Acids, Bases and Salts-Exercise
Chemistry - Exercise
Q1. A solution turns red litmus blue, its pH is likely to be
- 1
- 4
- 5
- 10
Answer:
- 1- Strongly acidic (would turn blue litmus red, not red litmus blue)
- 4- Weakly acidic (would turn blue litmus red, not red litmus blue)
- 5- Weakly acidic (would turn blue litmus red, not red litmus blue)
- 10- Basic/Alkaline (turns red litmus blue)
Since the solution turns red litmus blue, it must be basic, meaning its pH must be greater than 7. Among all the options, only 10 satisfies this condition.
Therefore, the correct answer is 10.
Q2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
- NaCl
- HCl
- LiCl
- KCl
Answer: HCl
Explanation:
When the solution reacts with crushed egg-shells (which contain calcium carbonate, CaCO₃), it produces a gas
that turns lime-water milky. This gas is carbon dioxide (CO₂), and the milky appearance is due to the
formation of calcium carbonate precipitate in lime-water.
Only an acid can react with calcium carbonate to produce CO₂ gas. Among the given options, HCl (hydrochloric
acid) is the only acid, while NaCl, LiCl, and KCl are all neutral salts.
Chemical Reaction \[\boxed{\scriptsize\mathrm{CaCO_3 + HCl\rightarrow CaCl_2 + H_2O + CO_2}}\]
Q3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be
- 4mL
- 8mL
- 12ml
- 16mL
Answer:
Since the concentration of both solutions remains the same, the volume of acid required to neutralize the
base is directly proportional to the volume of base used.
\[
{\scriptsize
\begin{aligned}
\frac{\text{Volume of HCl}}{\text{Volume of NaOH}} &= \text{Constant} \\\\
\implies \frac{v_{i(\mathrm{HCl})}}{v_{i(\mathrm{NaOH})}} &=
\frac{v_{f(\mathrm{HCl})}}{v_{f(\mathrm{NaOH})}} \\\\
\implies \frac{8}{10} &= \frac{v_{f(\mathrm{HCl})}}{20} \\\\
v_{f(\mathrm{HCl})} &= \frac{20 \times 8}{10} \\\\
&= 16\,\mathrm{mL}
\end{aligned}
}
\]
The neutralization reaction is: \[ \boxed{\scriptsize\mathrm{NaOH + HCl\rightarrow NaCl+H_2O}} \]
The molar ratio is 1:1, so doubling the amount of NaOH requires doubling the amount of HCl.
Therefore, 16 mL of HCl solution is required to neutralize 20 mL of NaOH solution.
Q4. Which one of the following types of medicines is used for treating indigestion?
- Antibiotic
- Analgesic
- Antacid
- Antiseptic
Answer: Antacid
Explanation:
Indigestion, or dyspepsia, occurs when excess stomach acid irritates the stomach lining, causing discomfort
such as heartburn, bloating, or nausea.
- Antacids are medicines that neutralize excess hydrochloric acid (HCl) in the stomach by reacting to form water and a salt.
- This neutralization relieves acidity-related irritation and discomfort.
Q5. Write word equations and then balanced equations for the reaction taking place when
- dilute sulphuric acid reacts with zinc granules.
- dilute hydrochloric acid reacts with magnesium ribbon.
- dilute sulphuric acid reacts with aluminium powder.
- dilute hydrochloric acid reacts with iron filings.
Dilute sulphuric acid reacts with zinc granules
Word equation:
Zinc + Dilute sulphuric acid → Zinc sulphate + Hydrogen
Balanced equation:
\[
\boxed{\scriptsize\mathrm{Zn+H_2SO_4\rightarrow ZnSO_4 + H_2}}
\]
Dilute hydrochloric acid reacts with magnesium ribbon
Word equation:
Magnesium + Dilute hydrochloric acid → Magnesium chloride + Hydrogen
Balanced equation:
\[
\boxed{\scriptsize\mathrm{Mg+2HCl\rightarrow MgCl_2 + H_2}}
\]
Dilute sulphuric acid reacts with aluminium powder
Word equation:
Aluminium + Dilute sulphuric acid → Aluminium sulphate + Hydrogen
Balanced equation:
\[
\boxed{\scriptsize\mathrm{Al+3H_2SO_4\rightarrow Al_2(SO_4)_3 + H_2}}
\]
Dilute hydrochloric acid reacts with iron filings
Word equation:
Iron + Dilute hydrochloric acid → Iron(II) chloride + Hydrogen
Balanced equation:
\[
\boxed{\scriptsize\mathrm{Fe+2HCl\rightarrow FeCl_2 + H_2}}
\]
Q6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Activity to Demonstrate That Alcohols and Glucose Are Not Acids
Objective:
Show that solutions of an alcohol (e.g., ethanol) and glucose do not exhibit acidic behavior unlike true
acids.
