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State Board · Class 9 · Science and Technology

Lesson 5: Acids, Bases and Salts

5 concepts · 40 practice questions with answers · Free to read

This lesson is also in: मराठी

Acids, bases, salts and Arrhenius theory

From daily life

Lemon juice and tamarind taste sour, while soap feels slippery, and the salt on the dining table is neither. These three kinds of substances, acids, bases and salts, are all around us, and chemists sort them by the ions they give in water. (Never taste or touch chemicals in a laboratory.)

Simple definition

An acid (आम्ल) gives H+ in water. A base (आम्लारी) gives OH− in water. A salt (क्षार) is an ionic compound that has neither H+ nor OH− as its ions.

An acid such as HCl gives H plus ions in water and a base such as NaOH gives OH minus ionsArrhenius: what an acid and a base give in waterAcidHClH₂OH⁺+Cl⁻only cation: H⁺BaseNaOHH₂ONa⁺+OH⁻only anion: OH⁻

In the figure : In the figure an acid gives H⁺ and a base gives OH⁻ in water.

Rule 1: Acids, bases and salts — Compounds with H+ as the basic radical are acids. Compounds with OH− as the acidic radical are bases. Ionic compounds with other basic radicals than H+ and other acidic radicals than OH− are salts.

  • Easy : HCl and H2SO4 are acids; NaOH and Ca(OH)2 are bases; NaCl is a salt.
  • Medium : Classify KOH, HNO3 and K2SO4: base, acid, salt.
  • Exam-type : CaCO3 has Ca2+ (not H+) and CO32− (not OH−), so it is a salt.

Rule 2: Arrhenius theory and dissociation — Arrhenius (Swedish, 1887) said an acid gives H+ as the only cation in water (HCl, H2SO4), and a base gives OH− as the only anion (NaOH, Ca(OH)2). Ionic compounds split into ions in water; this is dissociation (वियोजन), and the ions are written with (aq).

  • Easy : HCl → H+(aq) + Cl−(aq)
  • Medium : NaOH → Na+(aq) + OH−(aq) has the same charge on both sides: 0 = (+1) + (−1).
  • Exam-type : Ca(OH)2 → Ca2+ + 2OH−: charge is 0 = (+2) + 2 × (−1).
Remember

Acid: H⁺. Base: OH⁻. Salt: neither. Ionic compounds dissociate in water.

Common mistake

Thinking every compound with hydrogen is an acid. Water and methane have hydrogen but give no H⁺ as acids do.

Shortcut

Memory clue: "A-H, B-OH": Acid gives H⁺, Base gives OH⁻.

Spot the difference

SubstanceIon given in waterExample
AcidH⁺HCl, H₂SO₄
BaseOH⁻NaOH, Ca(OH)₂
SaltOther ionsNaCl, K₂SO₄

Practice questions

Easy
  1. 1What is an acid according to Arrhenius?
  2. 2What is a base according to Arrhenius?
  3. 3What is dissociation?
Medium
  1. 4Name two acids.
  2. 5Name two bases.
  3. 6Classify NaOH, HNO₃ and KCl.
Challenging
  1. 7Write the dissociation of HNO₃ in water.
  2. 8Write the dissociation of Ca(OH)₂ and show that the total charge is zero.
Show answers (try them yourself first)
  1. 1A substance that gives H⁺ as the only cation in water.
  2. 2A substance that gives OH⁻ as the only anion in water.
  3. 3Splitting of an ionic compound into ions in water.
  4. 4HCl and H₂SO₄ (or HNO₃).
  5. 5NaOH and KOH (or Ca(OH)₂).
  6. 6Base, acid, salt.
  7. 7HNO₃ → H⁺(aq) + NO₃⁻(aq)
  8. 8Ca(OH)₂ → Ca²⁺ + 2OH⁻; (+2) + 2 × (−1) = 0.

