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Acids, Bases, and Salts

Properties of acids

  • Acids have a sour taste
  • Acids change the colour of blue litmus to red

Properties of bases

  • Bases have a bitter taste
  • Bases change the colour of red litmus to blue

Indicators

It helps us to identify whether a substance is acidic or basic with the help of colour change. Different type of indicators are: 

    • Synthetic indicators: Commonly used indicators in laboratories. Examples are phenolphthalein, and methyl orange.
    • Olfactory Indicators: Substances which change their smell when mixed with acid or base are known as olfactory indicators. Examples are onion, vanilla, clove
    • Natural Indictors: Indicators obtained from natural sources. Examples are Litmus, Turmeric etc.
      • Litmus: Obtained from lichens. Litmus indicator solution turns red in acidic solutions, blue in alkaline solutions, and purple in neutral solutions.
      • Litmus paper comes in 2 colours:  red litmus paper and blue litmus paper. An acid changes blue litmus paper into red and a base changes red litmus paper into blue.

Indicators

Reaction of acid with metals

  • When an acid reacts with a metal, this results in the formation of a salt with the evolution of hydrogen gas.

Acid + Metal → Salt + Hydrogen gas

  • HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g): Here hydrochloric acid reacts with metal Zinc (Zn) to give ZnCl2 as salt and hydrogen gas. The Zn metal displaces the hydrogen in this reaction.
  • Zn + H2SO4→ ZnSO4 + H2: When dilute sulphuric acid reacts with zinc granules, then zinc sulphate (a salt) and hydrogen gas are formed. Zinc being more reactive than hydrogen, displaces it from sulphuric acid and evolves as hydrogen gas.

Reaction of base with metals

  • When a base reacts with a metal, this results in the formation of a salt with the evolution of hydrogen gas.

Base + Metal   → Salt + Hydrogen gas

  • 2NaOH(aq)+ Zn(s) → Na2ZnO2(s) + H2(g): In the above reaction Sodium hydroxide (NaOH), a base reacts with a metal Zinc (Zn) to give a salt, sodium zincate (Na2ZnO2), and hydrogen gas as the product.

Reaction of Metal Carbonates and Bicarbonates with Acids

  • Acids react with metal carbonates or metal bicarbonates to give carbon dioxide, metal salts and water.

Acid + Metal Carbonate/ Hydrogen Carbonate → Salt + Water + Carbon Dioxide

  • This carbon dioxide gas is allowed to pass through lime water, turning it milky.
  • Acid and Metal Carbonate reactions are exothermic.

Acid Base Reactions

  • When an acid and base react together, they react to neutralize the acid and base properties. When acid and base react together it will produce salt and water
  • Base + Acid → Salt + Water
  • Ex: NaOH + HCl → NaCl +H2O
  • The reaction between an acid and a base to give a salt and water is called as neutralisation reaction.

Reaction of metallic oxides with acids

  • Metallic oxides react with acids to form salt and water.
  • Metal Oxides + Acid → Salt + Water
  • Ex: CuO + 2HCl → CuCl2+ H2O: When copper oxide and dilute hydrochloric acid are mixed the blue- green solution is formed. Formation of blue- green solution is due to the formation of copper (II) chloride.

Reaction of non- metallic oxides with base

  • Bases react with non – metallic oxides to form a salt and water.
  • Non-metallic oxides + Bases → Salt + Water
  • Ex: Ca (OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
  • Non-metal oxides are acidic in nature

Acid in a water solution

  • Acid in presence of water produces hydrogen ions (H+).
  • Ex: : : HCl in presence of water produces H+ ion. This ion cannot exist alone and hence combines with water molecules and forms H3O+.

Base in water solution

  • Base when dissolved in water produces OH- ion.
  • All bases do not dissolve in water. Bases soluble in water are called alkalis.
  • Peculiarities of alkalis:  soapy in nature, bitter, corrosive.

Neutralisation reaction

  • A neutralization reaction is when an acid and a base react to form water and a salt.
  • In another word we can say H+ions and OH– ions are combined to form water.
  • Acid + Base → Salt + Water.

