Oracle IAS, the best coaching institute for UPSC/IAS/PCS preparation in Dehradun brings to you UKPCS Science Chemistry (paper 6).
Acid, Bases, Salts and buffer
- Acids have a pH of less than 7. Acid – substances that ionize (release charged particles when dissolved) in water solution to produce hydrogen ions, H+ , or free protons ie Donates protons.
- The stronger the acid, the lower the pH number. Acids turn blue litmus paper red. They turn universal indicator red if they are strong, and orange or yellow if they are weak.
- Bases have a pH of more than 7. Base – substances that ionize in water solution to produce hydroxide ions, OH- ie accepts protons. When bases are dissolved in water, they are known as alkalis.
- Substances that can react with acids and neutralise them to make a salt and water are called bases. They are usually metal oxides or metal hydroxides. For example, copper oxide and sodium hydroxide are bases.
- Bases that dissolve in water are called alkalis. Copper oxide is not an alkali because it does not dissolve in water. Sodium hydroxide is an alkali because it does dissolve in water.
- Alkaline solutions have a pH of more than 7. The stronger the alkali, the higher the pH number. Alkalis turn red litmus paper blue. They turn universal indicator dark blue or purple if they are strong, and blue-green if they are weak.
- Salts are made when an acid reacts with a base, carbonate or metal. Salt – a crystalline compound composed of the negative ion of an acid and the positive ion of a base. The name of the salt formed depends on the metal in the base and the acid used. For example, salts made using hydrochloric acid are called chlorides.
- When acids react with bases, a saltand water are made. This reaction is called neutralisation. In general:
acid + metal oxide → salt + water
acid + metal hydroxide → salt + water
- Buffer – a solution that can receive moderate amounts of either acid or base without the significant change in its pH.
A buffer solution is an aqueous solution made up of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. The solution resists changes in pH when a small amount of an acid or alkali is added to it. Buffer solutions are critical in keeping the correct pH for enzymes.
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