Strength, basicity, acidity and concentration

From daily life

A glass of sugar water and a glass of tea may both look dilute, but one tastes far sweeter. Likewise, the sourness of vinegar and of strong laboratory acid are very different. We need words for how strongly an acid acts and how much of it is dissolved. (Concentrated acids burn, so only a teacher handles them.)

Simple definition

A strong acid (तीव्र आम्ल) dissociates almost completely in water; a weak acid (सौम्य आम्ल) only partly. Concentration (संहती) is the amount of solute in a solution.

Rule 1: Strong and weak — Strong acids: HCl, HBr, HNO3, H2SO4. Weak acids: CH3COOH and CO2 (in water). Strong bases: NaOH, KOH, Ca(OH)2. Weak base: NH3. An alkali (विद्राव्य आम्लारी) is a highly soluble base (NaOH, KOH, NH3).

  • Easy : Vinegar contains acetic acid, a weak acid; it is safe in food at normal strength.
  • Medium : Of HCl and CH₃COOH, HCl is strong and CH₃COOH is weak, so HCl gives more H⁺ at equal concentration.
  • Exam-type : NaOH and NH₃ are both alkalis because both dissolve easily in water, but NaOH is strong and NH₃ is weak.

Rule 2: Basicity and acidity — Basicity of an acid is the number of H+ ions one molecule gives. Acidity of a base is the number of OH− ions one molecule gives.

  • Easy : HCl gives 1 H⁺, so its basicity is 1. NaOH gives 1 OH⁻, so its acidity is 1.
  • Medium : H₂SO₄ gives 2 H⁺, so basicity is 2. Ca(OH)₂ gives 2 OH⁻, so acidity is 2.
  • Exam-type : How many H⁺ do 3 molecules of H₂SO₄ give? 3 × 2 = 6 H⁺. How many OH⁻ do 5 units of Ca(OH)₂ give? 5 × 2 = 10 OH⁻.

Rule 3: Concentration and molarity — Concentration is the proportion of solute in a solution, in g/L or as molarity (मोलॅरिटी, mol/L). Molarity = mass ÷ (molar mass × volume in litres).

  • Easy : 117 g of NaCl (molar mass 58.5 g/mol) in 2 L: 117 ÷ 2 = 58.5 g/L, and 58.5 ÷ 58.5 = 1 mol/L = 1 M.
  • Medium : 11.7 g of NaCl in 1 L: 11.7 ÷ 58.5 = 0.2 mol/L.
  • Exam-type : 4 g of NaOH (molar mass 40 g/mol) in 500 mL (0.5 L): 4 ÷ 40 = 0.1 mol, and 0.1 ÷ 0.5 = 0.2 mol/L.
Remember

Strong acids dissociate fully, weak only partly. Basicity = H⁺ per molecule; acidity = OH⁻ per molecule.

Common mistake

Mixing up strength and concentration. A weak acid can be concentrated and a strong acid can be dilute.

Shortcut

Memory clue: "Strong = Splits fully, Weak = Wrings out a little".

Spot the difference

PointStrongWeak
DissociationAlmost completePartial
Acid exampleHCl, H₂SO₄CH₃COOH
Base exampleNaOH, KOHNH₃

Practice questions

Easy
  1. 1Name two strong acids.
  2. 2Name a weak acid and a weak base.
  3. 3What is an alkali?
Medium
  1. 4Define basicity of an acid.
  2. 5What is the basicity of H₂SO₄ and the acidity of NaOH?
  3. 6What is the molarity of 20 g NaOH (molar mass 40 g/mol) in 1 L?
Challenging
  1. 729.25 g of NaCl (58.5 g/mol) is dissolved to make 500 mL. Find the molarity.
  2. 8Explain with an example why a weak acid need not be a dilute acid.
Show answers (try them yourself first)
  1. 1HCl and H₂SO₄ (or HNO₃).
  2. 2CH₃COOH and NH₃.
  3. 3A base that dissolves highly in water, like NaOH or KOH.
  4. 4The number of H⁺ ions given by one molecule.
  5. 5Basicity of H₂SO₄ = 2; acidity of NaOH = 1.
  6. 620 ÷ 40 = 0.5 mol/L
  7. 729.25 ÷ 58.5 = 0.5 mol; 0.5 ÷ 0.5 = 1 mol/L.
  8. 8Strength is about dissociation; concentration is about how much is dissolved. Concentrated acetic acid is weak yet concentrated.

