ACIDS, BASES/ALKALIS AND SALTS: You have already read that a large number of compounds can be made by the combination of various elements. More than three million compounds are known to the scientists. It is practically impossible for anyone to learn about each of these compounds. Therefore, all these compounds are divided into different groups to make their studies easier.
In this chapter, you will learn about acids, bases and salts, their properties and uses. pH, its range in aqueous medium and indicators would also be discussed. 6.1 Acids The word acid is derived from Latin word ‘acidus’ means sour. In chemistry, the term acid has been used to name a group of compounds that have sour taste. Acids can be deﬁned as the compounds which produce +hydrogen ions (H ) in their aqueous solutions. sour taste due to citric acid. Hydrochloric acid is an important mineral acid. It is also found in gastric juice of the stomach. It acts as an antiseptic and is helpful in digestion of proteins. Sources of Common Acids Generally, acids are obtained from two different sources. Some acids occur in plants and animals and are known as organic acids while others are obtained from minerals and are called mineral acids. Some common organic acids and their sources are given in table 6.1.
Some examples of the acids which are prepared from mineral elements are given in Table 6.2.
Name source Name Source
Formic acid Ant’s sting Tartaric acid Tamarind, grapes
Acetic acid Vinegar Lactic acid Yoghurt
Oxalic acid Tomatoes Malic acid Apples
Citric acid Citrus fruit Stearic acid Fats
Table 6.1: Some important acids obtained from animals or plants
Mineral acid Formula
Hydrochloric acid HCl
Nitric acid HNO3
Sulphuric acid HSO 2 4
Phosphoric acid HPO 3 4
Properties of Acids Let us now study the properties, which are common to all acids. (i) All acids have a sour taste.
Strong acids are corrosive liquids. They burn skin and destroy fabrics and animal tissues. (iv) Aqueous solutions of acids are good conductors of electricity. (v) Acids react with reactive metals (Mg, Zn) to form salt and evolve hydrogen gas.
Hydrogen gas produced in the reaction burns with pop sound (Figure 6.2). This is a test for identiﬁcation of hydrogen gas.
(vi) Acids react with metal carbonates and metal hydrogen carbonates to liberate carbon dioxide. Figure 6.2: Reaction of zinc with dilute acid CaCO (s) 3 Calcium carbonate + 2HCl (aq) Hydrochloric acid CaCl (aq) 2 Calcium chloride + HO (l) 2 Water + CO (g) 2 Carbon dioxide
NaHCO (S) 3 + CHCOOH (aq) 3 CHCOONa (aq) 3 + HO 2 + CO (g) 2 Sodium hydrogen carbonate Acetic acid Sodium acetate Water Carbon dioxide
Figure 6.3: Reaction of acetic acid with sodium hydrogen carbonate
Acetic acid (vinegar) + Sodium hydrogen carbonate (baking powder)
Paints and pigments
Fibres and dyes
Soaps and detergents
Chemicals and plastics
(iv) for etching designs on metals like copper, brass and bronze. Sulphuric Acid Sulphuric acid is used: (i) as a dehydrating agent. (ii) in the manufacture of fertilizers like ammonium phosphate, calcium ammonium phosphate, calcium super phosphate, etc. (iii) in the manufacture of celluloid plastic, artiﬁcial silk, paints, drugs and detergents. (iv) in petroleum reﬁning, textile, paper, and leather industries. (v) in lead storage batteries. The uses of sulphuric acid are so large and so important that it is known as the king of chemicals Acetic Acid Acetic acid is used: (i) in the preparation of pickles. (ii) in the manufacture of synthetic ﬁbre. 6.2 Bases / Alkalis Many compounds have properties which are contrary to acids. Such compounds are termed as bases. The bases which are soluble in water are called alkalis. The word alkali has been taken form Arabic word “qali” which means “from ashes”. Alkalis are obtained from the ashes of plants. -Alkalis/bases are the compounds which produce hydroxide ions (OH) in their aqueous solutions. Sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide Ca(OH), etc., are the examples of bases / alkalis. Some important alkalis and their 2 formulae are given in Table 6.3. Table 6.3: Some common alkalis and their formulae Alkali Formula Sodium hydroxide NaOH Potassium hydroxide KOH Calcium hydroxide Ca(OH)2 Ammonium hydroxide NHOH 4 Magnesium hydroxide Mg(OH)2
Figure 6.5: Common alkalis used in laboratory
Commonly used alkalis as laboratory reagents are shown below in reagent bottles.
Properties of Bases / Alkalis (i) Aqueous solution of a base has a soapy touch.
