicine to remove acidity of the stomach

2. Used in making baking powder (Basic soda + tartaric acid)

NaHCO3 + H (from mild acid) → Na (sodium salt of acid) + CO­2 + H2O

The CO2 produced during the process gets trapped in wet dough and bubbles out slowly to make cake ‘rise’ so that it becomes soft and spongy.

Tartaric acid neutralizes it, and so it has pleasant taste.

3. Used in soda-acid fire extinguisher

Washing Soda (Na2CO3.10H2O)

(Sodium Carbonate)

Preparation: Na2CO3 + 10 H2O → Na2CO3.10H2O

Preparation of Na2CO3

{NaCl + NH3 + H2O + CO2 → NaHCO3 + NH4Cl

NaHCO3 → Na2CO3 + CO2 + H2O}

The anhydrous sodium carbonate obtained is called soda ash. recrystallisation of sodium carbonate gives washing soda. It is also a basic salt.


·         Transparent crystalline solid

·         Soluble in water

·         Turns red litmus blue

·         Detergent properties

1. Used in glass, soap and paper industries

2. Used in manufacturing of sodium compounds such as Borax

3. Cleaning agent for domestic purpose

4. Remove permanent hardness of water

Bleaching Powder (CaOCl2)

Calcium Oxychloride

Preparation: Ca(OH)2 + Cl2 → CaOCl2 + H2O

                         Slaked Lime     Calcium Oxychloride


·         Gives strong smell of chlorine

·         Soluble in cold water

·         The Cl2 produced by action of dilute acid acts as bleaching agent.

CaOCl2 + H2SO4 → CaSO4 + Cl2 + H2O

1. For bleaching cotton and linen in textile industry, for bleaching wood pulp in paper factories, for bleaching washed clothes in laundry

2. Oxidizing agent in chemical industries

3. Disinfecting drinking water

Plaster of Paris (P.O.P) (CaSO4.1/2 H2O)

(Calcium Sulphate Hemihydrate)


CaSO4.1/2H2O (Plaster of Paris) +3/2 H2O

* Heating of gypsum should not be done above 100oC as above that temperature, water of crystallization will eliminate 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.


Has 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 container 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 material

4. 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

Water of crystallisation

The water molecules which forms the part of the structure of a crystal(of a salt) are called water of crystallisation.

Copper sulphate crystals (CuSO4.5H2O) – contains 5 molecules of water of crystallisation.

Sodium carbonate crystals (Na2CO3.10H2O) – contains 10 molecules of water of crystallisation.

Calcium sulphate crystals (CaSO4.2H2O) – contains 2 molecules of water of crystallisation.

Iron sulphate crystals (FeSO4.7H2O) – contains 7 molecules of water of crystallisation.

The water of crystallisation is a part of crystal structure of a salt. Since, water of crystallisation is not free of free of water it does not wet the salt.

The water of crystallisation gives the salts their shape and, in some cases, imparts them colour.

When the hydrated salts are heated strongly, they lose their water of crystallisation. The which have lost their water of crystallisation is called anhydrous salts.


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