pH scale and indicators

From daily life

A doctor says blood is slightly basic, a dentist warns about sour drinks, and a gardener tests soil with a kit. They all use one scale to say how acidic or basic something is, the pH scale.

Simple definition

The pH scale (pH मापनी) runs from 0 to 14 and tells how acidic or basic a solution is.

A coloured pH scale from 0 to 14 with lemon juice, vinegar, water, blood, baking soda and caustic soda placed on itThe pH scale01234567891011121314lemon juice 2.5vinegar 3pure water 7blood 7.4baking soda 9caustic soda 13◄ more acidicneutral 7more basic ►

In the figure : In the figure acids are below 7, bases are above 7 and pure water is exactly 7.

Rule 1: The pH scale — Devised by Sorensen (1909). Pure water has pH 7 ([H+] = 1 × 10−7 mol/L at 25 °C); below 7 is acidic, above 7 is basic. Some values: lemon juice 2.5, vinegar 3.0, blood 7.4, baking soda 8.5, milk of magnesia 10.5, limewater 11.

  • Easy : Vinegar (pH 3.0) is acidic; baking soda (pH 8.5) is basic.
  • Medium : Order from most acidic to most basic: lemon juice 2.5, vinegar 3.0, blood 7.4, baking soda 8.5, limewater 11.
  • Exam-type : Each pH step is a factor of 10 in H⁺. A solution of pH 3 has 10 times the H⁺ of pH 4 and 100 times that of pH 5: 10⁻³ ÷ 10⁻⁵ = 100.

Rule 2: Indicators — A universal indicator (सार्वत्रिक दर्शक), which is a mixture of indicators, or pH paper shows colour changes with pH. The most accurate method is a pH meter.

  • Easy : pH paper turns a colour that you match with a chart.
  • Medium : A pH meter gives a number like 6.8, so it is more exact than a colour.
  • Exam-type : Rainwater of pH 5.6 is slightly acidic as pH is below 7, yet it is much less acidic than lemon juice (pH 2.5).
Live figure

pH scale and indicators

In the app you can move this figure with your finger.

Open in the app
Remember

pH 7 neutral; below 7 acid; above 7 base. Universal indicator and pH paper show colour; pH meter is most accurate.

Common mistake

Thinking pH 0 is neutral or pH 14 is an acid. The scale goes acid → neutral → base.

Shortcut

Memory clue: "7 is Heaven (neutral); below = Acid, above = Alkali".

Spot the difference

SolutionpHNature
Lemon juice2.5Acidic
Pure water7Neutral
Baking soda8.5Basic
Limewater11Basic

Practice questions

Easy
  1. 1What is the range of the pH scale?
  2. 2What is the pH of pure water at 25 °C?
  3. 3Who devised the pH scale and when?
Medium
  1. 4Is blood (pH 7.4) acidic or basic?
  2. 5Name the most accurate way to measure pH.
  3. 6Which is more acidic, vinegar (3.0) or lemon juice (2.5)?
Challenging
  1. 7How many times more H⁺ does a pH 2 solution have than a pH 4 solution?
  2. 8A solution turns pH paper to a value of 9. Is it acid or base, and can it be pure water? Explain.
Show answers (try them yourself first)
  1. 10 to 14
  2. 27
  3. 3Sorensen, 1909.
  4. 4Slightly basic.
  5. 5A pH meter.
  6. 6Lemon juice, as its pH is lower.
  7. 710⁻² ÷ 10⁻⁴ = 100 times
  8. 8Base, because 9 is above 7. It cannot be pure water, which has pH 7.

