Class 7 Science Acid Base and Salts Chapter Notes

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Acid, Base and Salts

Acids, bases and salts are among the most important groups of chemical substances. This chapter notes page covers the Arrhenius, Bronsted-Lowry and Lewis concepts of acids and bases, classification of solvents, the levelling effect, types of salts, pH and pOH, universal indicator colours, the preparation, properties and uses of bleaching powder, sodium hydroxide, plaster of Paris, baking soda and washing soda, followed by exercises with answers.

ACIDS AND BASES

1. Different workers gave different definitions for acids and bases from time to time.

i) Arrhenius Theory: Arrhenius defined acids as those compounds which give hydrogen ions in aqueous solution, and bases as those compounds which give hydroxide ions in solution, e.g.,
H₂SO₄ (aq) ⇌ 2H⁺ (aq) + SO₄²⁻ (aq)
The strength of an acid or base depends upon its tendency to furnish H⁺ or OH⁻ ions in solution.
This definition can be applied only for those reactions which take place in aqueous solutions. Further, this theory does not include substances that do not contain H⁺ and OH⁻ ions but still can neutralize acids and bases.

Basicity or protonicity of acids: It is the number of H⁺ ions furnished by a molecule of an acid.
Monobasic acid: An acid furnishing one H⁺ ion, e.g. HCl, HNO₃ etc.
Dibasic acid: An acid furnishing two H⁺ ions, e.g. H₂SO₄, H₃PO₃ etc.
Tribasic acid: An acid furnishing three H⁺ ions, e.g. H₃PO₄.

Acidity or hydroxicity of base: It may be defined as the number of OH⁻ ions furnished by a molecule of a base.
Monoacidic base: NaOH, KOH
Diacidic base: Ca(OH)₂, Ba(OH)₂

ii) Bronsted Lowry Theory: According to this theory, acids are defined as those substances which give up a proton and bases are substances which accept a proton, e.g.,
HCl + H₂O ⇌ H₃O⁺ + Cl⁻ (Equation 1)
(HCl is acid A₁, H₂O is base B₂, H₃O⁺ is acid A₂, Cl⁻ is base B₁)
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (Equation 2)
(NH₃ is base B₁, H₂O is acid A₂, NH₄⁺ is acid A₁, OH⁻ is base B₂)

From the above observations we may conclude that every acid has a conjugate base and every base has a conjugate acid.
In equation 1, the conjugate base of acid HCl is Cl⁻ and the conjugate acid of water is H₃O⁺ (hydroxonium ion).
In equation 2, the conjugate acid of base NH₃ is NH₄⁺ and the conjugate base of H₂O is OH⁻.

Thus according to this theory the following conclusions can be drawn.
a) A substance can act as an acid or base only when another substance capable of accepting a proton or donating a proton exists.
b) In aqueous solutions H⁺ ions exist as hydrated ions or H₃O⁺ (H⁺.H₂O) ions.
c) Even ions may act as acids or bases.
d) Water can act both as an acid and as a base because it can give off a proton as well as accept a proton.
H₂O + H₂O ⇌ H₃O⁺ + OH⁻ (one water molecule acts as base B₁, the other as acid A₁; H₃O⁺ is acid A₁ and OH⁻ is base B₂)
e) The strength of acids (or bases) depends on the medium which acts as base (or acid).
HCl + H₂O → H₃O⁺ + Cl⁻
HCl + NH₃ → NH₄⁺ + Cl⁻
HCl + Benzene → No reaction
Acidic nature of HCl is greatest in ammonia and least in benzene.

CLASSIFICATION OF SOLVENTS

i) Aprotic: Solvents which can neither accept nor donate a proton, e.g. CCl₄, CS₂, Benzene.
ii) Amphiprotic: Solvents which can both accept and donate a proton, e.g. H₂O, liquid ammonia.
iii) Protophillic: Solvents which have a greater tendency to accept a proton, e.g. H₂O, R-OH, liquid NH₃.

LEVELLING EFFECT

Water acts as a very strong base because it has a great tendency to accept protons from mineral acids. The ionization of strong acids in water may be represented as below:
HCl + H₂O ⇌ H₃O⁺ + Cl⁻
H₂SO₄ + H₂O ⇌ H₃O⁺ + HSO₄⁻

Thus all strong acids (e.g. HNO₃, HCl, H₂SO₄, HClO₄) react almost completely to form H₃O⁺ ion. Therefore all strong acids in aqueous solution appear equally strong, i.e. the acidic levels for all acids are the same. This is called the levelling effect and water is the levelling solvent. Thus the relative acidic strengths or basic strengths in aqueous solutions cannot be compared.

