Physical Science Classes VI-X for School Assistants and TET Paper 2A · Chapter 10

Acids, Bases, Salts and pH

What to remember

Acids give H⁺ (H3O⁺) ions in water and bases give OH⁻ ions; they react to form salt and water (neutralisation). pH = -log10[H⁺]; pH below 7 is acidic, 7 is neutral, above 7 is basic at 25 °C.
Common compounds: NaOH (chlor-alkali), bleaching powder CaOCl2, baking soda NaHCO3, washing soda Na2CO3·10H2O, plaster of Paris CaSO4·½H2O.
Each pH unit is a ten-fold change in H⁺ concentration; pH 3 is 100 times more acidic than pH 5.

Acids and bases: definitions

Theory	Acid	Base
Arrhenius	Gives H⁺ in water	Gives OH⁻ in water
Bronsted-Lowry	Proton (H⁺) donor	Proton acceptor
Lewis	Electron-pair acceptor	Electron-pair donor

Conjugate pair: an acid and the base formed when it loses H⁺ (HCl and Cl⁻; NH3 and NH4⁺). A strong acid has a weak conjugate base.
Water is amphoteric (amphiprotic): it can act as an acid or a base.
Alkali: a base that dissolves in water (NaOH, KOH). All alkalis are bases, but not all bases are alkalis (Cu(OH)2 is insoluble).
Strong acids ionise completely: HCl, H2SO4, HNO3. Weak acids ionise partly: acetic acid, carbonic acid, citric acid. Strong bases: NaOH, KOH. Weak bases: NH4OH.
Basicity is the number of replaceable H⁺ in one acid molecule: HCl 1, H2SO4 2, H3PO4 3. Acidity of a base is the number of OH⁻ it gives.
Natural acids: citric acid (lemon, orange), tartaric acid (tamarind, grapes), malic acid (apple), lactic acid (curd, sour milk), oxalic acid (spinach, tomato), formic acid (ant sting, nettle), acetic acid (vinegar), ascorbic acid (vitamin C).
Dilution of a strong acid is exothermic: always add acid to water slowly, never water to acid.
Acids in water conduct electricity because of ions; glucose and alcohol have hydrogen but do not give ions, so they are not acids.

Indicators

Indicator	In acid	In base
Litmus	Blue turns red	Red turns blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow
Turmeric	Yellow (no change)	Red-brown
China rose (hibiscus)	Dark pink or magenta	Green
Red cabbage extract	Red	Green or yellow

Olfactory indicators change smell: onion, vanilla and clove oil lose their smell in a base. Universal indicator shows a colour range for each pH value.

Reactions

Acid + metal gives salt + hydrogen (burns with a pop sound test).
Acid + metal carbonate or hydrogencarbonate gives salt + water + CO2. Limewater turns milky: Ca(OH)2 + CO2 gives CaCO3 + H2O. Excess CO2 clears it as calcium hydrogencarbonate.
Acid + base gives salt + water: neutralisation (HCl + NaOH gives NaCl + H2O).
Base + metal (Zn, Al) with NaOH gives hydrogen: 2NaOH + Zn gives Na2ZnO2 + H2.
Non-metal oxides are acidic (CO2 with water gives carbonic acid); metal oxides are basic.
Neutralisation calculation: for monobasic acid and monoacidic base, M1V1 = M2V2 at the end point. Example: 25 mL of 0.1 M HCl needs 25 mL of 0.1 M NaOH. For 20 mL of 0.2 M HCl, the volume of 0.1 M NaOH is (0.2 × 20)/0.1 = 40 mL.