Note

  • Acid/ Base + water reaction is exothermic reaction.
  • Mixing of acid/ base with water will decrease the concentration of per unit volume. This process is known as dilution.
  • We call the acid or base after dilution as diluted acid or base.

pH

  • When we move right of the pH scale the concentration of H+ ions decrease and OH– ions increase.
  • When we move left of the pH scale the concentration of H+ ions increase and OH– ions decrease.
  • The strength of acid depends on More the ions, then acids are said to be strong acids and vice versa.
  • The strength of base depends on concentration of More the concentration of ions then base is said to be strong base.

Importance of pH in everyday life

The_pH_scale

    • pH range in which our body works: 7.0 to 7.8.
    • If the pH of rain is less than 5.6, it is known as acid rain.
    • A pH range of 6 to 7 is generally most favourable for plant growth.
    • pH in our digestive system
      • Our stomach produces hydrochloric acid which helps in the digestion of food.
      • During indigestion, acidic level inside our stomach increases which cause irritation and pain.
      • To avoid irritation we use antacid, which is a base, and it neutralises excess acid.
    • Enamel of tooth decay if the pH of mouth is less than 5.5. This happens when bacteria inside the mouth produce acids. To avoid situation, clean the mouth using toothpastes, which is basic in nature.

Some Naturally occurring acids

Some naturally occurring acids

pH of salts

pH of salts

Sodium Hydroxide

  • Sodium hydroxide is made through chlor-alkali process.
  • In this process, electricity is passed through aqueous solution of sodium chloride (called brine) and it decomposes to produce sodium hydroxide, chlorine and hydrogen.
  • Chlorine gas is given off at the anode and hydrogen gas at the cathode.

  • Chemical equation of the process:

Bleaching powder

  • Chemical equation of the process:

  • Uses of bleaching powder
    • It is used for bleaching clothes
    • it is used as disinfectant and germicide
    • It is used for manufacturing of chloroform
    • It is used as an oxidising agent in many chemical industries to obtain various chemical products

Baking Soda

Baking soda used mainly in kitchen.

  • Chemical name: Sodium hydrogen carbonate (NaHCO3).

  • Chemical equation of the process:

  • Sodium hydrogen carbonate is a mild, non – corrosive basic salt.
  • When Sodium hydrogen carbonate is heated, it decomposes to give Sodium carbonate. Chemical equation of the process:

Uses of baking soda

  • It is used for making baking powder. It is a mixture of baking soda and mild edible acid like tartaric acid. The chemical equation when baking powder is heated or mixed in water:

  • CO2  produced during this reaction make the food product soft and spongy.
  • Sodium hydrogen carbonate is used as antacids.
  • It is used in soda acid fire extinguisher.

Washing Soda

  • The chemical name of washing soda is sodium carbonate Na2CO3. 10 H2
  • Sodium carbonate can be obtained by heating baking soda. Anhydrous sodium carbonate is mixed with water to form washing soda.
  • Na2CO3+   10H2O → NaCO3.10H2O

Uses of washing soda:

    • Washing soda is used for removing the permanent hardness of the water.
    • It is used in the manufacture of soap, glass, paper.
    • It is used in the manufacturing of sodium compounds, such as borax.

Water of crystallisation

  • The fixed number of water molecules present in one formula unit of a salt is called water of crystallization.
  • The chemical formula of hydrated copper sulphate is CuSO4. 5H2 The copper sulphate crystals contain five molecules of water. When we heat it, this water gets removed and salt becomes white. If we moisten the crystal with water, the blue colour of the crystal reappears.
  • Another salt which contains water of crystallization is gypsum. The chemical formula of gypsum is CaSO4. 2H2

Plaster of Paris

  • You must have seen doctors using Plaster of Paris for putting plaster for fractured bones. But do you know how it is obtained?
  • Heating gypsum at 373 K, it loses water molecules and become calcium sulphate hemihydrate (CaSO4. H2O). This is known as Plater of Paris.
  • It is a white powder.
  • If we mix Plaster of Paris with water, it changes again to gypsum.
    • CaSO4. H2O + 1  H2O      →     CaSO4 .2H2O

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