Class 10 Science - Previous Year Questions - Acids Bases and Salts

Ans: (a) Strong acid, weak acid, neutral, strong base
Strong acid: pH < 3 (e.g., HCl, pH ~1).
Weak acid: pH 4–6 (e.g., acetic acid, pH ~4–5).
Neutral: pH 7 (e.g., water or NaCl solution).
Strong base: pH > 11 (e.g., NaOH, pH ~13–14).
Option (a) matches typical pH ranges for these categories.

Ans: (c) FeSO₄
FeSO₄·7H₂O (ferrous sulphate heptahydrate) has 7 water molecules.
CuSO₄·5H₂O has 5 water molecules.
Na₂CO₃·10H₂O has 10 water molecules.
CaSO₄·2H₂O (gypsum) has 2 water molecules.

Ans: (a) Salt and water is formed
(i) HCl + NaOH → NaCl + H₂O (neutralization, forms salt and water).
(ii) MgO + 2HCl → MgCl₂ + H₂O (forms salt and water).
(iii) CO₂ + 2NaOH → Na₂CO₃ + H₂O (forms salt and water).
All reactions produce a salt and water, though salts may vary (neutral or basic).

Ans: (b) 5
Tooth enamel (Ca₅(PO₄)₃OH) corrodes in acidic conditions.
pH below 5.5 causes demineralization due to acid attack.
pH 7 is neutral, 10 and 14 are basic, not corrosive to enamel.

Ans:
Formation of a precipitate or gas.
Color change or temperature change.
Chemical changes produce new substances, e.g., gas bubbles (H₂ or CO₂) or precipitates.
Color or temperature changes (e.g., heat evolved) indicate a chemical reaction.

Ans: (c) Sodium
Sodium hydroxide (NaOH) is a strong, corrosive base, requiring a warning sign.
Aluminium hydroxide is insoluble, less hazardous.
Calcium and magnesium hydroxides are weaker bases, less corrosive.

Ans: (c) 7.0 to 7.8
Blood pH is tightly regulated between 7.35–7.45 for normal body function.
Option (c) (7.0–7.8) is the closest range.

Ans: (c) Na₂ZnO₂
Zinc reacts with NaOH to form sodium zincate (Na₂ZnO₂) and hydrogen:
Zn + 2NaOH → Na₂ZnO₂ + H₂.
Zinc is amphoteric, reacting with bases to form zincate ions.

Ans: (b) K₂SO₄, Na₂SO₄, CaSO₄
A family of salts shares the same anion.
Option (b) has sulphate (SO₄²⁻) as the common anion.
Others have mixed anions (Cl⁻, SO₄²⁻, CO₃²⁻, NO₃⁻).

Ans: (c) have fixed number of water molecules of crystallisation
Washing soda (Na₂CO₃·10H₂O), copper sulphate (CuSO₄·5H₂O), gypsum (CaSO₄·2H₂O), and ferrous sulphate (FeSO₄·7H₂O) all have fixed water molecules.
Not all are coloured or acidic/basic.

Ans: (b) Disinfectants
Chlorine from brine electrolysis is used in disinfectants (e.g., bleach).
Ammonia, plaster of Paris, and soaps do not primarily use chlorine.

Ans: (a) Carbon dioxide
Baking soda (NaHCO₃) reacts with tartaric acid to produce CO₂:
NaHCO₃ + H⁺ (from acid) → Na⁺ + H₂O + CO₂.
CO₂ causes the spongy texture in cakes.

Ans: (b) B < C < A
Potassium nitrate (KNO₃): Neutral (pH ~7).
Ammonium chloride (NH₄Cl): Acidic (pH < 7) due to NH₄⁺ hydrolysis.
Sodium carbonate (Na₂CO₃): Basic (pH > 7) due to CO₃²⁻ hydrolysis.
Order: NH₄Cl (lowest pH) < Na₂CO₃ < KNO₃.

Ans: (d) Na₂CO₃·10H₂O
Washing soda is sodium carbonate decahydrate (Na₂CO₃·10H₂O).
NaHCO₃ is baking soda, with no such hydrates.