Materials Needed:
- Ethanol solution (5% v/v in water)
- Glucose solution (10% w/v in water)
- Dilute hydrochloric acid (0.1 M HCl) as a positive control
- Red and blue litmus papers
- Small test tubes or beakers
- Glass stirring rods
- pH paper or a pH meter (optional)
Experimental Procedure
- Label three test tubes as A, B, and C.
- Add 5 mL of ethanol solution to Test Tube A.
- Add 5 mL of glucose solution to Test Tube B.
- Add 5 mL of dilute HCl to Test Tube C (acid control).
- Dip a strip of red litmus paper into each test tube in turn, then record any color change.
- Dip a strip of blue litmus paper into each test tube in turn, then record any color change.
- (Optional) Measure the pH of each solution with pH paper or a pH meter and record the values.
Observations
Test Tube A (Ethanol):
- Red litmus remains red
– Blue litmus remains blue
– pH ≈ 7 (neutral)
Test Tube B (Glucose):
– Red litmus remains red
– Blue litmus remains blue
– pH ≈ 7 (neutral)
Test Tube C (HCl Control):
– Red litmus turns blue
– Blue litmus remains blue
– pH < 7 (acidic)
Explanation and Conclusion
Litmus Test: Acids turn blue litmus red; they do not affect red litmus. In Test Tube C, HCl turned blue litmus red, confirming acidic behavior. Neither ethanol nor glucose altered either litmus paper, indicating they are not acidic.
pH Measurement: Both ethanol and glucose solutions showed pH values around 7, typical of neutral substances, whereas the acid control had pH below 7.
Chemical Basis: Although alcohols and glucose contain hydrogen atoms, these hydrogens are covalently bonded to carbon or oxygen and are not ionizable to release \(\mathrm{H^+}\)under normal conditions. True acids (like HCl) dissociate in water to produce free \(\mathrm{H^+}\) ions, which is what gives them acidic properties.
Thus, this simple activity confirms that alcohols and glucose do not behave as acids.
Q7. Why does distilled water not conduct electricity, whereas rain water does?
Distilled water conducts almost no electricity because the distillation process removes virtually all
dissolved salts and minerals, leaving only the minute ions from water’s slight self-ionization (about 10⁻⁷
M) which are far too few to carry a measurable current.
Rainwater, however, absorbs carbon dioxide and other airborne impurities as it falls, forming weak carbonic
acid and dissolving trace salts and pollutants. These
dissolved species release appreciable amounts of H⁺, HCO₃⁻, and other ions (typically on the order of 10⁻⁴
to 10⁻³ M), enabling rainwater to conduct electricity.
Q8. Why do acids not show acidic behaviour in the absence of water?
Acids exhibit acidic behavior by donating protons \((\mathrm{H^+})\) to a solvent. In the absence of water (or another polar solvent), they cannot ionize to release \((\mathrm{H^+})\) so they do not display typical acidic properties such as lowering pH or turning litmus paper red.
Q9. Five solutions A,B,C,D and E when tested with universal indicator showed pH as
4,1,11,7 and 9, respectively. Which solution is
- (a) neutral?
- (b) strongly alkaline?
- (c) strongly acidic?
- (d) weakly acidic?
- (e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Answer:
- (a) neutral: Solution D (pH 7)?
- (b) strongly alkaline: Solution C (pH 11)?
- (c) strongly acidic: Solution B (pH 1)?
- (d) weakly acidic: Solution A (pH 4)?
- (e) weakly alkaline: Solution E (pH 9)?