When acetic acid is used as solvent instead of water, it is found that the acidic strength of the acid lowers and now the given acid in acetic acid becomes weak, and thereby different acids will dissociate to different extents in glacial acetic acid. (This is because of the poor proton accepting character of acetic acid.)
HA + CH₃COOH ⇌ CH₃COOH₂⁺ + A⁻
In this reaction the equilibrium does not lie very much to the right and hence strong acids are feebly dissociated. The degree of dissociation of a number of acids dissolved in glacial acetic acid has been determined and it is found that the acidic strength order is
HClO₄ > HI > HBr > H₂SO₄ > HCl > HNO₃ > H₃O⁺ > HSO₄⁻ > H₃PO₄ > HF > CH₃COOH > H₂CO₃ > H₂S > NH₄⁺ > HCN

LEWIS CONCEPT OF ACIDS AND BASES

According to Lewis, an acid is any species which is capable of accepting a pair of electrons and a base is a species that has a lone pair of electrons for donation.
This statement covers a wide range of acids and bases. Thus all Bronsted-Lowry bases are Lewis bases while all Bronsted-Lowry acids are not Lewis acids.
Hence this theory includes acids which do not donate a proton or liberate H⁺ (aprotic).
NH₃ + BF₃ → H₃N → BF₃ (N carries δ+ and B carries δ−)
R-Cl + AlCl₃ → R⁺ AlCl₄⁻

Lewis acids
i) Compounds where the central atom has an incomplete octet. This means all electron deficient molecules can act as Lewis acids, e.g. BF₃, BBr₃, AlCl₃, FeCl₃, GaCl₃ etc.
ii) Compounds in which the central atom has available vacant orbitals, e.g. SnF₄, SnCl₂, SnCl₄, PF₃, PF₄, SF₄, TiCl₄ etc.
iii) Simple cations like Ag⁺, Cu²⁺, Al³⁺ etc., e.g. Cu²⁺ + 4NH₃ → [Cu(NH₃)₄]²⁺.

Lewis bases
i) All simple anions are Lewis bases, e.g. Cl⁻, NO₃⁻, OH⁻.
ii) Molecules containing one or more unshared pairs of electrons (lone pairs), e.g. H₂O, NH₃, ROH, NX₃, R₂S etc., are Lewis bases.
iii) Multiple bonded compounds which can donate the pair of electrons, e.g. C = O, N = O, CH₂ = CH₂, CH ≡ CH.

SALT

A substance which ionizes in water to produce ions other than H⁺ and OH⁻ is called a salt.

TYPES OF SALTS

Neutral Salts: Those salts whose aqueous solutions neither turn blue litmus red nor red litmus blue are called neutral salts. These are prepared by the neutralization of a strong acid and a strong base, e.g. NaCl, K₂SO₄, KNO₃ etc.
Acidic Salts: Those salts whose aqueous solutions turn blue litmus red are called acidic salts. These are prepared by neutralisation of a strong acid with a weak base, e.g., NH₄NO₃, NH₄Cl.
Basic Salts: Those salts whose aqueous solutions turn red litmus blue are called basic salts. These are formed by the neutralization of strong bases with weak acids, e.g. Na₂CO₃, CH₃COONa.
Mixed Salts: Salts formed by the neutralization of one acid by two bases or one base by two acids are called mixed salts, e.g. CaOCl₂.
Double Salts: A compound of two salts whose aqueous solution shows the tests for all constituent ions is called a double salt, e.g.
Mohr Salt: FeSO₄.(NH₄)₂SO₄.6H₂O
Potash Alum: K₂SO₄.Al₂(SO₄)₃.24H₂O
Complex Salts: A compound whose solution does not give tests for the constituent ions is called a complex salt, e.g.
K₄[Fe(CN)₆]
Li(AlH₄)

pH AND pOH

pH: The pH scale is used to measure the strengths of acidic and alkaline solutions. It may be defined in a number of ways. pH is defined as the negative logarithm of its [H⁺] ion concentration.
pH = −log[H⁺] = log \( \frac{1}{[H^{+}]} \)
pH values do not give an instantaneous idea about the relative strengths of the solution.