Ans: (d) Tartaric acid
Tamarind contains tartaric acid, which is acidic and turns blue litmus red.
Acetic acid (vinegar), methanoic acid (formic acid), and oxalic acid are not primary in tamarind.

Ans: (c) Neutral
KNO₃ is a salt of a strong acid (HNO₃) and strong base (KOH), forming a neutral solution (pH ~7).

Ans: (b) (ii) and (iii)

Ans:
(a) X: FeSO₄·7H₂O (ferrous sulphate heptahydrate); Y: Fe₂O₃ (ferric oxide).
(b) Green to brown.
FeSO₄·7H₂O (green) loses water, forming white FeSO₄, then decomposes to brown Fe₂O₃, SO₂, and SO₃ (pungent gases).
Reaction: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃.

Ans:
Formula: Na₂CO₃·10H₂O.
Preparation: NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (heat); Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
Uses:

1. In laundry detergents.
2. Softening hard water.

Explanation:

• Baking soda (NaHCO₃) is heated to form sodium carbonate, which is crystallized with water.
• Washing soda removes dirt and hardness-causing ions.

Ans: Dry HCl gas does not dissociate into H⁺ ions in the absence of water, so it cannot act as an acid to change litmus color.

• Acidity requires H⁺ ions, formed when HCl dissolves in water (HCl → H⁺ + Cl⁻).
• Dry HCl gas and dry litmus lack moisture for ion formation.

Ans: Baking powder produces CO₂ without needing an external acid, unlike baking soda, ensuring consistent rising.

• Baking powder contains NaHCO₃ and an acid (e.g., tartaric acid), reacting to release CO₂.
• Baking soda requires an acidic ingredient (e.g., buttermilk), which may be inconsistent.

Ans:

• X: Bleaching powder (Ca(OCl)Cl).
• Preparation: Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O.

Chlorine reacts with slaked lime to form bleaching powder, used for bleaching and disinfection.

Ans:

Equations:

• At anode: 2Cl⁻ → Cl₂ + 2e⁻
• At cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻
• Overall: 2NaCl + 2H₂O → Cl₂ (anode) + H₂ (cathode) + 2NaOH (solution).

Uses of Chlorine:

• Disinfecting water.
• Manufacturing bleach.

Explanation:

• Brine (NaCl solution) electrolysis produces chlorine, hydrogen, and sodium hydroxide.
• Chlorine is a disinfectant and bleaching agent.

Ans:

• A: NaHCO₃ (baking soda); B: Na₂CO₃ (washing soda).
• Equation: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.

Explanation:

• NaHCO₃ is mildly basic, used in cooking.
• Heating forms Na₂CO₃, which precipitates Ca²⁺/Mg²⁺ as carbonates, softening water.

Ans:
Definition: Fixed number of water molecules chemically bonded in a salt’s crystalline structure.
Examples:

1. CuSO₄·5H₂O (copper sulphate pentahydrate).
2. Na₂CO₃·10H₂O (washing soda).

Water molecules are integral to the crystal lattice, lost on heating.

Ans:
(a) P: pH < 3; Q: pH > 11.
(b) (i) NaOH (to increase pH); (ii) HCl (to decrease pH).
(c) (i) Hydronium ion concentration increases 100-fold (pH drop by 2 units); pH paper shows red.

• Strong acids (e.g., HCl) have low pH; strong bases (e.g., NaOH) have high pH.
• pH 7 is neutral; NaOH makes it basic, HCl makes it acidic.
• pH = -log[H₃O⁺]; pH 4 to 2 increases [H₃O⁺] from 10⁻⁴ to 10⁻² M.

Ans:
(a) A: Basic, pH 8–10; B: Acidic, pH 4–6.
(b) A is a metal; metal oxides (e.g., MgO) form basic solutions, while non-metal oxides (e.g., CO₂) form acidic solutions.

• Metal oxides dissolve to form bases (e.g., MgO → Mg(OH)₂).
• Non-metal oxides form acids (e.g., CO₂ + H₂O → H₂CO₃).

