This MCQ module is based on: Acids and Bases in Water and the pH Scale
TOPIC 6 OF 50
Acids and Bases in Water and the pH Scale
🎓 Class 10
Science
CBSE
Theory
Ch 2 — Acids, Bases and Salts
⏱ ~25 min
🌐 Language: [gtranslate]
🧠 AI-Powered MCQ Assessment
▲
Myaischool Lt Science Assessment
▲
[myaischool_lt_science_assessment grade_level="class_10" science_domain="chemistry" difficulty="intermediate"]
2.2 What Do All Acids and All Bases Have in Common?
We have seen that acids react with metals, bases, and carbonates. But what is the fundamental chemical reason behind the acidic behaviour? The answer lies in what happens when acids dissolve in water.
Acids Produce H+ Ions in Water
When an acid dissolves in water, it releases hydrogen ions — H+(aq). These H+ ions are responsible for all the typical properties of acids.
\(\text{HCl}(aq) \rightarrow \text{H}^+(aq) + \text{Cl}^-(aq)\)
\(\text{H}_2\text{SO}_4(aq) \rightarrow 2\text{H}^+(aq) + \text{SO}_4^{2-}(aq)\)
The H+ ion is actually a bare proton. In water, it immediately bonds to a water molecule forming the hydronium ion:
\(\text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+\)
Bases Produce OH- Ions in Water
Bases that dissolve in water release hydroxide ions — OH-(aq):
\(\text{NaOH}(aq) \rightarrow \text{Na}^+(aq) + \text{OH}^-(aq)\)
\(\text{KOH}(aq) \rightarrow \text{K}^+(aq) + \text{OH}^-(aq)\)
Alkali: A base that is soluble in water is called an alkali. All alkalis are bases, but not all bases are alkalis (e.g., Cu(OH)2 is a base but is insoluble in water, so it is not an alkali).
Activity 2.6 — Conductivity of Acid Solutions vs Glucose/Alcohol
L4 Analyse
Predict first: Both glucose and HCl dissolve in water. Do you think both solutions will conduct electricity equally well?
- Prepare separate solutions of glucose, alcohol, hydrochloric acid, and sodium hydroxide in water.
- Set up a circuit with a battery, bulb, and two electrodes dipping into each solution in turn.
- Observe which solutions make the bulb glow.
Observations:
The bulb glows for HCl and NaOH solutions, but does not glow for glucose and alcohol solutions.
Explanation: HCl and NaOH produce ions (H+/Cl- and Na+/OH-) in water. These ions carry electric current. Glucose and alcohol dissolve but do not produce ions — they exist as neutral molecules in solution. Since no ions are present, no current flows.
The bulb glows for HCl and NaOH solutions, but does not glow for glucose and alcohol solutions.
Explanation: HCl and NaOH produce ions (H+/Cl- and Na+/OH-) in water. These ions carry electric current. Glucose and alcohol dissolve but do not produce ions — they exist as neutral molecules in solution. Since no ions are present, no current flows.
Activity 2.7 — Dry vs Wet HCl Gas on Litmus
L4 Analyse
Predict first: If HCl is an acid, shouldn't it always turn blue litmus red? What if the HCl is in dry gas form (no water)?
- Place dry blue litmus paper in a gas jar filled with dry HCl gas.
- Observe any colour change.
- Now moisten another piece of blue litmus paper with water and place it in the same dry HCl gas.
- Compare the results.
Observations:
Dry litmus paper shows no colour change in dry HCl gas. But moist litmus paper immediately turns red.
Explanation: Dry HCl gas does not contain H+ ions — it is just HCl molecules. Only when HCl dissolves in water does it ionise to produce H+ ions. It is these H+(aq) ions that turn litmus red. Water is essential for acids to show their acidic character.
Dry litmus paper shows no colour change in dry HCl gas. But moist litmus paper immediately turns red.
Explanation: Dry HCl gas does not contain H+ ions — it is just HCl molecules. Only when HCl dissolves in water does it ionise to produce H+ ions. It is these H+(aq) ions that turn litmus red. Water is essential for acids to show their acidic character.
Critical Concept: Acids produce H+(aq) ions only in the presence of water. Without water, an acid cannot show its typical acidic properties. This is why dry HCl gas does not change the colour of dry litmus paper.
