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Acids, Bases and Salts

NCERT Class 10 · Science Based on NCERT Class 10 Science textbook · Free CBSE study kit

Chapter Notes

**ACIDS, BASES AND SALTS — COMPLETE CHEAT SHEET**

**SECTION 1: DEFINITION & IDENTIFICATION**

• **Acid**: A substance that is sour in taste, turns blue litmus paper red, and conducts electricity in aqueous solution

• **Base**: A substance that is bitter in taste, turns red litmus paper blue, and conducts electricity in aqueous solution

• **Salt**: A compound formed when an acid reacts with a base; composed of a metal cation and a non-metal anion

**DO NOT CONFUSE**: Litmus is purple when neutral. Red litmus → acid turns it red. Blue litmus → base turns it blue.

**Types of Indicators**:

• **Natural Indicators**: Litmus (purple dye from lichen), turmeric (yellow → reddish-brown in base), red cabbage leaves, flower petals (Hydrangea, Petunia, Geranium)

• **Synthetic Indicators**: Methyl orange (red in acid, yellow in base), phenolphthalein (colorless in acid, pink in base)

• **Olfactory Indicators**: Substances whose odor changes in acidic/basic media. Example: Onion (odor disappears in both acid and base)

**Key Property Used in Remedies**: Acids and bases neutralize each other's effects. Example: Baking soda (base) relieves acidity (excess acid in stomach)

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**SECTION 2: REACTIONS OF ACIDS WITH METALS**

**General Pattern**: Acid + Metal → Salt + Hydrogen gas (↑)

**Examples with Chemical Equations**:

1. **Dilute Sulphuric Acid + Zinc**:

H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g) ↑

2. **Dilute Hydrochloric Acid + Zinc**:

2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g) ↑

3. **Dilute Nitric Acid + Zinc**:

2HNO₃(aq) + Zn(s) → Zn(NO₃)₂(aq) + H₂(g) ↑

4. **Acetic Acid + Zinc**:

2CH₃COOH(aq) + Zn(s) → (CH₃COO)₂Zn(aq) + H₂(g) ↑

**Test for Hydrogen Gas**: Bring a burning candle near the gas bubble → a "pop" sound is heard (hydrogen burns with a distinctive pop)

**Important**: Not all metals react with acids. Reactivity depends on metal's position in the reactivity series. More reactive metals displace hydrogen more readily.

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**SECTION 3: REACTIONS OF BASES WITH METALS**

**Pattern**: Only certain metals (like Zn, Al) react with bases. Most metals do NOT react with bases.

**Example with Chemical Equation**:

**Sodium Hydroxide + Zinc** (with heat):

2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g) ↑

(Sodium zincate)

**Key Point**: Hydrogen gas is again produced (just like with acids), but bases react with only specific metals. This is why bases cannot be used universally as acid remedies through metal reactions.

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**SECTION 4: REACTIONS OF METAL CARBONATES & HYDROGENCARBONATES WITH ACIDS**

**General Pattern**: Metal Carbonate/Hydrogencarbonate + Acid → Salt + Water + Carbon Dioxide gas (↑)

**Examples with Chemical Equations**:

1. **Sodium Carbonate + Hydrochloric Acid**:

Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) ↑

2. **Sodium Hydrogencarbonate + Hydrochloric Acid**:

NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g) ↑

**Test for Carbon Dioxide Gas**: Pass the evolved gas through lime water [Ca(OH)₂(aq)]:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

**Observation**: Lime water turns white/milky (white precipitate of calcium carbonate forms). If excess CO₂ is passed, the white precipitate dissolves forming soluble calcium hydrogencarbonate: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

**DO NOT CONFUSE**: Carbonate produces CO₂ readily; hydrogencarbonate also produces CO₂ but is less stable. Both produce the same final products with acids.

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**SECTION 5: NEUTRALIZATION REACTIONS (ACID-BASE REACTIONS)**

**Definition**: A reaction in which an acid reacts with a base to form salt and water. H⁺ ions from acid combine with OH⁻ ions from base.