pOH: The solutions with pH less than 7 are acidic, the solutions with pH value more than 7 are alkaline and solutions with pH value equal to 7 are neutral. pOH may be defined as the negative logarithm of hydroxyl ion concentration.
pOH = −log[OH⁻] = log \( \frac{1}{[OH^{-}]} \)

Ionic product of water:
[H⁺][OH⁻] = 10⁻¹⁴ = K_w
−log[H⁺] + (−log[OH⁻]) = 14 = pK_w
pH + pOH = pK_w = 14

Indicators: Universal indicators give different colours with different strength of acidity and alkalinity. Universal indicators are mixtures of indicators that produce a range of colours. They come in liquid or paper form, e.g. pH paper. The different paper colours are:
Red for strong acids
Orange / Yellow for weak acids
Green for neutral substance
Turquoise for weak alkali
Deep blue for strong alkali

SOME IMPORTANT CHEMICALS

Bleaching Powder (CaOCl₂)

Preparation: In Hasen-Clever Plant
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Properties:
1. It is a yellowish white solid having a chlorine like smell.
2. Bleaching powder decomposes to form O₂.
2CaOCl₂ → (CoCl₂) 2CaCl₂ + O₂
3. When reacted with H₂O it liberates Cl₂.
CaOCl₂ + H₂O → Ca(OH)₂ + Cl₂
4. Bleaching powder is decomposed to Cl₂ by dilute H₂SO₄.
CaOCl₂ + H₂SO₄ → CaSO₄ + H₂O + Cl₂
5. It reacts with carbon dioxide to form calcium carbonate and Cl₂.
CaOCl₂ + CO₂ → CaCO₃ + Cl₂
6. The chlorine liberated in the above reaction can be used to oxidise I⁻ to I₂, thereby liberating I₂. This I₂ can be estimated by Na₂S₂O₃, thus we can measure the available chlorine in bleaching powder.
2KI + Cl₂ → 2KCl + I₂

Uses: Bleaching powder is used for the disinfection of drinking water or swimming pool water. For use in outdoor swimming pools, it can be used as a sanitizer in combination with a cyanuric acid stabilizer. The stabilizer will reduce the loss of chlorine because of UV radiation. Calcium does make the water 'hard' and tends to clog up some filters; for this reason sodium hypochlorite is preferred. Bleaching powder is also used for bleaching cotton and linen and in the manufacture of chloroform.

SODIUM HYDROXIDE (NaOH)

Preparation:
1. Soda lime process, causticisation process or Gossage process
Na₂CO₃ + Ca(OH)₂ → (80 - 90°C) 2NaOH + CaCO₃
2. Castner Kellner process - Electrolytic process
Electrolyte: Brine (NaCl solution in water)
Anode: Graphite
Cathode: Iron rods, and mercury acts as intermediate cathode by induction.

Reaction:
NaCl → Na⁺ + Cl⁻
At anode: Cl⁻ → Cl + e⁻; Cl + Cl → Cl₂
At cathode: Na⁺ + e⁻ + Hg → Na-Hg
2Na-Hg + 2H₂O → 2NaOH + H₂ + 2Hg (Na-Hg is sodium amalgam)

