NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text (Page-wise) Questions
You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer
Dip the red litmus paper into each test tube one by one.
• The solution that turns red litmus blue is the base.
• Now take the litmus paper that turned blue and dip it in the remaining two tubes. The one that turns the blue litmus back to red is the acid.
• The solution that causes no colour change in either case is distilled water.
Why should curd and sour substances not be kept in brass and copper vessels?
Answer
Curd and other sour substances contain acids (like lactic acid). These acids react with the metal of brass and copper containers, forming toxic metallic salts that contaminate the food and can be harmful if consumed. Therefore, acidic foods should be stored in non-reactive containers such as glass or stainless steel.
Which gas is usually liberated when an acid reacts with a metal? How will you test for the presence of this gas?
Answer
When an acid reacts with a metal, hydrogen gas (H₂) is evolved.
Example: Zn(s) + H₂SO₄(aq)
⇒ ZnSO₄(aq) + H₂(g)↑
Test: Bring a burning matchstick or candle near the mouth of the test tube. If hydrogen gas is present, it burns with a characteristic "pop" sound.
Metal compound 'A' reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Answer
The gas that produces effervescence and extinguishes a burning candle is carbon dioxide (CO₂). The metal compound A is calcium carbonate (CaCO₃).
CaCO₃(s) + 2HCl(aq)
⇒ CaCl₂(aq) + H₂O(l) + CO₂(g)↑
Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Answer
Acids like HCl and HNO₃ dissociate in water to release hydrogen ions (H⁺), or more precisely, hydronium ions (H₃O⁺). It is these H⁺/H₃O⁺ ions that are responsible for acidic behaviour.
HCl(aq) → H⁺(aq) + Cl⁻(aq)
Although alcohol (C₂H₅OH) and glucose (C₆H₁₂O₆) also contain hydrogen in their molecules, they do not ionise in water to release H⁺ ions. Their hydrogen atoms are bonded covalently and do not separate as ions. Hence, they do not show acidic character.
Why does an aqueous solution of acid conduct electricity?
Answer
When dissolved in water, acids dissociate into ions — positively charged hydrogen ions (H⁺) and negatively charged anions (like Cl⁻, SO₄²⁻, etc.). These free-moving ions act as carriers of electric charge, allowing the solution to conduct electricity. Pure water, which has very few ions, is a poor conductor.
Why does dry HCl gas not change the colour of dry litmus paper?
Answer
Dry HCl gas does not contain water, and without water, HCl cannot ionise to produce H⁺ ions. Since it is the H⁺ ions that give acids their characteristic properties (including the ability to turn litmus red), dry HCl has no effect on dry litmus paper. Acidic behaviour is only shown in the presence of water.
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer
The process of dissolving acid in water is highly exothermic (releases a lot of heat). If water is added to concentrated acid, the small amount of water may boil instantly due to the intense heat generated, causing the acid to splash out and potentially cause severe burns. When acid is added to water gradually, the large volume of water absorbs the heat safely, keeping the temperature under control. Remember: "Do as you oughta — add acid to water."
How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Answer
When an acid solution is diluted by adding more water, the total volume of the solution increases while the number of H₃O⁺ ions remains the same. As a result, the concentration of H₃O⁺ ions per unit volume decreases. The solution becomes less acidic (its pH moves closer to 7).
How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer
Sodium hydroxide is a strong base that fully dissociates in water: NaOH
⇒ Na⁺ + OH⁻. When more NaOH is dissolved in the existing solution, additional OH⁻ ions are released. Therefore, the concentration of OH⁻ ions increases, making the solution more strongly basic (pH moves further above 7).
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?
Answer
Solution A (pH 6) has a higher concentration of H⁺ ions because pH is inversely related to H⁺ concentration — a lower pH means more H⁺ ions.
• Solution A (pH 6) is acidic (pH below 7)
• Solution B (pH 8) is basic (pH above 7)
What effect does the concentration of H⁺(aq) ions have on the nature of the solution?
Answer
The nature of the solution directly depends on H⁺ concentration:
• Higher H⁺ concentration
⇒ solution is more strongly acidic (lower pH)
• Lower H⁺ concentration
⇒ solution is less acidic / more basic (higher pH)
• When H⁺ concentration equals OH⁻ concentration
⇒ solution is neutral (pH = 7)
Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?
Answer
Yes, basic solutions do contain H⁺ ions because water itself undergoes a small degree of self-ionisation: H₂O ⇌ H⁺ + OH⁻. However, in a basic solution, the number of OH⁻ ions is far greater than the number of H⁺ ions. It is this excess of hydroxide ions that gives the solution its basic character.
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
When the soil becomes too acidic (pH below the optimal range for crops), a farmer treats it with quick lime (CaO), slaked lime [Ca(OH)₂], or chalk (CaCO₃). These are all basic substances that neutralise the excess acid in the soil, raising its pH to a level suitable for healthy plant growth. This process is known as liming of soil.
What is the common name of the compound CaOCl₂?