**General Equation**: Acid + Base → Salt + Water

**Examples with Chemical Equations**:

1. **Hydrochloric Acid + Sodium Hydroxide**:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

2. **Sulphuric Acid + Potassium Hydroxide**:

H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

3. **Hydrochloric Acid + Calcium Hydroxide**:

2HCl(aq) + Ca(OH)₂(aq) → CaCl₂(aq) + 2H₂O(l)

**Indicator Behavior During Neutralization** (Using Phenolphthalein):

  • Initial: Base solution is pink (phenolphthalein in basic medium)
  • As acid is added drop by drop: Pink color gradually fades
  • Endpoint: Color becomes colorless when acid and base completely neutralize each other
  • If more NaOH is added again to the neutralized solution: Pink color reappears (basic condition restored)

    • **Cause-Effect Chain of Neutralization**:

    Acid provides H⁺ ions → Base provides OH⁻ ions → H⁺ + OH⁻ → H₂O (water formed) → Original properties of both acid and base disappear → New salt formed with different properties

    **Real-Life Applications**:

    • Treating acidity: Antacids (bases) neutralize excess stomach acid

    • Treating basic burns: Weak acids neutralize base on skin

    • Soil treatment: Adding lime (base) to acidic soil; adding sulfur to basic soil

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    **SECTION 6: KEY DIFFERENCES & DO NOT CONFUSE ALERTS**

    **Acids vs Bases**:

    | Property | Acid | Base |

    | Taste | Sour | Bitter |

    | Litmus (Red) | Red litmus stays red | Red litmus → blue |

    | Litmus (Blue) | Blue litmus → red | Blue litmus stays blue |

    | Metal Reaction | Produces H₂ gas | Only certain metals produce H₂ |

    | Metal Carbonate | Produces CO₂ gas | Does NOT produce CO₂ |

    | Phenolphthalein | Colorless | Pink |

    | Methyl Orange | Red | Yellow |

    **DO NOT CONFUSE**:

    • Neutral solution (pH = 7) is NOT the same as absence of acid/base — it contains both H⁺ and OH⁻ in equal amounts

    • Natural indicators show color change based on pH; they don't "create" acidity/basicity

    • Metal carbonates differ from metal hydrogencarbonates chemically but both produce CO₂ with acids

    • Hydrogen gas from metal-acid reaction ≠ Hydrogen gas from metal-base reaction (though both exist)

    • A compound can be salt without being sodium chloride (NaCl) — salts include ZnSO₄, K₂SO₄, CaCl₂, etc.

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    **SECTION 7: SUMMARY OF IMPORTANT CHEMICAL EQUATIONS**

    **Metal + Acid**: 2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g) ↑

    **Metal + Base**: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g) ↑

    **Metal Carbonate + Acid**: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) ↑

    **Metal Hydrogencarbonate + Acid**: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g) ↑

    **CO₂ + Lime Water**: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) [White precipitate]

    **Excess CO₂ + Lime Water Precipitate**: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) [Precipitate dissolves]

    **Acid + Base (Neutralization)**: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

    MCQs — 10 Questions with Answers

    Q1. A student adds zinc granules to dilute hydrochloric acid in a test tube and observes vigorous bubbling. When a burning candle is brought near the gas collected, it burns with a 'pop' sound. What does this observation indicate about the product formed?

    • A. Hydrogen gas is produced because the metal displaces hydrogen from the acid ✓
    • B. Oxygen gas is produced because the acid oxidizes the metal
    • C. Carbon dioxide gas is produced from the reaction between acid and carbonate impurities
    • D. Chlorine gas is produced from decomposition of hydrochloric acid

    Answer: A — The 'pop' sound is the characteristic test for hydrogen gas; zinc displaces hydrogen from HCl to form zinc chloride and H₂, not oxygen (no burning candle test) or CO₂ (no pop sound with burning candle).

    Q2. When red litmus paper is dipped into an unknown solution and turns blue, while blue litmus paper shows no change, which conclusion can be drawn?

    • A. The solution is basic because red litmus turns blue only in basic conditions ✓
    • B. The solution is acidic because it affects one type of litmus paper
    • C. The solution is neutral because only one litmus changes color
    • D. The solution is a salt because it turns red litmus blue

    Answer: A — Red litmus turning blue is the definitive test for a basic solution; blue litmus turning red would indicate acidic conditions, so no change in blue litmus confirms basicity, not neutrality or acidity.