Properties:
1. White hygroscopic solid.
2. Sodium hydroxide decomposes on heating to form sodium, hydrogen and oxygen.
2NaOH → (1300°C) 2Na + H₂ + O₂
3. It reacts with acids to form salt and water.
a) NaOH + HCl → NaCl + H₂O
b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
4. It reacts with acidic oxides to form salt and water.
a) 2NaOH + CO₂ → Na₂CO₃ + H₂O (Sodium carbonate)
b) 2NaOH + SO₂ → Na₂SO₃ + H₂O (Sodium sulphite)
c) 2NaOH + SiO₂ → Na₂SiO₃ + H₂O
5. Metals like Zn, Al, Sn, Pb etc. displace hydrogen from sodium hydroxide. Even silicon displaces hydrogen from NaOH.
a) Zn + 2NaOH → Na₂ZnO₂ + H₂ (Sodium zincate)
b) 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂ (Sodium meta-aluminate)
c) 2Al + 6NaOH → 2Na₃AlO₃ + 3H₂ (Sodium aluminate)
d) Si + 2NaOH + H₂O → Na₂SiO₃ + 2H₂ (Sodium silicate)
6. Non metals like P, S, Cl₂, Br₂ disproportionate in sodium hydroxide.
a) 4S + 6NaOH → 2Na₂S + Na₂S₂O₃ + 3H₂O (Sodium thiosulphate)
b) 4P + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃ (Sodium hypophosphite)
c) Cl₂ + 2NaOH (cold and dilute) → NaCl + NaClO + H₂O (Sodium hypochlorite)
d) 3Cl₂ + 6NaOH (hot and conc.) → 5NaCl + NaClO₃ + 3H₂O (Sodium chlorate)
7. It precipitates Fe³⁺, Cu²⁺, Cd²⁺ etc. as metal hydroxides.
a) FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl (Brown)
b) CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄ (Green)
c) CdCl₂ + 2NaOH → Cd(OH)₂ + 2NaCl
8. Metallic salts of Al³⁺ and Zn²⁺ are precipitated as Al(OH)₃ and Zn(OH)₂, but in excess of NaOH these precipitates dissolve to form aluminates and zincates respectively.
a) AlCl₃ + 3NaOH → Al(OH)₃ + 3NaCl
Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O
b) ZnSO₄ + 2NaOH → Zn(OH)₂ + Na₂SO₄
Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + H₂O

Uses: Used as a strong base in the chemical industry. Also used in detection of elements in chemical analysis, soap making and in making of biodiesel as a catalyst.

PLASTER OF PARIS

Quick-setting gypsum plaster consisting of a fine, white powder, calcium sulphate hemihydrate, which hardens when moistened and allowed to dry. Plaster of Paris is prepared by heating calcium sulphate dihydrate, or gypsum, to 120° - 180°C (248° - 356°F). With an additive to retard the set, it is called wall, or hard-wall, plaster.
CaSO₄.2H₂O → CaSO₄.½H₂O + 1½H₂O
(Gypsum → Plaster of Paris)

Uses: Used since ancient times, plaster of Paris is so called because gypsum was early used near Paris to make plaster and cement. Plaster of Paris is also used to precast and hold parts of ornamental plasterwork placed on ceilings and cornices and is used in medicine to make plaster casts to immobilize broken bones while they heal. Some modern sculptors work directly in plaster of Paris. The speed at which the plaster sets gives the work a sense of immediacy and enables the sculptor to achieve the original idea quickly.

WATER OF CRYSTALLISATION

Water chemically bonded to a salt in its crystalline state is called water of crystallisation. For example, in copper (II) sulphate, there are five moles of water per mole of copper sulphate; hence its formula is CuSO₄.5H₂O. This water is responsible for the colour and shape of the crystalline form. When the crystals are heated gently, the water is driven off as steam and a white powder of the anhydrous salt is formed.
CuSO₄.5H₂O (s) → CuSO₄ (s) + 5H₂O (g)

BAKING SODA

Sodium bicarbonate is the chemical compound with the formula NaHCO₃. Because it has long been known and is widely used, the salt has many other names including sodium hydrogencarbonate, sodium bicarbonate, baking soda, bread soda, cooking soda, bicarb soda, saleratus or bicarbonate of soda. It is soluble in water. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slight alkaline taste resembling that of sodium carbonate. It is a component of the mineral natron and is found dissolved in many mineral springs. The natural mineral form is known as nahcolite. It is also produced artificially by passing CO₂ through aqueous sodium carbonate solution.
Na₂CO₃ + H₂O + CO₂ → 2NaHCO₃

Uses: It is primarily used in cooking. Also used as an antacid in medicines and as an absorbent for moisture and odour. In toothpaste it is used as a whitener.

WASHING SODA

Sodium carbonate (also known as washing soda or soda ash), Na₂CO₃, is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate which readily effloresces to form a white powder, the monohydrate. It has a cooling alkaline taste, and can be extracted from the ashes of many plants. It is synthetically produced in large quantities from table salt in a process known as the Solvay process.

