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CHEMICAL REACTIONS

ACIDS, BASES AND SALTS

On the basis of their chemical properties, compounds can be classified as acids, bases, and salts.

Indicator

  • An indicator is a dye which gives different colours in acid and base.
  • Three common indicators used to test acids and bases are Litmus, Methyl Orange, and Phenolphthalein.
  • Litmus can be used in the form of litmus solution or in the form of litmus paper. It is of two types – blue litmus and red litmus. An acid turns blue litmus red and a base turns red litmus blue. (A water soluble base is called an alkali).
  • Litmus is a natural indicator. The neutral colour of litmus is purple.
  • Methyl orange and phenolphthalein are synthetic indicators. The neutral colour of methyl orange is orange, whereas that of phenolphthalein is colourless.
  • Methyl orange gives red colour in an acid solution and yellow colour in a basic solution.
  • Phenolphthalein remains colourless in an acid solution and gives pink colour in a basic solution. CHEMICAL REACTIONS
  • Turmeric is also a natural indicator, and contains a yellow dye. It turns red in presence of a base.
  • That is why yellow stains of turmeric on a cloth turn reddish-brown in contact with soap, which is basic in nature.
  • Extract of red cabbage is a natural indicator, and is red in colour. It remains red in acidic solutions but turns green in the presence of a base.
  • Substances which change their smell (odour) in acidic and basic medium are called olfactory indicators.
  • Examples: Onion and vanilla extracts. The smell of onion cannot be detected when a base is added to onion extract.
  • However, acids do not destroy the smell of onions. Similarly, the pleasant smell of vanilla is destroyed in presence of a base but not in presence of an acid. CHEMICAL REACTIONS

ACIDS | CHEMICAL REACTIONS

  • Acids are substances which turn blue litmus red. They have a sour taste.
  • The sour taste of lemon, orange, tamarind, raw mango and raw grapes is due to the presence of acids in them.
  • Citrus fruits like lemons and oranges contain citric acid, vinegar contains acetic acid, sour milk and curd contain lactic acid, tamarind and raw grapes contain tartaric acid, and tomatoes contain oxalic acid.
  • Formic acid is present in ant sting and nettle leaf sting. CHEMICAL REACTIONS
  • All these are organic acids and are weak acids. Hydrochloric acid, sulphuric acid, and nitric acid are called mineral acids because they are prepared from minerals of the earth.
  • These three acids are strong acids. Carbonic acid is a weak mineral acid.
  • Acetic acid, in the form of vinegar, is used in making pickles and tomato ketchup, tartaric acid is used in baking powder, and carbonic acid is used in soda water and fizzy soft drinks.
  • A contains much more water. The process of diluting a concentrated acid with water is highly exothermic (heat producing).
  • Therefore, dilution should be carried out by slowly adding concentrated acid to water and not by adding water to acid. CHEMICAL REACTIONS
  • If water is added to the acid, the large amount of heat produced converts water to steam which can splash the acid on the body or clothes and cause acid bums.
  • Acid solutions conduct electricity. Acids react with metals to form hydrogen gas, which burns making a ‘pop’ sound.
  • Sour foodstuffs, such as curd, lemon juice, etc., should not be kept in metal vessels because the acids present in these foodstuffs can react with the metal to form poisonous compounds which can cause food poisoning.
  • Acids react with bases (alkalis) to form salt and water. This is known as neutralisation reaction.
  • They also react with metal oxides-to form salt and water. Mineral acids are corrosive in nature.
  • They cause severe burns on the skin, make holes in clothes, burn wood, and corrode metal structures and stonework. That is why acids are stored in containers made of glass or ceramic and not in metal containers.

Strong and Weak Acids

  • An acid which is completely ionized in water to produce a large amount of H+ ions is called a strong acid.
  • Examples: HCI, H2SO4, HNO3. These acids have high reactivity and high electrical conductivity. Thus, they are strong electrolytes.
  • An acid which is partially ionized in water to produce a small amount of H+ ions is called a weak acid. Examples: CH3COOH, and H2CO3.
  • They have low reactivity and low electrical conductivity. Thus, they are weak electrolytes.