    Q3. Assertion (A): When granulated zinc is added to sodium hydroxide solution and the mixture is warmed, hydrogen gas is evolved. Reason (R): Bases react with all metals to produce hydrogen gas and a salt. Choose the correct option:

    • A. Both A and R are true and R is the correct explanation of A
    • B. Both A and R are true but R is not the correct explanation of A
    • C. A is true but R is false ✓
    • D. A is false but R is true

    Answer: C — The assertion is true (zinc reacts with NaOH to produce H₂ and sodium zincate), but the reason is false because bases do not react with all metals—only amphoteric metals like zinc and aluminum react with bases to produce hydrogen.

    Q4. A homeowner uses turmeric powder solution to test whether the dishwashing liquid is basic or acidic. The solution changes from yellow to reddish-brown. What does this color change indicate?

    • A. The dishwashing liquid is basic, making turmeric an effective indicator in basic media ✓
    • B. The dishwashing liquid is acidic, and turmeric only works in acidic conditions
    • C. The dishwashing liquid is neutral, and turmeric changes color at neutral pH
    • D. The turmeric has decomposed due to the temperature of the solution

    Answer: A — Turmeric turns reddish-brown in basic solutions; dishwashing liquids are indeed basic (alkaline), making this a correct application of natural indicators to identify basic substances in daily life.

    Q5. Assertion (A): Litmus solution is purple in color when it is in a neutral solution. Reason (R): Litmus changes color only in the presence of acids or bases, not in neutral solutions. Choose the correct option:

    • A. Both A and R are true and R is the correct explanation of A ✓
    • B. Both A and R are true but R is not the correct explanation of A
    • C. A is true but R is false
    • D. A is false but R is true

    Answer: A — Litmus solution is indeed purple in neutral conditions; this occurs precisely because litmus only changes color (to red in acid, blue in base) when pH deviates from neutral, making the reason the correct explanation.

    Q6. Two cloth strips soaked in onion juice show different odors when exposed to dilute HCl and dilute NaOH respectively. After rinsing with water, both strips regain their original onion smell. What does this observation reveal about onion as an olfactory indicator?

    • A. Onion can be used as an olfactory indicator because its odor is reversibly modified in acidic and basic solutions ✓
    • B. Onion cannot be used as an olfactory indicator because the odor change is temporary
    • C. Onion is a permanent indicator because the odor change never returns to normal
    • D. Onion only works as an indicator in acidic solutions, not basic ones

    Answer: A — The reversible change in odor upon exposure to acids/bases and recovery after rinsing demonstrates that onion is an effective olfactory indicator; permanence is not required for indicator use, only detectable change upon reaction.

    Q7. Assertion (A): When carbon dioxide gas is passed through lime water, a white precipitate forms, but if excess carbon dioxide is continued to be passed, the precipitate dissolves. Reason (R): Calcium carbonate is insoluble in water, but calcium bicarbonate formed by excess CO₂ is soluble. Choose the correct option:

    • A. Both A and R are true and R is the correct explanation of A ✓
    • B. Both A and R are true but R is not the correct explanation of A
    • C. A is true but R is false
    • D. A is false but R is true

    Answer: A — Both statements are true: Ca(OH)₂ + CO₂ → CaCO₃ (white ppt) + H₂O, and with excess CO₂, CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (soluble); the reason correctly explains why the precipitate dissolves.

    Q8. A student performs Activity 2.3 using three different acids: dilute HCl, dilute H₂SO₄, and dilute CH₃COOH. In all three cases, zinc granules produce bubbles that pop when a burning candle is brought near. What conclusion about acid strength can be drawn from identical observations?

    • A. All three acids have the same strength because they all produce hydrogen gas with zinc
    • B. Strong acids (HCl and H₂SO₄) are more reactive than weak acids (CH₃COOH), though all produce hydrogen
    • C. The observation alone cannot determine acid strength; identical gas production does not indicate identical reactivity rates ✓
    • D. Weak acids like acetic acid do not produce hydrogen gas, so the observation contradicts the results

    Answer: C — While all three acids produce hydrogen gas (same product), the rate and vigor of the reaction differ based on acid strength; observation of gas production alone is insufficient to rank acid strengths without measuring reaction rates.