pH scale

Sorensen introduced pH. pH = -log10[H⁺] and pOH = -log10[OH⁻]. At 25 °C, Kw = [H⁺][OH⁻] = 10⁻¹⁴, so pH + pOH = 14.
Worked examples:
0.01 M HCl: [H⁺] = 10⁻² so pH = 2.
0.001 M NaOH: [OH⁻] = 10⁻³, pOH = 3, pH = 11.
0.05 M H2SO4 (strong, both H⁺): [H⁺] = 0.1 so pH = 1.
pH 3 vs pH 6: [H⁺] differs by 10³ = 1000 times.
pH of pure water at 25 °C is 7 because [H⁺] = [OH⁻] = 10⁻⁷.
Diluting an acid 10 times raises pH by 1 (strong acid, pH 2 to pH 3).
Approximate values: gastric juice about 1.5 to 3; lemon juice about 2.2; vinegar about 3; tomato juice about 4; black coffee about 5; milk slightly below 7; pure water 7; blood about 7.4; sea water about 8; baking soda solution about 8.3; milk of magnesia about 10; household bleach and caustic soda very high.
pH in daily life:
Digestion: the stomach makes HCl; over-acidity is treated with antacids (milk of magnesia, Mg(OH)2).
Tooth decay starts when mouth pH falls below about 5.5. Toothpastes are basic.
Soil: plants grow best in a suitable pH; farmers add lime (basic) to acidic soil and organic matter to basic soil.
Acid rain: pH below 5.6 (CO2 itself gives rain pH near 5.6). It damages marble (Taj Mahal), aquatic life and crops.
Insect stings: bee stings (acid) are eased by baking soda; wasp stings (basic) by vinegar. Ant bites and nettle contain formic acid.
Body fluids: blood pH is kept at about 7.4 by buffers.
Buffer: a solution that resists a change in pH (acetic acid + sodium acetate).

Salts

Salt: compound formed from an acid and a base. Family: salts with the same cation or anion (NaCl, Na2SO4 are sodium salts).
pH of salt solutions: strong acid + strong base gives neutral (NaCl, pH 7); strong acid + weak base gives acidic (NH4Cl, pH below 7); weak acid + strong base gives basic (sodium carbonate and sodium acetate, pH above 7).
Common salt (NaCl): from sea water and rock salt. Raw material for NaOH, Cl2, H2, baking soda and washing soda.
Chlor-alkali process: electrolysis of brine. 2NaCl + 2H2O gives 2NaOH + Cl2 + H2. Chlorine at the anode, hydrogen at the cathode, NaOH near the cathode. Uses: Cl2 for bleaching and water treatment, H2 as fuel, NaOH in soaps and paper.
Bleaching powder: Ca(OH)2 + Cl2 gives CaOCl2 + H2O. Uses: bleaching, disinfecting drinking water, oxidiser.
Baking soda (sodium hydrogencarbonate, NaHCO3): NaCl + H2O + CO2 + NH3 gives NH4Cl + NaHCO3. Mild non-corrosive base. On heating: 2NaHCO3 gives Na2CO3 + H2O + CO2. Uses: antacid, baking powder (baking soda + tartaric acid; the CO2 makes cakes rise), soda-acid fire extinguishers.
Washing soda (Na2CO3·10H2O): from heating baking soda to get sodium carbonate, then recrystallising with water. Uses: glass, soap, paper, softening hard water.
Water of crystallisation: fixed number of water molecules in one formula unit of a salt. Examples: blue vitriol CuSO4·5H2O (blue; on heating turns white, restored by water), washing soda 10 H2O, gypsum CaSO4·2H2O.
Plaster of Paris (CaSO4·½H2O): made by heating gypsum at 373 K. CaSO4·2H2O gives CaSO4·½H2O + 1½H2O. It sets into gypsum when mixed with water. Used for supporting broken bones, toys, decoration, smoothing surfaces. It should be stored in moisture-proof containers.

Substance	Formula	Common name or use
Sodium hydroxide	NaOH	Caustic soda
Sodium hydrogencarbonate	NaHCO3	Baking soda
Sodium carbonate decahydrate	Na2CO3·10H2O	Washing soda
Calcium oxychloride	CaOCl2	Bleaching powder
Calcium sulphate hemihydrate	CaSO4·½H2O	Plaster of Paris
Calcium hydroxide	Ca(OH)2	Slaked lime
Calcium oxide	CaO	Quick lime
Magnesium hydroxide	Mg(OH)2	Milk of magnesia

Classroom angle

Prepare a red cabbage or turmeric indicator in class and test lemon juice, soap and baking soda solution. Show the ten-fold change in H⁺ for each pH unit with serial dilution of lemon juice.

