PAGE 18 – Activity Question

Q 1. You have been provided with three test tubes… identify the contents using only red litmus paper.

Answer:

• Dip red litmus paper into each solution.

• Solution which turns red litmus blue is a base.

• Solution which does not change colour could be water or acid.

• Now dip the same strip (already blue) into the two remaining test tubes:

  • If blue litmus turns red → acidic solution

  • If no change → distilled water

Explanation:
Acids turn blue litmus red, bases turn red litmus blue, and neutral water shows no change.

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PAGE 22 – Short Questions

Q1. Why should curd and sour substances not be kept in brass or copper vessels?

Answer:
Curd and sour foods contain acids which react with copper or brass (metals) forming toxic salts that may cause food poisoning.
This is due to metal reacting with acid to form salt + hydrogen gas.

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Q2. Which gas is usually liberated when acid reacts with metal? How do you test it?

Answer:
Hydrogen gas is liberated.
Test: Bring a burning splinter near the gas bubbles. It burns with a ‘pop’ sound.
Reaction Example:

$${\text{Zn + H}}_2{\text{SO}}_4\to {\text{ZnSO}}_4+{\text{H}}_2\uparrow$$

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Q3. Metal compound A reacts with HCl producing a gas that extinguishes flame; CO₂ is produced. Write balanced equation.

Answer:

$${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{CO}}_2+{\text{H}}_2\text{O}$$

Explanation: Gas formed turns lime water milky → indicates CO₂.

PAGE 25 – pH Section Questions

Q1. Why do HCl, HNO₃ show acidic character in aqueous solution but glucose & alcohol do not?

Answer:

HCl and HNO₃ show acidic character in aqueous solution because they ionise in water and produce hydrogen ions (H⁺ / H₃O⁺).
These free ions are responsible for acidic properties.

Example:

$$\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$$
$${\text{HNO}}_3+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{NO}}_3^{-}$$

Q2. Why does an aqueous solution of an acid conduct electricity?

Answer:
An aqueous acid conducts electricity because acids dissociate in water to produce ions, particularly hydrogen ions (H⁺ / H₃O⁺). These ions are charged particles that carry electric current through the solution.

Example:

$$\text{HCl (aq)}\to {\text{H}}^{+}(\text{aq})+{\text{Cl}}^{-}(\text{aq})$$

Explanation:
Electric current in a solution is carried by free-moving ions. When an acid dissolves in water, it breaks into positive and negative ions, allowing electricity to flow.
That is why the bulb glows in acid solution during the conductivity test.

Q3. Why does dry HCl gas not change the colour of dry litmus paper?

Answer:
Dry HCl gas does not change the colour of dry litmus paper because it does not produce hydrogen ions (H⁺) in the absence of water.
Acids show acidic behaviour only when they ionise in water.

$$\text{HCl (gas)}\text{does not ionise}\Rightarrow {\text{No H}}^{+}\text{ions}\Rightarrow \text{No acidic property}$$

When moisture (water) is present:

$$\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$$

These H⁺ (H₃O⁺) ions are responsible for colour change of litmus.

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Conclusion

Q4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer:
While diluting an acid, acid must always be added slowly to water because the process of mixing acid with water is highly exothermic (releases a lot of heat).
If water is added directly to a concentrated acid, the heat produced may cause the mixture to splash out suddenly, which can cause severe burns or break the container.

However, when acid is added gradually to a larger volume of water, the heat gets absorbed safely by water and no splashing occurs.

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Explanation

$$\text{Acid + Water → Heat released (exothermic)}$$

 

Q5. How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?

Answer:
When a solution of an acid is diluted by adding water, the concentration of hydronium ions (H₃O⁺) decreases.
This happens because the number of H₃O⁺ ions per unit volume reduces as more water molecules are added, spreading the ions over a larger volume.

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Explanation

$$\text{Acid}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+\text{ions}$$

When water is added:

• The total volume increases

• The number of H₃O⁺ ions remains the same

• Therefore, ionic concentration (H₃O⁺ per unit volume) decreases

Q6. How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?

Answer:
When excess base is added to a solution of sodium hydroxide (NaOH), the concentration of hydroxide ions (OH⁻) increases because more OH⁻ ions are released into the solution.