    Q9. Assertion (A): Phenolphthalein is colorless in acidic solutions and pink in basic solutions. Reason (R): Phenolphthalein is a synthetic indicator that responds to pH changes because its molecular structure changes in different pH environments. Choose the correct option:

    • A. Both A and R are true and R is the correct explanation of A ✓
    • B. Both A and R are true but R is not the correct explanation of A
    • C. A is true but R is false
    • D. A is false but R is true

    Answer: A — The assertion correctly describes phenolphthalein's color changes; the reason correctly explains that structural changes in the indicator molecule at different pH values cause the color shift from colorless to pink.

    Q10. A laboratory assistant accidentally mixed sodium carbonate and sodium bicarbonate solutions. When dilute HCl is added to the mixture, vigorous bubbling occurs and gas is evolved. After collecting the gas and passing it through lime water, a white precipitate forms. What is the gas produced and why does lime water confirm its identity?

    • A. Hydrogen gas is produced because metals in carbonates displace hydrogen from HCl
    • B. Carbon dioxide gas is produced; lime water test confirms it because CO₂ reacts with Ca(OH)₂ to form white CaCO₃ precipitate ✓
    • C. Oxygen gas is produced from decomposition of the carbonates at room temperature
    • D. Chlorine gas is produced from the reaction between HCl and carbonate salts

    Answer: B — Both Na₂CO₃ and NaHCO₃ react with HCl to produce CO₂; the white precipitate with lime water (Ca(OH)₂ → CaCO₃) is the classic confirmatory test for CO₂, not H₂, O₂, or Cl₂.

    Flashcards

    What do you observe when a burning candle is brought near H₂ gas produced from metal + acid?

    The candle burns with a pop sound because H₂ gas is flammable.

    Why does phenolphthalein turn pink in a basic solution?

    Phenolphthalein is a synthetic indicator that changes color in the presence of OH⁻ ions (bases).

    Write the general reaction equation for metal + acid.

    Acid + Metal → Salt + Hydrogen gas (e.g., HCl + Zn → ZnCl₂ + H₂).

    What is the difference between a natural and synthetic acid-base indicator?

    Natural indicators like turmeric and litmus come from plants, while synthetic indicators like phenolphthalein and methyl orange are chemically manufactured.

    When Na₂CO₃ reacts with dilute HCl, what gas is produced and how is it identified?

    Carbon dioxide gas (CO₂) is produced; it turns lime water milky white as a positive test.

    Why does zinc react with both dilute HCl and dilute NaOH to produce H₂ gas?

    Zinc is amphoteric—it reacts with acids to displace H⁺ ions and with bases to displace H⁻ ions, both producing H₂ gas.

    What does an olfactory indicator do?

    An olfactory indicator changes its odor in acidic or basic media (e.g., onion smell changes in acid/base).

    If you have only red litmus paper and three unknown solutions (acid, base, water), how will you identify each?

    The acid will turn red litmus red, the base will NOT change red litmus, and water will NOT change red litmus (same as base result, so test base with blue litmus separately).

    What happens to the color of turmeric stain when basic soap is scrubbed on it?

    Turmeric stain turns reddish-brown in basic solution and returns to yellow when washed with water.

    Write the equation for the reaction of sodium hydroxide with zinc metal.

    2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g) (sodium zincate is formed).

    Important Board Questions

    What do you observe when dilute hydrochloric acid is added to zinc granules? Write the equation for this reaction and name the gas produced. [2 marks]

    Observe bubbling/effervescence on zinc surface; identify the gas using burning candle (pop sound means H₂); write Metal + Acid → Salt + H₂.

    Explain why a student who has heartburn (excess stomach acid) is advised to take a spoonful of baking soda solution instead of lemon juice. Write the relevant chemical equation. [3 marks]

    Baking soda is basic and neutralizes excess acid; lemon juice is acidic and worsens the problem; use Acid + Base → Salt + H₂O equation (NaHCO₃ + HCl → NaCl + H₂O + CO₂).

    You are provided with three test tubes containing: (A) dilute HCl, (B) dilute NaOH, and (C) distilled water. Using only phenolphthalein solution, explain how you would identify each solution. Also describe what happens when the solution from test tube A is added dropwise to the solution from test tube B. [5 marks]

    Phenolphthalein turns pink in base (B) only; colorless in acid (A) and water (C); when A is added to B, neutralization occurs—pink color fades as OH⁻ ions are consumed by H⁺ ions, forming salt and water; write the neutralization equation.

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