Exam traps

A strong acid has a weak conjugate base.
Sulphuric acid is dibasic: in simple problems 0.05 M H2SO4 gives [H⁺] = 0.1 M, so pH is 1, not 1.3.
Baking soda is NaHCO3; washing soda is Na2CO3·10H2O.
Plaster of Paris is a hemihydrate (½ H2O), gypsum is a dihydrate.
Pure water is neutral at 25 °C; its pH is lower than 7 at higher temperature though still neutral.
Phenolphthalein is pink in base, but methyl orange is yellow in base.
Always add acid to water, never water to acid.
Sodium chloride solution is neutral; ammonium chloride is acidic; sodium carbonate is basic.
An alkali is a water-soluble base; do not equate alkali and base.
Acid rain pH is below 5.6, not 7.

One-liners

1. Arrhenius acid gives H⁺; Bronsted acid donates a proton; Lewis acid accepts an electron pair.
2. pH + pOH = 14 at 25 °C.
3. pH of pure water is 7.
4. Lemon juice has citric acid; tamarind has tartaric acid; curd has lactic acid.
5. Baking soda releases CO2 on heating.
6. Bleaching powder is CaOCl2.
7. Plaster of Paris is CaSO4·½H2O.
8. Brine electrolysis gives NaOH, Cl2 and H2.
9. Blood pH is about 7.4.
10. Tooth decay begins below pH of about 5.5.
11. Antacids such as milk of magnesia are bases.
12. Copper sulphate crystals have 5 water molecules.