Solvay's ammonia process: In this process brine (NaCl) solution is saturated with ammonia and carbon dioxide to get the precipitate of NaHCO₃, which on heating gives Na₂CO₃. The various reactions taking place are given below:
CO₂ + H₂O → H₂CO₃
H₂CO₃ → H⁺ + HCO₃⁻
NH₃ + H⁺ → NH₄⁺
NH₄⁺ + HCO₃⁻ + NaCl → NaHCO₃ + NH₄Cl
2NaHCO₃ → (250°C) Na₂CO₃ + H₂O + CO₂ (recycled)
2NH₄Cl + CaO → CaCl₂ + H₂O + 2NH₃ (recycled)

Uses: Sodium carbonate: its most important use is in the chemical make-up of glass. In chemistry, sodium carbonate is often used as an electrolyte. Domestically it is used as a water softener during laundry. It competes with the ions magnesium and calcium in hard water and prevents them from bonding with the detergent being used. Without using washing soda, additional detergent is needed to soak up the magnesium and calcium ions. Called Washing Soda or Sal Soda in the detergent section of stores, it effectively removes oil, grease, and alcohol stains. Sodium carbonate is also used as a descaling agent in boilers such as found in coffee pots, espresso machines, etc. Sodium carbonate is widely used in photographic processes as a pH regulator to maintain stable alkaline conditions necessary for the action of the majority of developing agents. It is also used for making fusion mixture (Na₂CO₃ + K₂CO₃) and black ash (Na₂CO₃ + CaS).

EXERCISES

I. Multiple Choice Questions

1. H₃PO₂ is a
(A) monobasic acid
(B) dibasic acid
(C) tribasic acid
(D) tetrabasic acid

Answer: (A) monobasic acid

1. H₃PO₂ (hypophosphorous acid) has the structure H₂P(O)OH.
2. Only the hydrogen attached to oxygen can be released as H⁺.
3. Hence, the correct option is (A).

Teacher's Note:
a) Basicity counts only the O-H hydrogens, not all hydrogens in the formula.
b) Do not call it tribasic just because three hydrogens are written.

2. AlCl₃ is
(A) Arrhenius acid
(B) Bronsted acid
(C) Lewis acid
(D) None of these

Answer: (C) Lewis acid

1. In AlCl₃ the central aluminium atom has only six electrons, an incomplete octet.
2. It can accept a pair of electrons, so it is a Lewis acid.
3. It has no H⁺ to donate, so it is neither an Arrhenius nor a Bronsted acid.

Teacher's Note:
a) Electron-deficient molecules like BF₃ and AlCl₃ are classic Lewis acids.

3. Aqueous solution of C₆H₅COONa is
(A) Acidic
(B) Basic
(C) Neutral
(D) Amphoteric

Answer: (B) Basic

1. C₆H₅COONa is the salt of a strong base (NaOH) and a weak acid (benzoic acid).
2. Its anion undergoes hydrolysis and produces OH⁻ ions in water.
3. Hence, the solution is basic and the correct option is (B).

Teacher's Note:
a) Strong base plus weak acid gives a basic salt.
b) Compare with CH₃COONa, which behaves the same way.

4. Bleaching powder acts as
(A) oxidising agent
(B) reducing agent
(C) both of these
(D) none of these

Answer: (A) oxidising agent

1. Bleaching powder releases chlorine and oxygen that oxidise other substances.
2. For example, it oxidises I⁻ to I₂.
3. Hence, the correct option is (A).

Teacher's Note:
a) Its bleaching and disinfecting action is due to oxidation.

5. Soaps are sodium salts of
(A) carbonic acid
(B) benzoic acid
(C) lower fatty acids
(D) higher fatty acids

Answer: (D) higher fatty acids

1. Soaps are made by boiling fats or oils with NaOH (saponification).
2. The products are sodium salts of long-chain (higher) fatty acids such as stearic and palmitic acid.
3. Hence, the correct option is (D).

Teacher's Note:
a) Soap = sodium salt of a long-chain fatty acid.

6. Which of the following is a weak electrolyte ?
(A) NH₄Cl
(B) NH₄OH
(C) NaOH
(D) HCl

Answer: (B) NH₄OH

1. NH₄Cl is a salt and ionises almost completely.
2. NaOH and HCl are a strong base and a strong acid, so they ionise completely.
3. NH₄OH is a weak base and ionises only slightly, so it is a weak electrolyte.

Teacher's Note:
a) Weak acids and weak bases are weak electrolytes.