Answer
The common name is bleaching powder (calcium oxychloride). It is prepared by passing chlorine gas through dry slaked lime:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Name the sodium compound which is used for softening hard water.
Answer
Washing soda — sodium carbonate decahydrate (Na₂CO₃·10H₂O). It removes the calcium and magnesium salts present in hard water by forming insoluble carbonates.
What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved.
Answer
On heating, sodium hydrogencarbonate (baking soda) decomposes to form sodium carbonate, water, and carbon dioxide gas:
2NaHCO₃(s)
⇒ heat Na₂CO₃(s) + H₂O(l) + CO₂(g)↑
Write an equation to show the reaction between Plaster of Paris and water.
Answer
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
(Plaster of Paris) → (Gypsum)
Plaster of Paris absorbs water and sets into a hard solid mass of gypsum. This setting reaction is exothermic.
Section B: NCERT Exercise Questions
Q1. A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Ans. (d) 10. A solution that turns red litmus blue is basic. Basic solutions have pH greater than 7. Among the given options, only pH 10 is above 7.
A solution reacts with crushed egg-shells to give a gas that turns lime water milky. The solution contains:
Answer
(a) NaCl (b) HCl (c) LiCl (d) KCl
Ans. (b) HCl. Egg shells contain calcium carbonate. HCl (an acid) reacts with CaCO₃ to release CO₂ gas, which turns lime water milky:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
NaCl, LiCl, and KCl are neutral salts and would not react with egg shells.
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 of HCl solution required to neutralise it will be:
Answer
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Ans. (d) 16 mL. Since concentrations remain the same, the volume relationship is directly proportional. If 10 mL NaOH needs 8 mL HCl, then 20 mL NaOH will need 2 × 8 = 16 mL HCl.
Which one of the following types of medicines is used for treating indigestion?
Answer
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Ans. (c) Antacid. Indigestion occurs when the stomach produces excess hydrochloric acid. Antacids are mild bases (like magnesium hydroxide or sodium hydrogencarbonate) that neutralise the excess acid, providing relief.
Write word equations and then balanced equations for the reaction of:
Answer
(a) dilute sulphuric acid with zinc (b) dilute hydrochloric acid with magnesium ribbon (c) dilute sulphuric acid with aluminium (d) dilute hydrochloric acid with iron
Ans.
(a) Zinc + Sulphuric acid
⇒ Zinc sulphate + Hydrogen
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑
(b) Magnesium + Hydrochloric acid
⇒ Magnesium chloride + Hydrogen
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)↑
(c) Aluminium + Sulphuric acid
⇒ Aluminium sulphate + Hydrogen
2Al(s) + 3H₂SO₄(aq)
⇒ Al₂(SO₄)₃(aq) + 3H₂(g)↑
(d) Iron + Hydrochloric acid
⇒ Ferrous chloride + Hydrogen
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑
Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Answer
Take two beakers — one with dilute HCl and another with glucose solution. Set up an electrical circuit with a battery, bulb, and two electrodes in each beaker.
• In the HCl beaker, the bulb glows because HCl ionises in water to produce H⁺ and Cl⁻ ions that conduct electricity.
• In the glucose beaker, the bulb does not glow because glucose does not dissociate into ions in water.
This proves that although glucose contains hydrogen, it does not produce H⁺ ions in solution and therefore is not an acid.
Why does distilled water not conduct electricity, whereas rain water does?
Answer
Distilled water is pure and contains negligible ions, making it a very poor conductor of electricity. Rain water, on the other hand, absorbs gases like CO₂ and SO₂ from the atmosphere as it falls:
CO₂ + H₂O → H₂CO₃ (carbonic acid)
SO₂ + H₂O → H₂SO₃ (sulphurous acid)
These dissolved acids dissociate to produce ions in the rain water, enabling it to conduct electricity.
Why do acids not show acidic behaviour in the absence of water?
Answer
Acids show their characteristic properties only when they can release H⁺ ions. This ionisation process requires water as a medium. In the absence of water, the acid molecules remain intact and do not dissociate into ions. Without free H⁺ ions, no acidic behaviour (such as sour taste, colour change with indicators, or reactivity with metals) can be observed.
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:
Answer
(a) neutral (b) strongly alkaline (c) strongly acidic (d) weakly acidic (e) weakly alkaline
Arrange them in increasing order of hydrogen ion concentration.
Ans.
(a) Neutral → D (pH 7)
(b) Strongly alkaline → C (pH 11)
(c) Strongly acidic → B (pH 1)
(d) Weakly acidic → A (pH 4)
(e) Weakly alkaline → E (pH 9)
Increasing order of H⁺ ion concentration (lower H⁺ to higher H⁺):
C (pH 11) < E (pH 9) < D (pH 7) < A (pH 4) < B (pH 1)
Equal lengths of magnesium ribbons are taken in two test tubes 'A' and 'B'. Hydrochloric acid is added to 'A' while acetic acid is added to 'B'. Amount and concentration of both acids are same. In which test tube will the fizzing occur more vigorously and why?