Ans:
(i) Acid: HCl; Base: NaOH.
(ii) NaCl is neutral; formed from a strong acid and strong base, no hydrolysis.
(iii) (a) Electrolysis of brine:

• Products: Cl₂ (anode), H₂ (cathode), NaOH (solution).
• NaCl is formed by neutralization: HCl + NaOH → NaCl + H₂O.
• Electrolysis: 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH.

Ans:
Property: NaHCO₃ reacts with acids to produce CO₂.
(i) Neutralizes stomach acid (HCl) to relieve indigestion.
(ii) Reacts with tartaric acid in baking powder to release CO₂, making dough rise.
(iii) Reacts with H₂SO₄ in fire extinguishers to produce CO₂, smothering fire.
NaHCO₃ + H⁺ → Na⁺ + H₂O + CO₂ is the key reaction.

Ans:
(i) Zn + 2HCl → ZnCl₂ + H₂
(ii) HCl + NaOH → NaCl + H₂O
(iii) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

• Metals produce H₂ gas.
• Bases form salt and water (neutralization).
• Carbonates release CO₂.

Ans:
(i) H₃O⁺ maximum: D (red); OH⁻ maximum: B (blue).
(ii) D: HCl; B: NaOH; A: Acetic acid; C: NaCl.
(iii) pH ~7; equal volumes of strong acid (d) and strong base (b) neutralize, while A and C are near neutral.

• Red (pH < 3): High H₃O⁺; Blue (pH > 11): High OH⁻.
• Neutralization balances H₃O⁺ and OH⁻.

Ans:
(i) Gas: Hydrogen; Tested by burning with a pop sound.
(ii) (1) Zn + 2HCl → ZnCl₂ + H₂
(2) Zn + 2NaOH → Na₂ZnO₂ + H₂

• Acids with metals produce H₂, confirmed by the pop test.
• Zinc is amphoteric, reacting with both acid (HCl) and base (NaOH).

Ans: (b)

• Turmeric and litmus are both natural indicators.
• They change colour when they come into contact with acids or bases:
• Turmeric turns red in alkaline solutions.
• Litmus turns red in acidic solutions and blue in alkaline solutions.
• This makes them useful for testing the pH of substances.

Ans: (a)

• Washing soda, or sodium carbonate, is a chemical compound that is used in making glass, as a cleaning agent in soaps, and in the paper-making process.
• It's important for its ability to soften water and enhance cleaning.

Ans: (d)

• When hydrochloric acid (HCl) reacts with ammonium hydroxide (NH4OH), it forms ammonium chloride, which is an acidic salt.
• This acid can turn blue litmus paper red, indicating that the solution is acidic.

Ans: (d)

• Sodium carbonate (Na₂CO₃) can exist as a hydrated compound called washing soda, which has 10 water molecules attached to each formula unit (Na₂CO₃·10H₂O). 
•  This means it has the highest number of water of crystallization compared to the other compounds listed. 

Ans: (a)

• Tooth enamel is primarily composed of a mineral known as hydroxyapatite, which is a form of calcium phosphate.
• This mineral plays a crucial role in: Strengthening teeth and making them resistant to decay
• Thus, the correct answer is (a) Calcium phosphate.

Ans: (c)

Sodium chloride (NaCl) is a neutral salt, formed from a strong acid (HCl) and a strong base (NaOH). When NaCl dissolves in water, it dissociates into sodium (Na⁺) and chloride (Cl⁻) ions, both of which do not react with water to affect the pH. As a result, the solution remains neutral, with a pH of 7.

Ans: (d)

• The reaction of solid calcium oxide with water is exothermic, meaning it releases heat.
• This reaction produces calcium hydroxide, which is a strong base.
• As a result, the pH of the solution formed is greater than 7, indicating it is alkaline.

Ans: (c)

The juice of tamarind contains tartaric acid, a natural organic acid. This acid:

• Causes the solution to be acidic.
• Turns blue litmus paper red, indicating acidity.
  Thus, the correct answer is (c) tartaric acid.