7. Which of the following is the best conductor of electricity ?
(A) 1 M HCl
(B) 1 M H₂CO₃
(C) 1 M H₃SO₄
(D) 1 M H₂SO₄

Answer: (D) 1 M H₂SO₄

1. Conductivity depends on the concentration of ions in solution.
2. 1 M H₂SO₄ is a strong dibasic acid and gives about 2 M H⁺ ions, more than 1 M HCl.
3. H₂CO₃ is a weak acid and gives very few ions.
4. Hence, the correct option is (D).

Teacher's Note:
a) Option (C) is a misprint, as no acid H₃SO₄ exists.
b) A strong dibasic acid gives twice the H⁺ of a strong monobasic acid.

8. The reaction of water with ammonia is given by the following equation
H₂O + NH₃ ⇌ NH₄⁺ + OH⁻
In this reaction water behave as -
(A) neutral
(B) base
(C) acid
(D) both acid and base

Answer: (C) acid

1. In the forward reaction water donates a proton (H⁺) to NH₃.
2. A proton donor is a Bronsted-Lowry acid.
3. Hence, the correct option is (C).

Teacher's Note:
a) Water is amphiprotic; here ammonia is the base, so water acts as the acid.
b) Decide the role by asking who gives the proton in the forward reaction.

9. The compound that is not a Lewis acid is
(A) BF₃
(B) BaCl₂
(C) AlCl₃
(D) SnCl₄

Answer: (B) BaCl₂

1. BF₃ and AlCl₃ have an incomplete octet, and SnCl₄ has vacant d-orbitals.
2. BaCl₂ is an ionic salt of a Group 2 metal with no electron-accepting tendency.
3. Hence, the correct option is (B).

Teacher's Note:
a) Check for an incomplete octet or vacant orbitals on the central atom.

10. Which of the following will not change red litmus blue ?
(A) B(OH)₃
(B) NaOH
(C) Ca(OH)₂
(D) Ba(OH)₂

Answer: (A) B(OH)₃

1. NaOH, Ca(OH)₂ and Ba(OH)₂ are bases and turn red litmus blue.
2. B(OH)₃ is boric acid, a weak Lewis acid, and does not give OH⁻ ions itself.
3. Hence, the correct option is (A).

Teacher's Note:
a) Boric acid has OH in its formula but is an acid, not a base.

II. Fill in the blanks

i) Washing soda solution has pH _____________ than 7.

Answer: higher (greater)

Teacher's Note:
a) Washing soda is a basic salt, so its pH is above 7.
b) Basic salts come from a strong base and a weak acid.

ii) When Equal amounts of strong acid and base are mixed, the solution will be _____________.

Answer: neutral

Teacher's Note:
a) Equal equivalents of a strong acid and a strong base give a neutral salt and water.
b) The amounts must be chemically equivalent, not merely equal in volume.

iii) Famous "Dandi March" is related to chemical compound ____________.

Answer: NaCl (common salt)

Teacher's Note:
a) The Dandi March of 1930 was the Salt Satyagraha led by Mahatma Gandhi.
b) The salt involved is sodium chloride.

iv) Sodium bicarbonate is also called _____________________.

Answer: sodium hydrogen carbonate or baking soda

Teacher's Note:
a) The formula is NaHCO₃.
b) Do not confuse it with washing soda, Na₂CO₃.

v) HSO₄⁻ is a conjugate acid of ____________.

Answer: SO₄²⁻

Teacher's Note:
a) A conjugate acid is formed when a base gains one proton.
b) SO₄²⁻ + H⁺ → HSO₄⁻.

III. True / False

i) The pH of neutral water at 90ºC is 7.

Answer: False

Teacher's Note:
a) K_w increases with temperature, so at 90°C neutral water has pH below 7 (about 6.2).
b) Water stays neutral because [H⁺] = [OH⁻], even when pH is not 7.

ii) BF₃ is strong lewis acid than BBr₃.

Answer: False

Teacher's Note:
a) The Lewis acid strength order is BBr₃ > BCl₃ > BF₃.
b) Back-bonding from fluorine lone pairs makes BF₃ the weakest.

iii) NaBH₄ is a complex salt.

Answer: True

Teacher's Note:
a) It contains the complex ion BH₄⁻, like Li(AlH₄) in the notes.
b) Complex salts do not give tests for all their constituent ions.

iv) Organic acids are weak acids generally.

Answer: True

Teacher's Note:
a) Acids such as acetic acid ionise only partly in water.
b) Exceptions exist, such as trichloroacetic acid, which is fairly strong.

v) The blood is slightly alkaline.