2.3 How Strong Are Acid or Base Solutions?
Not all acids are equally strong. Hydrochloric acid (HCl) is much stronger than acetic acid (vinegar). Scientists measure the strength of an acidic or basic solution using the pH scale.
The pH Scale (0 to 14)
| pH Range | Nature | H+ Ion Concentration |
|---|---|---|
| pH < 7 | Acidic | High (more H+ ions) |
| pH = 7 | Neutral | Equal H+ and OH- |
| pH > 7 | Basic (alkaline) | Low (fewer H+ ions, more OH-) |
The lower the pH, the stronger the acid. The higher the pH (above 7), the stronger the base.
Interactive: Explore the pH Scale L3 Apply
Drag the slider to explore different pH values and see common substances at each level.
pH 7 — Neutral
Example: Pure water. Neither acidic nor basic. Equal concentration of H+ and OH- ions.
Example: Pure water. Neither acidic nor basic. Equal concentration of H+ and OH- ions.
Activity 2.8 — Testing pH with Universal Indicator
L3 Apply
Predict first: If you test lemon juice, soap solution, and pure water with universal indicator, which colours would you expect?
- Collect samples: dilute HCl, dilute NaOH, lemon juice, water, baking soda solution.
- Put a drop of universal indicator on each sample (or dip pH paper into each).
- Compare the colour with the standard pH chart and record the pH value.
Observations:
| Solution | Colour | Approx. pH | Nature |
|---|---|---|---|
| Dilute HCl | Red | 1 | Strongly acidic |
| Lemon juice | Red-orange | 2-3 | Acidic |
| Pure water | Green | 7 | Neutral |
| Baking soda | Blue-green | 8-9 | Mildly basic |
| Dilute NaOH | Violet/purple | 13-14 | Strongly basic |
Importance of pH in Everyday Life
The pH value is crucial in many aspects of our daily lives, in nature, and in industry:
| Context | pH Role | Details |
|---|---|---|
| Digestive system | Stomach acid | Our stomach produces HCl (pH ~ 1.2 to 2). It helps in digestion of food and killing bacteria. Excess acid causes acidity — antacids (mild bases like Mg(OH)2) are used to neutralise it. |
| Tooth decay | pH < 5.5 | Bacteria in the mouth produce acids from sugar. When pH drops below 5.5, tooth enamel (calcium phosphate) begins to corrode. Using basic toothpaste helps neutralise the acid. |
| Bee/ant sting | Formic acid | Bee stings inject methanoic acid (formic acid). Applying baking soda (NaHCO3, a mild base) neutralises the acid and relieves pain. |
| Soil pH | Agriculture | Most plants grow best in soil pH 6-7. Too acidic soil is treated with quicklime (CaO) or slaked lime Ca(OH)2. Too basic soil may need organic matter. |
| Factory effluents | Water treatment | Factory waste must be neutralised before discharge into rivers. Acidic effluents are treated with bases, and vice versa. |
Dilution of Acids — A Safety-Critical Process
Activity 2.9/2.10 — Safe Dilution of Acids
L5 Evaluate
Predict first: When concentrated sulphuric acid is mixed with water, a lot of heat is released. What is the safe way to perform this dilution?
- Take about 10 mL of water in a beaker.
- Slowly add a small quantity of concentrated sulphuric acid to the water, drop by drop, while stirring continuously.
- Touch the outer wall of the beaker carefully. Note the temperature change.
Observations:
The beaker becomes very hot. The process of mixing acid with water is highly exothermic.
Safety Rule: Always add acid to water, never water to acid. If water is added to concentrated acid, the enormous heat generated can cause the mixture to splash out violently, causing severe burns. When acid is added to a large volume of water, the heat is absorbed by the water, making the process safer.
The beaker becomes very hot. The process of mixing acid with water is highly exothermic.
Safety Rule: Always add acid to water, never water to acid. If water is added to concentrated acid, the enormous heat generated can cause the mixture to splash out violently, causing severe burns. When acid is added to a large volume of water, the heat is absorbed by the water, making the process safer.