Ans: (b)

Aluminum oxide (Al₂O₃) is an amphoteric oxide, meaning it can react with both acids and bases to form salts and water.

• Reaction with acid (HCl): Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
  Aluminum oxide reacts with hydrochloric acid to form aluminum chloride and water.
• Reaction with base (KOH): Al₂O₃ + 2KOH + 3H₂O → 2KAl(OH)₄
  Aluminum oxide reacts with potassium hydroxide in aqueous solution to form potassium aluminate, demonstrating its ability to react with bases.

Ans: (a)

The product 'X' obtained from the reaction is quick lime.

Here’s a brief explanation of the process:

• Heating calcium carbonate (CaCO₃) leads to its decomposition.
• This decomposition produces calcium oxide (CaO) and carbon dioxide (CO₂).
• This process is known as calcination, commonly used in making cement and lime.

Thus, the answer is (a) quick lime.

Ans: (i)  The taste of tomato juice will be slightly sour; The pH 4.6 indicates that tomato juice is an acid and acids are sour in taste.
(ii) Acids that give more H⁺ions / H₃O⁺ are Strong Acids Bases that give less OH^- ions are Weak Bases.
(iii) Living animals can survive within a pH range of 7·0 to 7·8. So, if the pH of river water becomes low due to acid rain (pH < 5·6), then survival of aquatic animals becomes difficult.

Ans:  (a) Acid – HCl, Base – NaOH

(b) Cation – Ca²⁺ , Anion – SO₄^2-

(c) Salts that share the same cation but have different anions are considered part of the same family. For example:

• Sodium chloride (NaCl)
• Washing soda (sodium carbonate, Na₂CO₃)

Both contain the Na⁺  cation.

OR

 (c) The pH scale measures how acidic or basic a solution is, ranging from 0 (acidic) to 14 (basic), with 7 being neutral. When Potassium hydroxide (KOH) reacts with Sulfuric acid (H₂SO₄), it produces Potassium sulfate (K₂SO₄), which is a neutral salt. Its aqueous solution has a pH of approximately 7.

Ans: (i) The gas evolved is HCl gas.

(ii) When tested with:

• Dry blue litmus paper: No change in colour.
• Wet blue litmus paper: Turns red.

Conclusion: The HCl gas is acidic in nature.

Ans: (a) (i)The water droplets appear due to the evaporation of water of crystallisation in ferrous sulphate crystals when heated.
(ii) The colour change observed during heating is from green to white.

(iii) Each molecule of FeSO₄ crystal has seven water molecules attached, represented as FeSO₄ · 7H₂O. The molecular formulas for the crystalline forms are:

• (I) Copper sulphate: CuSO₄ · 5H₂O
• (II) Sodium carbonate: Na₂CO₃ · 10H₂O

(iv) Plaster of Paris is obtained by heating gypsum (CaSO₄ · 2H₂O) at 373 K, which causes it to lose water molecules.

Two uses of Plaster of Paris are:

• It is used by doctors to support fractured bones.
• It is also used in making decorative items.

OR
(b) (i) X-Tartaric acid , Y-Baking soda , Z- Baking powder Y- NaHCO₃
(ii) NaCl + H₂O +CO₂ +NH₃ → NH₄Cl +NaHCO₃
NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt of acid CO₂ released during heating makes the cake soft and spongy. 
(iii) Magnesium hydroxide; Mg(OH)₂

Ans: Common Name: Plaster of Paris

Chemical Name: Calcium sulphate hemihydrate

Preparation Method:

• Plaster of Paris is made from gypsum (CaSO₄ · 2H₂O).
• It is prepared by heating gypsum at 373 K.

Chemical Equation:

When water reacts with Plaster of Paris:

CaSO₄ · ½H₂O + 1½ H₂O → CaSO₄ · 2H₂O

Ans: (a) The test tube with magnesium had the fastest rate of bubble formation and the highest temperature.

(b) All three metals react with dilute hydrochloric acid as they are more reactive than hydrogen.