Some Common Uses of Acids

  • Sulphuric acid is used in the manufacture of, fertilizers, paints, dyes, plastics, synthetic fibres, detergents, car batteries, etc.
  • Nitric acid is used for making fertilizers, dyes, plastics, and explosives like Trinitro Toluene (TNT).
  • Hydrochloric acid is used in dyes, textiles, and leather industry. It is used for removing deposits from inside boilers.. It is used for making plastics like Polyvinyl Chloride (PVC). It is used in medicines and cosmetics.

Antacids and Acid Inhibitors | CHEMICAL REACTIONS

  • Gastric juice is an acidic digestive fluid secreted by glands in the mucous membrane that lines the stomach.
  • It contains hydrochloric acid (HCI). Overeating and emotional factors can cause the stomach to produce too much HCI.
  • This leads to hyperacidity, commonly known as ‘acid indigestion’ or ‘heartburn’.

Two approaches are used to combat the problem of excess stomach acid:

  • Removal of excess acid through neutralisation, which involves the use of antacids like Digene and Gelusil [Mg(OH)2. Al(OH)3], Milk of Magnesia [Mg(OH)2], and Turns [CaCO3]. These are basic substances capable of neutralising the HCI present in gastric juice. Neutralisation involving sodium bicarbonate and calcium carbonate produces carbon dioxide, which causes a person to belch often. CHEMICAL REACTIONS
  • Decrease in the production of stomach acid, which involves the use of acid inhibitors like Pepcid, Tagamet, and Zantac. These substances inhibit the production of gastric acid by blocking the action of histamine, a gastric acid secretion regulator.

BASES | CHEMICAL REACTIONS

  • Bases are substances which turn red litmus blue.
  • They have a bitter taste and are soapy to touch. CHEMICAL REACTIONS
  • They neutralise acids. All metal oxides and metal hydroxides are bases.
  • For example, Na2O, CaO, NaOH, KOH, Ca(OH)2, Mg(OH)2, Ba(OH)2 are bases. NH4OH, Na2CO3, CaCO3 and NaHCO3 are also considered as bases because they neutralise acids.
  • Water soluble bases are called alkalis. For example, NaOH, KOH, NH4OH, Mg(OH)2, and Ca(OH)2.

All bases produce hydroxide ions (OH ions) when dissolved in water. For example,

              KOH (aq) –> K+ (aq) + OH (aq)

M g (OH)2(aq) –> Mg2+ (aq) + 2OH (aq)

Strong and Weak Base

  • A base which is completely ionized in water to produce a large amount of OH ions is called a strong base. Examples: NaOH and KOH.
  • A base which is partially ionized in water to produce a small amount of OH” ions is called a weak base. Examples: NH4OH, Mg(OH)2, and Ca(OH)2.

Some Common Uses of Bases

  • Sodium hydroxide is used in the manufacture of soap, paper, and rayon (synthetic fibre). It is also used in oil refining and making dyes and bleaches.
  • Potassium hydroxide is used in the manufacture of shampoos and shaving creams.
  • Magnesium hydroxide is used as an ‘antacid’.
  • Calcium hydroxide is used in the manufacture of bleaching powder. CHEMICAL REACTIONS

The pH Scale | CHEMICAL REACTIONS

  • The concentrations of W and OH ions are equal in pure water. Acidic solutions have excess of W ions, whereas basic solutions have excess of OHions.
  • The strengths of acid solutions and basic solutions can be represented by means of a scale, known as the pH scale, which was devised by Sorenson.
  • This is done by making use of H+ ion concentrations in these solutions. The pH of a solution is inversely proportional to the concentration of H+ ions in it. A solution with high concentration of H+ ions has a low pH value, and vice versa.
  • The letter ‘p’ in the term ‘pH’ stands for the German word ‘potenz’ (which means power) and ‘H’ stands for H+ ion concentration. The pH scale has values from 0 to 14. pH value is a number and has no units.
  • Neutral substances have a pH of exactly 7. For example, pure water, salt solution and sugar solution have a pH value of 7, i.e., they are neutral. Acids have a pH value less than 7.
  • The lower the pH value, the stronger is the acid. Bases have a pH of more than 7. The higher the pH value, the stronger is the base. pH values of some common substances is shown in the table below.

universal indicator:  | CHEMICAL REACTIONS

  • A universal indicator is used to obtain an idea about how acidic or basic a substance is.
  • It is a mixture of different indicators which give different colours at different pH values  of the entire range on the pH scale.
  • The universal indicator gives green colour with a neutral solution, yellow, orange or red with an acidic solution, and blue, purple or violet with a basic solution, the exact colour depending on the pH of the solution. CHEMICAL REACTIONS

