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Revision Notes on Acids, Bases and Salts

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

Acids

  • 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.

Reactions with Acids

1. Reaction of Acid with Metal

Acid + Metal → Salt + Hydrogen gas

Mg + H2 SO4 → H2 + Mg SO4

2. Reaction of Acid with Carbonates

Na2 CO3 (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 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.

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. For 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.

Sulphuric Acid

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

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 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 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 mole 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 they 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 produce 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 → 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 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-

 weak base

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

HA is weak acid

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.

pH scale

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 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.

Salts

When acid and base neutralize, salts are formed. Strong acid and strong base combines 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 which 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 turn pink in presence of bases.

Some Important Chemical Compounds and their uses

Common salt

  • Known as Sodium Chloride of NaCl

  • It is obtained from sea water

  • It is also found rocks which is known as Rock Salt

  • Baking soda, washing soda etc are prepared using NaCl

  • Common salt is added to prickles to prevent bacteria formation

Sodium hydroxide

  • Represents as NaOH.

  • When electricity is passed through aqueous solution of sodium chloride, sodium hydroxide is produced. 

  • It is used in water treatment and metal processing.

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