(c)(i) Hydrogen gas is not produced when a metal reacts with dilute nitric acid because nitric acid is a strong oxidising agent. It oxidises the hydrogen gas to water. The ultimate products are water and nitrogen oxides.
OR

(c)(i) The type of reaction that determines the reactivity of metals is a displacement reaction. If metal X displaces metal Y from its salt solution, then metal X is more reactive than metal Y, and vice versa.

Ans: (c)
Sol: When sodium bicarbonate reacts with dilute hydrochloric acid, carbon dioxide gas is liberated. The reaction that occurs is shown below:
NaHCO₃ + HCI → NaCl + H₂O + CO₂↑
Carbon dioxide when passed into lime water gives a milky solution. This is due to the formation of an insoluble suspension of calcium carbonate:

Ans: (a)

• Clove oil and vanilla essence are the correct pair of olfactory indicators.
• They change their smell in the presence of acids or bases.
• Other options like onion and turmeric do not serve as olfactory indicators.
• Litmus paper and methyl orange are not olfactory indicators.

Ans: (b)
Sol: Sodium hydroxide is classified as an alkali, while ferric hydroxide is not due to the following reasons:

• Sodium hydroxide is a strong base, whereas ferric hydroxide is a weak base.
• Sodium hydroxide is soluble in water, while ferric hydroxide is not.
• Sodium hydroxide is not a strong acid; it is a base, while ferric hydroxide does not fit this classification.
• Both are bases, but sodium hydroxide has a higher solubility in water compared to ferric hydroxide.

Ans: (c)
Sol: According to Arrhenius theory, acids are substances which dissociate in aqueous solution to give hydrogen ions (or hydronium ions).

Ans: (d)
Sol: 

Fresh milk has a pH of about 6 and can curdle when bacteria convert lactose into lactic acid, lowering the pH further. Sodium hydrogen carbonate (NaHCO₃), commonly called baking soda, is a mild base that neutralizes this acid, maintaining the pH closer to neutral and delaying curdling.

• Reaction:
  NaHCO₃ + H⁺ (from lactic acid) → Na⁺ + H₂O + CO₂
  This neutralizes the acid produced, preventing the milk from souring quickly.

Ans: (c)
Sol: Na₂CO₃.10H₂O is used to remove permanent hardness of water. Washing soda (Na₂CO₃.10H₂O) reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates, that can be removed by filtration and then water becomes soft.

Ans: (i) The compound formed is copper chloride (CuCl₂) and its colour is green in aqueous solution.
(ii) The balanced chemical equation for the reaction is: CuO + 2HCl → CuCl₂ + H₂O

Ans: (a) (i) 2AI + 3H₂SO₄ → AI₂(SO₄)₃ + 3H₂
(ii) 2AI + 2NaOH + 2H₂O → 2NaAIO₂ + 3H₂
(b) 

• The solution formed when copper oxide reacts with hydrochloric acid is a bluish-green colour.
• This occurs because copper oxide reacts with hydrochloric acid to produce copper chloride and water.
• The copper chloride dissolves in water, resulting in the bluish-green colour of the solution.

Ans: (i)  

•  During the dilution of a strong concentrated acid, always add acid to water and not the water to acid. 
•  The dissociation of an acid in water is a highly exothermic process, as the acid has a strong affinity for water. 
•  Adding water to acid can cause a violent reaction due to the rapid generation of heat. 
•  By adding acid to water slowly and with constant stirring, the heat generated can be dissipated more effectively, ensuring a safe dilution process. 

• The colour change requires the presence of hydrogen ions (H⁺).
• HCl gas can only produce these ions in an aqueous solution, where it dissociates.
• Without water, HCl gas does not release H  ions, hence no colour change occurs.

Ans: (i) Solution M gives a red color with a universal indicator, indicating that it is an acidic solution with a higher hydrogen ion concentration. Solution N gives a blue color with a universal indicator, indicating that it is a basic solution with a lower hydrogen ion concentration.
(ii) When solutions M and N are mixed, the resulting green color with a universal indicator indicates that the mixture is neutral. This suggests that a salt solution is formed, which is neither acidic nor basic.