Reactions of acids and bases

From daily life

When you feel acidity in your stomach, a doctor may advise an antacid tablet. The tablet is a mild base, and it cancels the extra acid. Also, a fizzy effervescent tablet gives off bubbles of gas when it meets water. These are two reactions of acids and bases.

Simple definition

Neutralisation (उदासीनीकरण) is the reaction of an acid with a base to give a salt and water.

An acid and a base react to give a salt and waterNeutralisationHCl(acid)+ NaOH(base)NaCl(salt)+ H₂O(water)

In the figure : In the figure acid + base gives salt + water.

Rule 1: Neutralisation — Acid + base → salt + water. Example: HCl + NaOH → NaCl + H2O. The net ionic reaction is H+ + OH− → H2O.

  • Easy : HNO3 + KOH → KNO3 + H2O.
  • Medium : H2SO4 + 2NaOH → Na2SO4 + 2H2O has H 4, S 1, O 6, Na 2 on both sides.
  • Exam-type : 1 mol HCl needs 1 mol NaOH, and 1 mol H₂SO₄ needs 2 mol NaOH, so 0.5 mol H₂SO₄ needs 1 mol NaOH.

Rule 2: Other reactions — Acid + metal → salt + hydrogen (Mg + 2HCl → MgCl2 + H2). Metal oxide + acid → salt + water (metal oxides are basic). Nonmetal oxide + base → salt + water (nonmetal oxides are acidic). Carbonate or bicarbonate + acid → salt + CO2 + water; CO2 turns limewater milky (CaCO3).

  • Easy : Zn + 2HCl → ZnCl2 + H2: hydrogen bubbles appear.
  • Medium : CaCO3 + 2HCl → CaCl2 + H2O + CO2 has Ca 1, C 1, O 3, H 2, Cl 2 on both sides.
  • Exam-type : CuO + H2SO4 → CuSO4 + H2O gives a blue solution; the metal oxide acts as a base.

Rule 3: Amphoteric oxides — Oxides such as Al2O3 and ZnO react with both acids and bases, so they are amphoteric (उभयधर्मी).

  • Easy : Al2O3 and ZnO are amphoteric.
  • Medium : CuO reacts only with acids, but ZnO reacts with both acids and bases.
  • Exam-type : A test to know if an oxide is amphoteric: it dissolves in an acid and also in a base.
Remember

Neutralisation: acid + base → salt + water. Metal + acid → salt + H₂. Carbonate + acid → CO₂.

Common mistake

Forgetting to balance equations like H₂SO₄ + 2NaOH. Two OH⁻ are needed.

Shortcut

Memory clue: "Acid + Base = Salt + Water (SW)" and "CO₂ makes limewater milky".

Spot the difference

ReactionProducts
Acid + baseSalt + water
Acid + metalSalt + hydrogen
Acid + carbonateSalt + CO₂ + water
Acid + metal oxideSalt + water

Practice questions

Easy
  1. 1What is neutralisation?
  2. 2Write the net ionic reaction of neutralisation.
  3. 3Which gas is formed when an acid reacts with a metal?
Medium
  1. 4How can you test the CO₂ formed in a reaction?
  2. 5Name two amphoteric oxides.
  3. 6Complete and balance: Mg + HCl → ?
Challenging
  1. 7Complete and balance: H₂SO₄ + NaOH → ?
  2. 8Why does an antacid tablet relieve acidity in the stomach? Give the type of reaction.
Show answers (try them yourself first)
  1. 1The reaction of an acid and a base to give a salt and water.
  2. 2H⁺ + OH⁻ → H₂O
  3. 3Hydrogen.
  4. 4Pass it through limewater; it turns milky.
  5. 5Al₂O₃ and ZnO.
  6. 6Mg + 2HCl → MgCl₂ + H₂
  7. 7H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
  8. 8The tablet is a mild base that neutralises the extra acid; it is a neutralisation reaction.