Bases turn red litmus blue, colourless phenolphthalein pink and methyl orange yellow. They turn turmeric paper brown. (iii) Aqueous solution of bases are good conductor of electricity. (iv) Bases react with acids to form salts and water. The reaction is called neutralization reaction.
v. Alkalis when heated with ammonium salts produce ammonia gas (Figure 6.6). We can identify ammonia gas by its pungent smell. Ammonia also turns moist red litmus paper blue.
Activity 6.4 Apparatus / Material required: Test tube, sodium hydroxide, water, etc. Procedure: 3§ Take 10 cm of water in a test tube. § Add a few pellets of sodium hydroxide and shake it. § Touch the solution with your ﬁngers. How do you feel?
Activity 6.5 Apparatus / Material required: Test tube, sodium hydroxide solution, dilute hydrochloric acid, phenolphthalein, etc. Procedure: 3§ Take 3 cm of sodium hydroxide solution in a test tube. § Add a drop of phenolphthalein solution to it. It turns pink. § To this, add dilute hydrochloric acid slowly until the colour is discharged. Transfer the solution to a china dish and evaporate it to dryness. What do you observe?
NaOH (aq) + HCl (aq) NaCl (aq) + HO (l) 2 Sodium hydroxide Hydrochloric acid Sodium chloride Water Ca(OH) (aq) 2 + 2NHCl (aq) 4 CaCl (aq) 2 + 2HO (l) 2 + 2NH (g) 3 Calcium hydroxide Ammonium chloride Calcium chloride Water Ammonia.
Uses of Bases / Alkalis Some common uses of bases are: Sodium hydroxide (NaOH) Sodium hydroxide is largely used in: (i) soap, textile and plastic industries. (ii) petroleum reﬁning. (iii) making rayon. (iv) the manufacture of paper pulp and medicines.
Calcium hydroxide Calcium hydroxide is called slaked lime. It is used: (i) in the manufacture of bleaching powder. (ii) as a dressing material for acid burns. (iii) in making lime sulphur sprays to be used as fungicide. (iv) as a water softener. (v) for neutralizing acidity present in soil. Ammonium hydroxide Ammonium hydroxide is used: (i) to remove grease from window panes. (ii) to remove ink spots from clothes. (iii) as a reagent in laboratory. (iv) for the treatment of bees’ stings. Salts A salt is a compound formed by the neutralization of an acid with a base. A large variety of compounds exists as salts. Sodium chloride is a common salt which we use in our food.
Table 6.4: Some common salts and their formulae Salt Formula Salt Formula Sodium chloride NaCl Sodium nitrate NaNO3 Potassium chloride KCl Potassium nitrate KNO3 Ammonium chloride NHCl 4 Ammonium nitrate NHNO 4 3 Calcium chloride CaCl2 Calcium sulphate CaSO4 Sodium carbonate NaCO 2 3 Calcium carbonate CaCO3 Sodium hydrogen carbonate NaHCO3 Copper sulphate CuSO4
Properties of Salts (i) Salts exist in solid states. They are found in crystalline or in powder forms. They have high melting and boiling points. (ii) Generally, salts are soluble in water. However, the salts like calcium carbonate, lead chloride and cadmium sulphate, etc., are insoluble in water. (iii) Aqueous solutions of metal salts or their molten forms conduct electricity. (iv) Many of the salts contain water molecules in their crystals which are responsible for the shape of the crystals. (v) Carbonates and bicarbonates react with acids to liberate carbon dioxide gas.
NaCO (s) 2 3 + 2HCl (aq) 2NaCl (aq) + HO (l) 2 + CO (g) 2 Sodium carbonate Hydrochloric acid Sodium chloride Water Carbon dioxide
FeCl (aq) 3 + 3NaOH (aq) Fe(OH) (ppt) 3 + 3NaCl (aq) Ferric chloride Sodium hydroxide Ferric hydroxide Sodium chloride
NaCO (s) 2 3 + 2HO (l) 2 2NaOH (aq) + HCO (aq) 2 3 Sodium carbonate Water Sodium hydroxide Carbonic acid.
Uses of Salts (i) Role of salts in human body Salts of sodium, potassium, calcium, magnesium and iron are needed for the normal working of our body (Figure 6.8). They perform the following functions:
NaCO Sodium and potassium salts are needed for the proper functioning of muscles and the nervous system.