Ans: (i) X is gypsum (CaSO₄ · 2H₂O) and Y is Plaster of Paris (CaSO₄ · 0.5H₂O). V is the substance doctors use for supporting fractured bones in the right position.
(ii) X can be reobtained from Y by:

• Adding water to Plaster of Paris.
• This process converts it back to gypsum.
  The reaction is: CaSO₄ · 0.5H₂O + 1.5H₂O → CaSO₄ · 2H₂O

Ans: (a) If in yCI, y is sodium, the salt formed is NaCI. NaCI is a salt of a strong acid (HCI) and a strong base (NaOH), making it a neutral salt. Hence, the pH of the salt solution would be 7.
(b) If in salt NH4X, X is nitrate, the salt formed is NH₄NO₃. NH₄NO₃ is a salt of a weak base (NH₄OH) and a strong acid (HNO₃). It is an acidic salt and will give an orange-yellow colour with a universal indicator.
(c) Potassium carbonate (K₂CO₃) is a basic salt formed from a strong base (KOH) and a weak acid (H₂CO₃). In solution, it hydrolyzes to produce OH⁻ ions, making the solution basic (pH > 7):

K₂CO₃ + H₂O ⇌ 2K⁺ + HCO₃⁻ + OH⁻
Since blue litmus paper remains blue in basic solutions and only turns red in acidic solutions, adding K₂CO₃ will not change its color—it stays blue.

Ans: Compound X: Sodium chloride (NaCl).

• Gas Y: Hydrogen gas (H₂) is liberated at the cathode.
• Gas Z: Chlorine gas (Cl₂) is liberated at the anode.
• Compound B: Bleaching powder (CaOCl₂) is formed when chlorine gas reacts with dry slaked lime (Ca(OH)₂).

Ans: (c)
Washing soda is chemically known as sodium carbonate decahydrate (Na₂CO₃·10H₂O). It is commonly used as a cleaning agent.
The other options represent different compounds:
(a) NaHCO₃: This is sodium bicarbonate, also known as baking soda.
(b) Na₂CO₃·5H₂O: This is not a common form of sodium carbonate; washing soda specifically has 10 molecules of water.
(d) NaOH: This is sodium hydroxide, also known as caustic soda.
Therefore, the correct answer is (c) Na₂CO₃·10H₂O.

Ans: (b)
(a) Lemon juice: Lemon juice is acidic with a pH around 2-3, but it is acidic, not basic. So, this option is incorrect.
(b) Milk of magnesia: This is basic with a pH around 10, which would turn the pH paper blue, indicating a basic nature. This option is correct.
(c) Gastric juice: Gastric juice is acidic with a pH around 1-3, not 6. A pH of 6 is close to neutral, which is not correct for gastric juice. So, this option is incorrect.
(d) Pure water: Pure water has a neutral pH of 7, but it would usually turn the pH paper green, not yellow. So, this option is also incorrect.
Therefore, the correct answer is (b) Milk of magnesia - Blue, pH 10, Basic.

​Ans: (A) The compound ‘X’ is Plaster of Paris. It is prepared by heating gypsum at 373 K. Its chemical formula is  CaSO₄ · 0.5H₂O.
(B) The differences between baking soda and baking powder are:
Baking Soda: Sodium bicarbonate (NaHCO₃), a single compound.Pure NaHCO₃, a base that releases CO₂ when heated or combined with an acid:
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (on heating).
Baking Powder: A mixture of sodium bicarbonate (NaHCO₃) and acid salts (e.g., monocalcium phosphate, Ca(H₂PO₄)₂), often with a filler like starch.
A leavening agent containing NaHCO₃ and an acid salt (e.g., monocalcium phosphate). When moistened, the acid reacts with NaHCO₃ to release CO₂ without needing an external acid:
NaHCO₃ + H⁺ (from acid salt) → Na⁺ + H₂O + CO₂.