Importance of pH Changes

Plant growth is best when the soil has a pH value close to 7.  If the soil is too acidic then materials like quicklime, slaked lime or chalk can be added to reduce its acidity.

pH Values of Some Common Substances

pH Value Example pH Value Example
0 battery acid 8 sea water, eggs
  sulfuric acid 9 baking soda
2 lemon juice, vinegar 10 Great Salt Lake, milk of magnesia
3 orange juice, soda 11 ammonia solution
4 tomato juice, acid rain 12 soapy water
5 black coffee, bananas 13 bleach, oven cleaner
6 urine, milk 14 liquid drain cleaner
7 pure water    

 

Basicity of the soil can be reduced by adding decaying organic matter (manure or com­post) which contains acidic materials.

  • When the pH of rain water is about 5.6, it is called acid rain. Too much acid rain can lower the pH of water in lakes and rivers. This can make the survival of aquatic animals difficult. Calcium carbonate can be added to reduce the acidity of water in lakes and rivers. CHEMICAL REACTIONS
  • Life does not exist on planet Venus because it is covered with thick white and yellowish clouds of sulphuric acid.
  • When a honey-bee stings a person, it injects an acidic substance into the skin which causes immense pain and irritation. Rubbing a mild base, like baking soda solution, on the affected area gives relief. When a wasp stings, it injects an alkaline liquid into the skin. Therefore, rubbing a mild acid, like vinegar, on that area gives relief. An ant’s sting injects methanoic acid into the skin, which can be neutralised by rubbing baking soda solution. Some plants also give painful stings. The stinging hair of nettle plant leaves inject methanoic acid into the skin which causes burning pain. It can be relieved with the help of baking soda, or by rubbing the leaf of a ‘dock’ plant (which contains some basic chemical).
  • The bacteria present in our mouth break down the sugar to form acids. Tooth decay starts when the pH of acid formed in the mouth fall’s below 5.5. Using the toothpastes, which are basic, for cleaning the teeth can neutralise the excess acid in the mouth and prevent tooth decay.

SALTS | CHEMICAL REACTIONS

Salts are formed when acids react with bases. Salts are ionic compounds and solutions of salts conduct electricity.

  • The aqueous solutions of most salts are neutral (pH = 7), but some salts produce acidic or basic solutions when dissolved in water due to their hydrolysis.
  • Hydrolysis is degradation of a compound by the action of water.
  • The salts of strong acids and strong bases give neutral solutions (pH = 7). Examples: Sodium Chloride and Potassium Sulphate. CHEMICAL REACTIONS
  • The salts of strong acids and weak bases give acidic solutions (pH < 7). Example: Ammonium Chloride.
  • The salts of weak acids and strong bases give basic solutions (pH > 7). Example: Sodium carbonate.

Some Salts Used in Everyday Life | CHEMICAL REACTIONS

Common Salt

Common salt (sodium chloride or NaCI) is obtained from sea water by the process of evaporation. Rock salt (large crystals of common salt) is mined from underground deposits just like coal.]

Rock salt was formed when the seas dried up due to evaporation thousands of years ago.

Common salt is used:

  • For making useful chemicals like sodium hydroxide, sodium carbonate, sodium bicarbonate, hydrochloric acid, hydrogen, chlorine, and sodium metal.
  • To improve the flavour of food. It is required for the proper functioning of the nervous system, movement of muscles, and generation of hydrochloric acid in the stomach for digestion of food.
  • As a preservative in pickles, and for preserving meat and fish.
  • In the manufacture of soap.
  • In cold countries to melt ice which gets deposited on the roads. CHEMICAL REACTIONS

Three useful products are obtained by the electrolysis of sodium chloride solution (also called brine): Sodium hydroxide, chlorine, and hydrogen. The process of electrolysis of sodium chloride solution is called chlor-alkali process.