Answer
Fizzing will be more vigorous in test tube A (containing HCl). Hydrochloric acid is a strong acid that ionises completely in water, producing a higher concentration of H⁺ ions. Acetic acid is a weak acid that only partially ionises. Since the rate of hydrogen gas evolution depends on H⁺ concentration, HCl reacts faster with magnesium, causing more vigorous fizzing.
Fresh milk has a pH of 6. How do you think the pH will change as it turns to curd? Explain your answer.
Answer
As milk converts to curd, its pH decreases (becomes more acidic). This happens because bacteria in the milk convert lactose (milk sugar) into lactic acid during the curdling process. The formation of lactic acid increases the H⁺ ion concentration, lowering the pH below 6.
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) Fresh milk is slightly acidic (pH ~6) and can turn sour quickly due to lactic acid formation. Baking soda (NaHCO₃) is a mild base that neutralises the lactic acid as it forms, preventing the milk from becoming sour too quickly. This keeps the milk consumable for a longer time.
(b) Since baking soda makes the milk slightly alkaline, the lactic acid produced by bacteria is continuously neutralised. It takes much longer for enough acid to accumulate to lower the pH sufficiently for the milk proteins to curdle and set as curd.
Plaster of Paris should be stored in a moisture-proof container. Explain why.
Answer
Plaster of Paris (CaSO₄·½H₂O) readily absorbs moisture from the atmosphere. If exposed to moisture, it reacts with water and converts to gypsum (CaSO₄·2H₂O), becoming a hard solid mass:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Once this happens, the plaster loses its ability to set when mixed with water later, making it useless for moulding and casting. Hence, it must be stored in airtight, moisture-proof containers.
What is a neutralisation reaction? Give two examples.
Answer
A neutralisation reaction occurs when an acid reacts with a base to form a salt and water. The H⁺ ions from the acid combine with OH⁻ ions from the base to form water, effectively cancelling out each other's properties.
Example 1: NaOH(aq) + HCl(aq)
⇒ NaCl(aq) + H₂O(l)
Example 2: 2KOH(aq) + H₂SO₄(aq)
⇒ K₂SO₄(aq) + 2H₂O(l)
Give two important uses of washing soda and baking soda.
Answer
Washing soda (Na₂CO₃·10H₂O)
(i) Used in the manufacture of glass, soap, and paper
(ii) Used as a cleansing agent for domestic purposes and for softening hard water
Baking soda (NaHCO₃)
(i) Used as an ingredient in baking powder for making fluffy cakes, bread, and biscuits (it releases CO₂ when heated, causing dough to rise)
(ii) Used as an antacid to relieve acidity and indigestion by neutralising excess stomach acid
Section C: Important Extra Questions for Board Exam
What is the pH scale? What does it indicate?
Answer
The pH scale is a measurement scale that ranges from 0 to 14 and indicates the acidic or basic nature of a solution.
• pH < 7
⇒ Acidic solution (lower pH = stronger acid)
• pH = 7 → Neutral solution
• pH > 7
⇒ Basic/alkaline solution (higher pH = stronger base)
pH stands for "potential of hydrogen" and is a measure of the hydrogen ion concentration in a solution.
How is bleaching powder prepared? Give its chemical formula and two uses.
Answer
Bleaching powder is prepared by passing chlorine gas through dry slaked lime:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Chemical formula: CaOCl₂ (Calcium oxychloride)
Uses:
(i) For bleaching cotton, linen, and wood pulp in textile and paper industries
(ii) For disinfecting drinking water to make it germ-free
What happens when zinc granules are treated with sodium hydroxide solution? Write the chemical equation.
Answer
Zinc is an amphoteric metal — it reacts with both acids and bases. When treated with NaOH, it dissolves to form sodium zincate and hydrogen gas:
Zn(s) + 2NaOH(aq)
⇒ Na₂ZnO₂(aq) + H₂(g)↑
This reaction demonstrates that certain metals can react with strong bases as well.
How is baking powder different from baking soda?
Answer
Baking soda is pure sodium hydrogencarbonate (NaHCO₃).
Baking powder is a mixture of baking soda and a mild edible acid such as tartaric acid or cream of tartar.
When baking powder is heated or mixed with water, the tartaric acid reacts with NaHCO₃ to produce CO₂ gas, which makes the bread/cake soft and fluffy. If only baking soda were used, the excess sodium carbonate formed would give a bitter taste to the food.
State the role of pH in everyday life (any four examples).
Answer
(i) Digestion: The stomach maintains a pH of about 1.5–3.5 using HCl to digest food. Excess acid causes acidity, treated with antacids.
(ii) Tooth decay: Bacteria in the mouth produce acids that lower pH below 5.5, causing tooth enamel (calcium phosphate) to corrode. Toothpaste (basic) helps neutralise the acid.
(iii) Bee/wasp stings: Bee sting injects formic acid (treat with baking soda); wasp sting is alkaline (treat with vinegar).
(iv) Soil pH: Plants grow best in a specific pH range. If soil is too acidic, farmers add lime; if too basic, they add organic matter to adjust pH.