Answer: True

Teacher's Note:
a) Normal blood pH is about 7.4.
b) Buffers in blood keep this value nearly constant.

IV. Subjective Type Questions

1. Calculate the pH of 0.1 M NaOH.

Answer:
1. NaOH is a strong base, so [OH⁻] = 0.1 M = 10⁻¹ M.
2. pOH = −log(10⁻¹) = 1
3. pH = 14 − pOH = 14 − 1 = 13

Teacher's Note:
a) For a base, find pOH first and then use pH + pOH = 14.
b) Do not put 0.1 directly into the pH formula; that gives the pH of an acid.

2. What will be the nature of solution obtained by mixing 200 ml of 0.1 M NaOH with 100 ml of 0.2 M H₂SO₄ ?

Answer:
1. Moles of OH⁻ = 0.1 × 0.200 = 0.02 mol
2. Moles of H₂SO₄ = 0.2 × 0.100 = 0.02 mol, and each mole gives 2 mol H⁺, so moles of H⁺ = 0.04 mol
3. H⁺ left after neutralisation = 0.04 − 0.02 = 0.02 mol, which is in excess
4. Hence, the solution is acidic.

Teacher's Note:
a) Compare moles of H⁺ and OH⁻, not just the moles of acid and base.
b) H₂SO₄ is dibasic, so its moles of H⁺ are doubled.

3. Out of SO₃ and SO₂ which is stronger and why ?

Answer:
1. SO₃ is the stronger acid (acidic oxide).
2. Sulphur has oxidation state +6 in SO₃ and +4 in SO₂, so sulphur is more electron-deficient in SO₃.
3. It therefore accepts a pair of electrons more readily and gives the stronger acid H₂SO₄, while SO₂ gives the weaker H₂SO₃.

Teacher's Note:
a) The higher the oxidation state of the central atom, the more acidic the oxide.
b) Compare H₂SO₄ (strong) and H₂SO₃ (weak) to support the answer.

4. Write the reaction for formation of sodium hypophosphite.

Answer: 4P + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃

Teacher's Note:
a) Phosphorus disproportionates in hot NaOH, forming both NaH₂PO₂ and phosphine.
b) Check the balance: 4 P, 3 Na, 6 O and 9 H on each side.

5. Which of the following is a Bronsted-Base ?
I⁻, H₃O⁺, HCl, NH₄⁺

Answer: I⁻

Teacher's Note:
a) A Bronsted base accepts a proton; I⁻ can become HI.
b) H₃O⁺, HCl and NH₄⁺ all donate protons, so they are acids.

6. What are indicators? How does it helps in describing nature of substance?

Answer:
1. Indicators are substances that show different colours in acidic, basic and neutral solutions.
2. Litmus turns red in acid and blue in base; universal indicator gives a range of colours with the strength of acidity or alkalinity.
3. Universal indicator paper shows red for strong acids, orange or yellow for weak acids, green for neutral, turquoise for weak alkali and deep blue for strong alkali.
4. By matching the colour we can tell whether a substance is acidic, basic or neutral and how strong it is.

Teacher's Note:
a) Indicators tell the nature of a substance, while pH gives a number for its strength.
b) Universal indicator is a mixture of indicators.
c) Learn the colour sequence red, orange, yellow, green, turquoise, blue.
d) Always name at least one example, such as litmus or phenolphthalein.

7. Complete the reaction :

Answer:
i) Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
ii) 2CaSO₄.2H₂O → (heat, 373 K) 2CaSO₄.H₂O + 3H₂O
iii) NaCl + NH₃ + H₂O + CO₂ → NaHCO₃ + NH₄Cl
iv) 2NaHCO₃ → (heat) Na₂CO₃ + H₂O + CO₂
v) CuSO₄.5H₂O → (heat) CuSO₄ + 5H₂O
vi) NaOH + HCl → NaCl + H₂O
vii) H₂SO₄ (aq) → H⁺ + HSO₄⁻
viii) H₂SO₄ + pH indicator paper → Red (strong acid)
ix) CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ + H₂O
x) Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O

Teacher's Note:
a) Balance every equation and write the condition (heat or temperature) above the arrow.
b) In (ii) the product is plaster of Paris, CaSO₄.½H₂O, written doubled to avoid the fraction.
c) Al(OH)₃ dissolves in excess NaOH because it is amphoteric.