Salts of calcium are present in bones. They are responsible for the strength of bones. These salts are responsible for preventing heart attacks. Plaster of Paris (CaSO HO ) is used for broken limbs. 4 2 (c) Potash alum is used to coagulate the blood coming out of a wound. It is also used for the puriﬁcation of water. (d) Salts of iodine are needed for the proper functioning of thyroid glands. They are also used for the treatment of goiter. (ii) Uses of salts in our daily life (a) In our daily life, we use common salt for seasoning food.
preservative for ﬁsh and pickles (Figure 6.8). (b) Baking soda is used for giving softness to bread and cake. (c) Washing soda is used for washing clothes. (d) Sodium potassium tartrate is used as a laxative. (iii) Uses of salts in industries (a) Sodium chloride is used for the manufacture of chlorine, hydrogen chloride, caustic soda, washing soda and sodium hydrogen carbonate. (b) Sodium carbonate is used for softening hard water and for the manufacture of glass and soap. (c) Potassium nitrate is used for the preparation of gun powder and ﬁreworks. It is also used as a fertilizer. (d) Potash alum is used for puriﬁcation of water, in dyeing cloth and for tanning hides. (e) Copper sulphate is used as a fungicide, in calico printing and in electroplating. 6.4 pH scale The scale which is used to measure the strength of acidic or alkaline solution is known as pH scale. The pH of a solution can be found with the help of universal indicator or pH paper. A universal indicator paper has a mixture of several dyes coated on it. It shows different colours for different pH values of the solutions. In an acidic solution, colour changes from yellow to orange and then red as the pH decreases. The colour changes from indigo to violet when pH changes from 7 to 14. You will observe that different shades of colour appear on each strip of pH paper. By comparing the colours with the chart provided with the pH paper you can ﬁnd the pH of different solutions. Strong acids have pH value 0 to 2. pH of weak acids is in between 3 and 6. pH of strong alkalis is 12 to 14.
6.4.1 pH and its Range (0 –14) in Aqueous Medium: Pure water ionizes very slightly into hydrogen (H ) and hydroxide (OH ) ions. + – However, the concentrations of hydrogen ions (H ) and hydroxide ions (OH ) in pure water are equal. Hydrogen ion concentration increases, when acids are dissolved in water. Alkalis on dissolving in water decrease the concentration of hydrogen ions in water as compared to hydroxide ion. The greater the concentration of hydrogen ions + (H ) in a solution, the stronger the acid it is. The lesser the concentration of hydrogen ions as compared to hydroxide ions in a solution, the stronger the alkali it is. Hence, the scale which is used to measure the strength of an acid or alkali in an aqueous + solution is based on the concentration of hydrogen ions (H ) which is termed as pH.
pH values range from 0 – 14 (Figure 6.9). The solutions having equal concentrations of + – hydrogen ions (H ) and hydroxide ions (OH) are neutral solutions. They have pH = 7. pH = 7 is the midpoint of the pH scale. The solutions with higher concentration of hydrogen ions will have lower than 7 value of pH. The solutions with lower concentration of hydrogen ions than that of hydroxide ions will have greater than 7 value of pH. Solution with lower pH values are stronger acids. The solutions with higher pH values are stronger alkalis. The higher the pH value of the solutions, the stronger the alkalis they are.
pH Meter The instrument which is used to measure the exact pH of the solutions is called pH meter (Figure 6.10). When the electrode of pH meter is dipped in the solution, the reading of its pH appears on the digital display of pH meter. 6.5 Indicators Majority of acids and bases are colourless. It is not possible to identify them by their appearance. In order to identify whether a substance is an acid or alkali indicators are used. An indicator is a substance that shows different colours in acidic and basic solutions (Table 6.6). Some examples of indicators are phenolphthalein, methyl orange, litmus, turmeric, china rose and red cabbage.
Test the above samples with methyl orange and phenolphthalein and record the observation. Natural indicators Red cabbage, turmeric, china rose and litmus.
Turmeric (Haldi Powder) You will observe that: Turmeric paper remains yellow in acidic and neutral solutions but turns brown in alkaline solution.
You will observe that: The purple colour of cabbage indicator turns red in acidic solutions and green in basic solutions. Neutral solutions do not change the colour of red cabbage indicator.
Acids are substances which have sour taste. They change blue litmus red. They also react with active metals producing salts and hydrogen gas. § Acids act on metal carbonates and hydrogen carbonates liberating carbon dioxide. § Acids neutralize bases to form salts and water. § Acids have many uses in laboratories and industries. § Hydroxides like NaOH, KOH, Ca(OH), NHOH are examples of bases. 2 4 § Bases have bitter taste and turn red litmus blue, colourless phenolphthalein to pink. § Bases neutralize acids to form salts and water. § Bases have many uses in laboratories in homes and in industries. Many salts are commonly used in our daily life.