  • Chlorine is a disinfectant and is used to sterilize drinking water supply and the water in swimming pools. It is used in the manufacture of bleaching powder and hydrochloric acid. It is used to make plastics, such as polyvinyl chloride (PVC), pesticides, chlorofluorocarbons (CFCs), chloroform, carbon tetrachloride, paints, dyes, and solvents for dry cleaning (like trichloroethane).
  • Hydrogen is used in the manufacture of ghee or margarine, hydrochloric acid, ammonia, and methanol. Liquid hydrogen is used as a fuel for rockets.
  • Sodium hydroxide and chlorine combine to form sodium hypochlorite (NaCIO) which is a bleaching agent.

Washing Soda | CHEMICAL REACTIONS

  • Washing soda is chernivally sodium carbonate containing 10 molecules of water of crystallisation (Na2CO3.10H2O). CHEMICAL REACTIONS
  • Anhydrous sodium carbonate (Na2CO3) is known as ‘soda ash’.
  • Washing soda has detergent (or cleansing) properties. It attacks dirt and grease to form water soluble products, which are washed away with water.
  • It is also used for removing permanent hardness of water. It is used in the manufacture of glass, soap, paper, and compounds such as borax. CHEMICAL REACTIONS

Baking Soda | CHEMICAL REACTIONS

  • Baking soda is chemically sodium hydrogen carbonate or sodium bicarbonate.
  • It is used as an antacid. It is sometimes added for faster cooking of food. Baking powder, used for making cakes, bread, etc., is a mixture of baking soda and a mild acid, like tar­taric acid or citric acid.
  • As long as baking powder is dry, baking soda, NaHCO3, and tartaric acid do not react.
  • When it mixes with water present in the dough (for bread or cake), reaction occurs between NaHCO3 and the acid to produce carbon dioxide gas, which makes the cake or bread soft and spongy.
  • Without the baking powder the cake obtained is hard and small in size.
  • If baking soda is used instead of baking powder, then Na2CO3 formed during baking will give a bitter taste to the cake.
  • Tartaric acid present in baking powder neutralises NazCO3 to give sodium tartarate which has a pleasant taste.
  • Baking soda is used in fire extinguishers. When the knob of the fire extinguisher is pressed, sulphuric acid gets mixed with NaHCO3 to produce carbon dioxide gas, which forces a stream of liquid to fall on the burning substance. CHEMICAL REACTIONS
  • Carbon dioxide itself forms a blanket around the burning substance and cuts off its supply of air, thus extinguishing the fire.

Bleaching Powder | CHEMICAL REACTIONS

  • Bleaching powder is chemically calcium oxychloride, Ca0C12. It is also called chloride of lime.
  • The real bleaching agent present in bleaching powder is chlorine.  CHEMICAL REACTIONS
  • Bleaching agent is a substance which removes colour from coloured substances and makes them colourless.
  • Bleaching powder is used for bleaching cotton and linen (textile industry), and wood pulp (paper industry).
  • It is used for disinfecting drinking water supply, and for making wool unshrinkable. It is also used as an oxidising agent. CHEMICAL REACTIONS

Plaster of Paris | CHEMICAL REACTIONS

  • Plaster of Paris (P.O.P.) is calcium sulphate hemihydrate (half hydrate), CaSO4.1/2HZO.
  • It was initially made by heating gypsum (CaSO4.2H2O), which was mainly found in Paris.
  • It is a white powder, which sets into a hard mass on wetting with water due to its conversion to gypsum.
  • Plaster of Paris is used for setting fractured bones in the right position.
  • It is used for making casts in dentistry. It is also used as a fire-proofing material.
  • It is also used for making walls and ceilings smooth before painting, and for making toys, decorative materials, chalks, and casts tor statues. CHEMICAL REACTIONS

Water of Crystallisation | CHEMICAL REACTIONS

  • Some salts contain a few water molecules as a part of their crystal structure. This water is known as water of crystallisation.
  • Salts which contain water of crystallisation are called hydrated salts. For example, CuSO4.5H2O, FeSO4.7H2O, CaSO4.2H2O, and Na2CO3.10HzO.
  • The shape of the crystals, and, in some cases, their colour, is due to the presence of water of crystallisation.
  • Hydrated salts lose their water of crystallisation on strong heating to give anhydrous salts.
  • On strong heating, blue copper sulphate crystals (CuSO4.5H2O) lose their water of crystallisation to give anhydrous copper sulphate (CuSO4). When water is added to anhydrous copper sulphate, it gets hydrated and turns blue. CHEMICAL REACTIONS

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