Safety Warning: The process of dissolving an acid in water is highly exothermic. Always add acid to water in a thin stream while stirring. The memory trick: "Do as you oughta — add acid to water." Never do it the other way around!
Competency-Based Questions
In a school lab, the teacher asked students to test five solutions (P, Q, R, S, T) with universal indicator paper. The results were: P turned red (pH 1), Q turned orange (pH 4), R turned green (pH 7), S turned blue (pH 10), and T turned violet (pH 14).
Q1. L1 Remember A solution has pH = 7. What is its nature?
Answer: C. Neutral. A pH of 7 indicates that the solution has equal concentrations of H+ and OH- ions, making it neither acidic nor basic. Pure water has pH = 7.
Q2. L2 Understand Arrange solutions P, Q, R, S, T in order of increasing hydrogen ion concentration. (2 marks)
Answer: T (pH 14) < S (pH 10) < R (pH 7) < Q (pH 4) < P (pH 1).
Hydrogen ion concentration increases as pH decreases. So the solution with the lowest pH (P, pH 1) has the highest H+ concentration, while T (pH 14) has the lowest.
Hydrogen ion concentration increases as pH decreases. So the solution with the lowest pH (P, pH 1) has the highest H+ concentration, while T (pH 14) has the lowest.
Q3. L4 Analyse Dry HCl gas does not change the colour of dry litmus paper, but moist HCl gas does. Explain why, relating your answer to the role of water. (3 marks)
Answer: Dry HCl gas consists of HCl molecules that have not ionised. Without water, HCl cannot dissociate into H+ and Cl- ions. Since it is the H+(aq) ions that are responsible for the acidic behaviour (including changing litmus colour), dry HCl has no effect on dry litmus. When the litmus is moistened, the HCl dissolves in the water film and ionises: HCl → H+ + Cl-. The H+ ions then turn blue litmus red. This proves that water is essential for acids to show acidic behaviour.
Q4. L3 Apply A person is stung by a bee. The sting injects formic acid. Suggest a household remedy and explain how it works chemically. (2 marks)
Answer: Applying baking soda (NaHCO3) paste on the sting area provides relief. Baking soda is a mild base. It neutralises the formic acid injected by the bee sting through a neutralisation reaction, reducing the burning sensation. The acid is converted into a harmless salt and water.
Q5. L5 Evaluate A student claims: "Since glucose dissolves in water, it must produce ions like HCl does." Evaluate this claim using evidence from Activity 2.6. (3 marks)
Answer: The claim is incorrect. In Activity 2.6, the conductivity test shows that glucose solution does not conduct electricity (the bulb does not glow), while HCl solution does. This proves that dissolving in water does not automatically mean ion production. HCl is an electrolyte — it dissociates into H+ and Cl- ions in water. Glucose, however, dissolves as intact neutral molecules (C6H12O6) and does not produce ions. Solubility and ionisation are two different phenomena.
Assertion-Reason Questions
Assertion (A): Acids produce H+(aq) ions only in aqueous solution.
Reason (R): Water molecules help in the ionisation of acid molecules by stabilising the ions formed.
Answer: A. Both are true and R explains A. Acids need water to ionise because water molecules surround and stabilise the H+ and anions produced, enabling the dissociation. Without water, the acid remains as undissociated molecules (as demonstrated by Activity 2.7 with dry HCl gas).
Assertion (A): When pH of a solution decreases from 7 to 1, the solution becomes increasingly acidic.
Reason (R): Lower pH means higher concentration of OH- ions.
Answer: C. The assertion is true — decreasing pH from 7 to 1 does mean increasing acidity. However, the reason is false. Lower pH corresponds to a higher concentration of H+ ions, not OH- ions. OH- concentration increases as pH rises above 7.
Assertion (A): While diluting an acid, acid should be added to water and not water to acid.
Reason (R): Dissolving acid in water is an exothermic process, and adding acid to water allows the large volume of water to absorb the heat safely.
Answer: A. Both are true and R correctly explains A. Adding concentrated acid to water allows the heat to be distributed across a large volume of water. If water were added to acid, the small amount of water would boil instantly due to the intense heat, causing the acid to splash out dangerously.
AI Tutor
Science Class 10 — NCERT (2024-25)
Ready