Practice questions

According to Arrhenius, an acid is a substance that gives which ion in water?
1. OH⁻
2. H⁺
3. Na⁺
4. Cl⁻
Answer
B. H⁺
Arrhenius acids release hydrogen ions in aqueous solution.
The indicator that turns pink in a basic solution is
1. methyl orange
2. blue litmus
3. turmeric
4. phenolphthalein
Answer
D. phenolphthalein
Phenolphthalein is colourless in acid and pink in base.
The pH of pure water at 25 °C is
1. 0
2. 14
3. 1
4. 7
Answer
D. 7
[H⁺] = [OH⁻] = 10⁻⁷ M.
The chemical formula of baking soda is
1. Na2CO3
2. NaHCO3
3. Na2CO3·10H2O
4. NaOH
Answer
B. NaHCO3
Baking soda is sodium hydrogencarbonate.
The chemical formula of washing soda is
1. NaHCO3
2. CaCO3
3. Na2CO3·10H2O
4. NaOH
Answer
C. Na2CO3·10H2O
Washing soda is hydrated sodium carbonate with ten water molecules.
The chemical formula of bleaching powder is
1. CaCO3
2. CaOCl2
3. Ca(OH)2
4. CaCl2
Answer
B. CaOCl2
It is made by passing chlorine over slaked lime.
Plaster of Paris has the formula
1. CaCO3
2. CaSO4
3. CaSO4·2H2O
4. CaSO4·½H2O
Answer
D. CaSO4·½H2O
It is calcium sulphate hemihydrate made by heating gypsum.
The acid present in tamarind is
1. acetic acid
2. lactic acid
3. tartaric acid
4. formic acid
Answer
C. tartaric acid
Lactic acid is in curd and formic acid in ant stings.
In the chlor-alkali process, the gas liberated at the cathode is
1. hydrogen
2. nitrogen
3. oxygen
4. chlorine
Answer
A. hydrogen
Chlorine is at the anode and hydrogen at the cathode.
In the chlor-alkali process, the gas liberated at the anode is
1. carbon dioxide
2. hydrogen
3. oxygen
4. chlorine
Answer
D. chlorine
2Cl⁻ loses electrons to form Cl2 at the anode.
A bee sting (acidic) can be relieved by applying
1. baking soda
2. lemon juice
3. tamarind pulp
4. vinegar
Answer
A. baking soda
A mild base neutralises the formic acid.
The number of water molecules in one formula unit of blue vitriol is
1. 5
2. 2
3. 10
4. 7
Answer
A. 5
Blue vitriol is CuSO4·5H2O.
The pH of human blood is about
1. 9.5
2. 7.4
3. 2.0
4. 5.5
Answer
B. 7.4
Buffers keep blood slightly basic.
According to Bronsted-Lowry, a base is
1. a proton donor
2. an electron-pair acceptor
3. a proton acceptor
4. an electron donor only
Answer
C. a proton acceptor
Acid donates H⁺; base accepts it.
The conjugate base of HCl is
1. HCl2⁻
2. H3O⁺
3. OH⁻
4. Cl⁻
Answer
D. Cl⁻
HCl loses H⁺ to leave Cl⁻.
Which of these is commonly used as an antacid?
1. Acetic acid
2. Citric acid
3. Magnesium hydroxide
4. Hydrochloric acid
Answer
C. Magnesium hydroxide
Milk of magnesia neutralises excess stomach acid.
Tooth decay begins when the mouth pH falls below about
1. 7.5
2. 9
3. 5.5
4. 3
Answer
C. 5.5
Below this, enamel begins to corrode.
Acid rain has a pH of less than about
1. 7
2. 5.6
3. 8.3
4. 9
Answer
B. 5.6
CO2 alone gives rain a pH near 5.6.
The correct way to dilute concentrated sulphuric acid is to
1. add water to the acid
2. mix both rapidly
3. heat the acid first
4. add the acid slowly to water
Answer
D. add the acid slowly to water
Dissolution is highly exothermic; adding water to acid can splash it.
An aqueous solution of ammonium chloride is acidic because it is a salt of a
1. strong acid and a weak base
2. weak acid and a strong base
3. strong acid and a strong base
4. weak acid and a weak base
Answer
A. strong acid and a weak base
NH4⁺ hydrolyses to give H⁺.
Water is called amphoteric because it
1. is neutral
2. can act as both an acid and a base
3. dissolves salts
4. has pH 14
Answer
B. can act as both an acid and a base
It can donate or accept a proton.
Baking powder makes cake batter rise because it
1. releases nitrogen
2. releases carbon dioxide on heating
3. releases hydrogen
4. absorbs oxygen
Answer
B. releases carbon dioxide on heating
NaHCO3 and tartaric acid produce CO2 bubbles.
Plaster of Paris should be stored in a moisture-proof container because it
1. loses all its mass
2. burns in air
3. sets into gypsum when water is added
4. dissolves in air
Answer
C. sets into gypsum when water is added
CaSO4·½H2O + 1½H2O gives CaSO4·2H2O.
Aqueous glucose does not conduct electricity because it
1. contains hydrogen but gives no ions in water
2. is a salt
3. is an acid
4. is a gas
Answer
A. contains hydrogen but gives no ions in water
Only substances that give H⁺ ions are acids.
The pH of 0.01 M HCl (a strong acid) is
1. 1
2. 0.01
3. 12
4. 2
Answer
D. 2
[H⁺] = 10⁻², pH = 2.
The pH of 0.001 M NaOH solution at 25 °C is
1. 3
2. 11
3. 10
4. 12
Answer
B. 11
pOH = 3, so pH = 14 - 3 = 11.