The hydrogen-ion concentration \((\mathrm{H^+})\) varies as \((\mathrm{10^{-pH}})\) , so higher pH means lower \((\mathrm{H^+})\). Thus, in order of increasing \((\mathrm{H^+})\) (from lowest to highest concentration): 11, 9, 7, 4, 1.
Q10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3 COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Answer
Fizzing is more vigorous in test tube A (with HCl) because hydrochloric acid is a strong acid that fully
dissociates into ((\mathrm{H^+})\) ions, producing a high ((\mathrm{H^+})\) and rapid reaction with Mg:
\[
{\scriptsize
\mathrm{Mg} + 2\,\mathrm{H}^+ \longrightarrow \mathrm{Mg}^{2+} + \mathrm{H}_2
}
\]
Acetic acid is weak and only partially ionizes, yielding far fewer((\mathrm{H^+})\) ions and a slower
fizzing rate.
Q11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer:
The pH will decrease (become more acidic) as fresh milk turns into curd. Lactic acid bacteria ferment
lactose into lactic acid, increasing((\mathrm{H^+})\) and lowering the pH from about 6 in fresh milk to
around 4.5 in curd.
Q12.A milkman adds a very small amount of baking soda to fresh milk.
- (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
- (b) Why does this milk take a long time to set as curd?
Answer:
- (a) The milkman raises the pH of fresh milk from about 6 to slightly alkaline by adding a pinch of baking soda (sodium hydrogen carbonate) because increasing the pH slows down the activity of the naturally occurring lactic acid bacteria. These microbes thrive in mildly acidic conditions, so making the milk slightly alkaline inhibits their growth and fermentation rate.
- (b) Because the bacteria are less active at higher pH, it takes longer for them to convert lactose into lactic acid. Slower acid production delays the drop in pH needed for casein proteins to coagulate, so the milk remains liquid for a longer time and “sets” into curd more slowly.
Q13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer:
Plaster of Paris is calcium sulfate hemihydrate \(\mathrm{CaSO_4.\frac{1}{2}H_2O}\) which reacts with water
to form gypsum \(\mathrm{CasO_4.2H_2O}\) and sets hard:
\[\scriptsize\mathrm{\underset{\text{Plaster of Paris}}{CaSO_4.\frac{1}{2}H_2O} + \frac{3}{2}H_2O\rightarrow
\underset{\text{Gypsum}}{CaSO_4.2H_2O}}\]
If exposed to moisture in air, it will prematurely hydrate and lose its powdery form, reducing its
effectiveness and workability. Storing it in a moisture-proof container prevents accidental contact with
water vapor and ensures it remains dry and usable.
Q14. What is a neutralisation reaction? Give two examples.
Answer:
A neutralization
reaction is a chemical process in which an acid reacts with a base to produce a salt and
water, usually resulting in a solution with a pH near 7; for example, hydrochloric acid reacting with sodium
hydroxide:
\[\scriptsize\mathrm{HCl} + \mathrm{NaOH} \rightarrow \mathrm{NaCl} + \mathrm{H}_2\mathrm{O}\]
and sulfuric acid reacting with potassium hydroxide:
\[\scriptsize\mathrm{H}_2\mathrm{SO}_4 + 2\,\mathrm{KOH} \rightarrow \mathrm{K}_2\mathrm{SO}_4 +
2\,\mathrm{H}_2\mathrm{O}\]
Q15. Give two important uses of washing soda and baking soda.
Washing Soda (Sodium Carbonate, \(\mathrm{Na_2CO_3}\))
- Water Softening: Washing soda precipitates calcium and magnesium ions from hard water as insoluble carbonates, preventing soap scum and improving cleaning efficiency.
- Heavy-Duty Cleaning: Its high alkalinity makes it effective at removing grease, oil, and tough stains from laundry, kitchen surfaces, and drains.
Baking Soda (Sodium Bicarbonate, \(\mathrm{NaHCO_3}\))
- Baking Leavening Agent: Upon heating or reacting with acids (e.g., cream of tartar), it decomposes to release \(\mathrm{CO_2}\) gas, causing doughs and batters to rise and become light and porous.
- Odor Neutralizer and Mild Abrasive: It reacts with acidic and basic odor molecules to neutralize smells in refrigerators, carpets, and shoes, and its fine particles provide gentle scrubbing action for dental and household cleaning.