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Explanation

Sodium hydroxide is a strong base, and it dissociates completely in water:

$$\text{NaOH (aq)}\to {\text{Na}}^{+}+{\text{OH}}^{-}$$

If more base (e.g., NaOH, KOH) is added, the solution gets additional OH⁻ ions, increasing the strength of alkalinity.

Page No. 28 – Answers

Q1. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8.
Which solution has more hydrogen ion concentration? Which of these is acidic and which one is basic?

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Answer:

• Solution A (pH 6) has more hydrogen ion (H⁺) concentration than solution B (pH 8).

• Solution A is acidic (pH < 7).

• Solution B is basic (pH > 7).

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Explanation

The pH scale indicates acidity or alkalinity of a solution.

• Lower pH value → higher H⁺ ion concentration → stronger acidity

• Higher pH value → lower H⁺ ion concentration → stronger basicity

Therefore:

$$\text{pH 6 (A)}<\text{pH 8 (B)}\Rightarrow [{\text{H}}^{+}]\text{in A}>[{\text{H}}^{+}]\text{in B}$$Solution A (pH 6) has more hydrogen ions and is acidic. Solution B (pH 8) has fewer hydrogen ions and is basic.

Q2. What effect does the concentration of H⁺(aq) ions have on the nature of the solution?

Answer:
The nature of a solution depends on the concentration of H⁺(aq) ions:

• Higher concentration of H⁺ ions makes the solution more acidic.

• Lower concentration of H⁺ ions makes the solution less acidic (or more basic).

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Explanation

On the pH scale:

$$\text{Higher }[{\text{H}}^{+}]\Rightarrow \text{Lower pH}\Rightarrow \text{Stronger acid}$$
$$\text{Lower }[{\text{H}}^{+}]\Rightarrow \text{Higher pH}\Rightarrow \text{Weaker acid / Basic}$$

Q3. Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?

Answer:

Yes, basic solutions also contain H⁺(aq) ions, but they are basic because the concentration of hydroxide ions (OH⁻)is much higher than the concentration of hydrogen ions (H⁺).
The excess OH⁻ ions make the solution alkaline.

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Explanation with Equations

Water undergoes slight ionisation:

$${\text{H}}_2\text{O}\rightleftharpoons {\text{H}}^{+}+{\text{OH}}^{-}$$

When a base like sodium hydroxide dissolves in water:

$$\text{NaOH (s)}\stackrel{{\text{H}}_2\text{O}}{\to }{\text{Na}}^{+}+{\text{OH}}^{-}$$

This produces a large amount of OH⁻ ions.
So, although a small amount of H⁺ is always present (from water ionisation), the solution remains basic because:

$$[{\text{OH}}^{-}]>[{\text{H}}^{+}]$$

Q4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Answer:
A farmer would treat the soil with quick lime (CaO), slaked lime (Ca(OH)₂), or chalk (CaCO₃) when the soil is too acidic (low pH).

These substances are basic in nature and neutralise excess acids present in the soil, helping to maintain the optimal pH required for plant growth.

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Explanation

When the soil becomes acidic due to excessive use of chemical fertilisers or acid rain, plant growth is hampered.
Adding lime compounds increases the pH of the soil, making it less acidic and more suitable for agriculture.

Neutralisation reaction example:

$${\text{Ca(OH)}}_2+2\text{HCl}\to {\text{CaCl}}_2+2{\text{H}}_2\text{O}$$
$${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{CO}}_2+{\text{H}}_2\text{O}$$

PAGE 33 – Answers

Q1. What is the common name of the compound Ca(ClO)₂?

Answer:
The common name of Ca(ClO)₂ is Bleaching Powder.

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Q2. Name the substance which on treatment with chlorine yields bleaching powder.

Answer:
Slaked lime (Calcium hydroxide, Ca(OH)₂)

Reaction:

$$2{\text{Ca(OH)}}_2+2{\text{Cl}}_2\to {\text{Ca(ClO)}}_2+{\text{CaCl}}_2+2{\text{H}}_2\text{O}$$

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Q3. Name the sodium compound which is used for softening hard water.

Answer:
Washing soda (Sodium carbonate – Na₂CO₃ · 10H₂O)

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Q4. What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved.

Answer:
On heating, sodium hydrogencarbonate decomposes to form sodium carbonate, carbon dioxide and water.

$$2{\text{NaHCO}}_3\stackrel{\text{Heat}}{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$$

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Q5. Write an equation to show the reaction between Plaster of Paris and water.

Answer:

$${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+\frac{3}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$$