If the pH of a solution is 4, its pOH at 25 °C is
1. 4
2. 14
3. 10
4. 7
Answer
C. 10
pH + pOH = 14.
A solution of pH 3 has how many times more H⁺ than a solution of pH 6?
1. 1000
2. 3
3. 100
4. 10
Answer
A. 1000
The difference of three pH units equals 10³.
What volume of 0.1 M NaOH neutralises 20 mL of 0.2 M HCl?
1. 80 mL
2. 20 mL
3. 10 mL
4. 40 mL
Answer
D. 40 mL
M1V1 = M2V2: 0.2 × 20 = 0.1 × V, V = 40 mL.
The pH of 0.05 M H2SO4 (assuming complete ionisation) is
1. 2
2. 1.3
3. 1
4. 0.05
Answer
C. 1
[H⁺] = 2 × 0.05 = 0.1 M, pH = 1.
A strong acid of pH 2 is diluted ten times. The new pH is about
1. 1
2. 3
3. 2.5
4. 12
Answer
B. 3
[H⁺] falls to one tenth, so pH rises by 1.
The hydroxide ion concentration in a solution of pH 9 at 25 °C is
1. 10⁻⁵ M
2. 10⁻⁷ M
3. 10⁻⁹ M
4. 10⁹ M
Answer
A. 10⁻⁵ M
pOH = 5, so [OH⁻] = 10⁻⁵.
The mass of water of crystallisation in one mole of CuSO4·5H2O (H = 1, O = 16) is
1. 180 g
2. 18 g
3. 36 g
4. 90 g
Answer
D. 90 g
5 × 18 = 90 g.
What volume of 0.5 M HCl neutralises 50 mL of 0.1 M NaOH?
1. 10 mL
2. 50 mL
3. 5 mL
4. 25 mL
Answer
A. 10 mL
0.5 × V = 0.1 × 50, V = 10 mL.
Which of the statements is/are correct? 1. Sodium chloride solution is neutral. 2. Sodium carbonate solution is acidic.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Sodium carbonate is a salt of a weak acid and a strong base, so it is basic.
Which of the statements is/are correct? 1. Plaster of Paris is CaSO4·2H2O. 2. Gypsum is CaSO4·½H2O.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
Plaster of Paris is the hemihydrate and gypsum is the dihydrate.
Which of the statements is/are correct? 1. A lower pH means a higher H⁺ concentration. 2. A solution of pH 7 is neutral at 25 °C.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Which of the statements is/are correct? 1. All bases are alkalis. 2. Alkalis are water-soluble bases.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Only water-soluble bases are alkalis.
Which of the statements is/are correct? 1. Brine electrolysis gives caustic soda. 2. Hydrogen is released at the cathode in this process.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Which of the statements is/are correct? 1. Baking soda gives sodium carbonate on heating. 2. Washing soda is anhydrous sodium carbonate.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Washing soda is Na2CO3·10H2O, not anhydrous.
Which of the statements is/are correct? 1. Acetic acid is a strong acid. 2. HCl is a strong acid.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Acetic acid is a weak acid.
Which of the statements is/are correct? 1. Turmeric is yellow in acid. 2. Turmeric turns red-brown in base.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Match substance with use: (a) NaHCO3 (b) CaOCl2 (c) CaSO4·½H2O (d) Na2CO3·10H2O; (i) disinfecting water (ii) antacid (iii) setting broken bones (iv) softening hard water
1. a-i, b-ii, c-iii, d-iv
2. a-iii, b-i, c-ii, d-iv
3. a-ii, b-i, c-iv, d-iii
4. a-ii, b-i, c-iii, d-iv
Answer
D. a-ii, b-i, c-iii, d-iv
Baking soda antacid, bleaching powder disinfectant, POP for plasters, washing soda softens water.
Match the acid with its natural source: (a) lactic acid (b) tartaric acid (c) formic acid (d) citric acid; (i) lemon (ii) curd (iii) tamarind (iv) ant sting
1. a-iii, b-ii, c-iv, d-i
2. a-iv, b-iii, c-ii, d-i
3. a-ii, b-iii, c-i, d-iv
4. a-ii, b-iii, c-iv, d-i
Answer
D. a-ii, b-iii, c-iv, d-i
Curd, tamarind, ant sting, lemon respectively.
Which of these does NOT release carbon dioxide when added to dilute HCl?
1. Sodium hydrogencarbonate
2. Sodium chloride
3. Sodium carbonate
4. Calcium carbonate
Answer
B. Sodium chloride
Only carbonates and hydrogencarbonates give CO2 with acids.

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1.The Contact Process is used for the manufacturing of which of the following chemicals?
2.The correct way of making a solution of acid in water is to ________.
3.The foul smell of rancid butter is due to -
4.The general reaction between a metal oxide and an acid can be written as:
5.Substances whose odour changes in acidic or basic media are called-
6.The addition of which of the following adds fizz to soft drinks?
7.The acid and base which form sodium hydrogen carbonate are:
8.The acid present in the cells of our body is ___________.
9.The action of chlorine on dry Ca(OH) 2, produces which of the following?
10.Sodium hydroxide (NaOH) contains __________ as its anion.

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