Types of salts, water of crystallisation and conductivity

From daily life

Table salt tastes neutral, while washing soda feels slippery and an alum piece used to clear muddy water is slightly sour. Different salts behave differently in water, because they come from different acids and bases.

Simple definition

A neutral salt comes from a strong acid and a strong base. An acidic salt comes from a strong acid and a weak base. A basic salt comes from a weak acid and a strong base.

Neutral, acidic and basic salts and how they formThree kinds of saltsNeutral saltstrong acid +strong basee.g. NaCl, pH 7Acidic saltstrong acid +weak basepH below 7Basic saltweak acid +strong basepH above 7

In the figure : In the figure a salt is neutral, acidic or basic depending on the acid and base it comes from.

Rule 1: Types of salts — Neutral salt: strong acid + strong base, pH 7. Acidic salt: strong acid + weak base, pH below 7. Basic salt: weak acid + strong base, pH above 7.

  • Easy : NaCl (HCl + NaOH, both strong) is a neutral salt.
  • Medium : NH₄Cl comes from HCl (strong) and NH₃ (weak), so it is acidic.
  • Exam-type : CH₃COONa comes from CH₃COOH (weak) and NaOH (strong), so its solution has pH above 7.

Rule 2: Water of crystallisation and conductivity — Water of crystallisation (स्फटिकजल) is the fixed number of water molecules in a crystal: CuSO4·5H2O (blue vitriol, turns white on heating), FeSO4·7H2O, Na2CO3·10H2O, alum. Ions conduct electricity in solution, so solutions of salts, strong acids and strong bases conduct. The cathode is joined to the negative terminal and the anode to the positive terminal.

  • Easy : Blue CuSO₄·5H₂O turns white CuSO₄ on heating as it loses its water of crystallisation.
  • Medium : In CuSO₄·5H₂O there are 5 water molecules for every CuSO₄; in 3 units there are 3 × 5 = 15.
  • Exam-type : Pure sugar solution does not conduct, but salt water does, because salt gives ions and sugar does not.
Remember

Neutral: strong + strong. Acidic: strong acid + weak base. Basic: weak acid + strong base.

Common mistake

Thinking all salts are neutral. Only a salt of a strong acid and a strong base is.

Shortcut

Memory clue: "Whoever is stronger wins": strong acid → acidic salt, strong base → basic salt.

Spot the difference

SaltAcid usedBase usedNature
NaClHCl (strong)NaOH (strong)Neutral
NH₄ClHCl (strong)NH₃ (weak)Acidic
CH₃COONaCH₃COOH (weak)NaOH (strong)Basic

Practice questions

Easy
  1. 1What kind of salt comes from a strong acid and a strong base?
  2. 2What is the nature of a salt from a strong acid and a weak base?
  3. 3What is water of crystallisation?
Medium
  1. 4What is the colour change when blue vitriol is heated?
  2. 5Why do salt solutions conduct electricity?
  3. 6Which terminal is the cathode joined to?
Challenging
  1. 7Classify Na₂CO₃ as acidic, basic or neutral given that it comes from H₂CO₃ (weak) and NaOH (strong).
  2. 8Why does a solution of NH₄Cl turn blue litmus red while NaCl does not change it?
Show answers (try them yourself first)
  1. 1A neutral salt (pH 7).
  2. 2Acidic (pH below 7).
  3. 3The fixed number of water molecules present in a crystal.
  4. 4Blue to white.
  5. 5They contain free ions.
  6. 6The negative terminal.
  7. 7Basic, since weak acid + strong base.
  8. 8NH₄Cl is an acidic salt (strong acid HCl + weak base NH₃), while NaCl is neutral.

Quick revision

  • Acid: H+; base: OH-; salt: other ions.
  • pH 7 neutral; less than 7 acid; more than 7 base.
  • Acid + base gives salt + water.