Ans: . (A) (i) To get relief from this acidic pain, one must take antacids. These contain bases to neutralise the excess acids. 
Examples: Magnesium hydroxide (milk of magnesia) i.e., Mg(OH)₂ or Aluminium hydroxide. 
(ii)  

• A bee injects an acidic fluid into a person's skin when it stings them causing pain and irritation.
• Applying a baking soda solution, a base made of sodium hydrogen carbonate, is the cure.
• By neutralising the acid, the solution relieves the pain.
• Accidental contact with nettle plant leaves causes the stinging hair to inject formic or methanoic acid into the person's epidermis, causing burning discomfort.
• As the nettle sting is acidic, it can be relieved by applying a basic substance to the skin, such as baking soda.

Ans: (c)
Sol: 

• Orange is a natural source of citric acid.
• Tamarind contains tartaric acid.

Ans: (a)
Sol: Colour of red litmus remains red in neutral solutions, become dark red in acidic and blue in presence of basic solutions. So, there will be no change in colour of red litmus in distilled water (test tube A). In test tube B, it becomes dark red and in test tube C, red litmus turns blue.

Ans: (b)
Sol: 

• If you add water to acid, you form an extremely concentrated solution of acid initially.
• The reaction of conc. acid with water is highly exothermic, and the solution may boil very violently, splashing concentrated acid.
• If you add acid to water, the solution that forms is very dilute and a small amount of heat released.

Ans: (d)
Sol: Bases are substances which have bitter taste, soapy touch and turn red litmus solution to blue. Bases have pH more than 7. Bases give pink colour with phenolphthalein.

Ans: (a)
Sol: Few drops of universal indicators in a solution give yellowish orange colour to solution. This indicates that the solution used was acidic in nature. For example: lemon juice.

Ans: (a)
HCl is stronger acid than CH₃COOH so, pH of acetic add is more than that of hydrochloric acid.

Ans: Litmus is used in laboratories as an indicator to identify whether a substance is acidic or basic. It works as follows:

• Litmus changes colour in response to acidity or basicity.
• Red litmus paper turns blue in basic solutions.
• Blue litmus paper turns red in acidic solutions.
• When neutral, litmus remains purple.

Ans: Hydrochloric acid is considered a strong acid because it completely dissociates in water, releasing H⁺  ions. In contrast, acetic acid only partially dissociates, making it a weak acid.

Another example of a strong acid is H₂SO₄ (sulphuric acid).

Ans: (b)
Sol: Clove oil can be used as acid-base indicator by visually challenged student. Clove oil gives different odour in acidic and basic solution.

Ans: (c)
Sol: As the aqueous solution of A turns phenolphthalein solution pink, hence A is basic in nature. On adding an acidic solution, the pink colour will disappear. Hence, B is an acid.

Ans: (a)

Hydrochloric acid (HCl) is produced in the stomach and plays a vital role in digestion. Its key functions include:

• Breaking down food effectively.
• Creating an acidic environment that helps kill harmful bacteria.
• Ensuring proper digestion and maintaining stomach health.

Ans: (i) To prepare hydrogen chloride gas in the laboratory:

• Set up a test tube with some solid sodium chloride (NaCl).
• Add concentrated sulphuric acid (H₂SO₄) to the test tube.
• Observe the gas evolved from the delivery tube.

 
(ii) Testing the gas with litmus paper:

• Dry blue litmus paper: No colour change.
• Moist blue litmus paper: Turns red.

This indicates that hydrogen chloride (HCl) gas is not acidic without water but becomes acidic when in contact with moisture.

(iii) When hydrogen chloride (HCl) gas dissolves in water, it forms hydrochloric acid solution (HCl(aq)). In the solution, HCl dissociates and produces H⁺ or H₃O⁺ ions. The presence of these ions makes HCl solution acidic. The reaction can be represented as: HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl^-(aq)

Ans: (i) 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)
(ii) CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)
(iii) HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl^-(aq)

Ans: (a) To prepare a dilute solution of sulphuric acid, follow these steps:

• Start with a beaker filled with a sufficient amount of water.
• Slowly add the concentrated acid to the water while stirring continuously.
• Always add acid to water, never the other way around, to minimise the risk of splashing or glass breakage due to heat.

(b) The effect of dilution on the (H₃O⁺/OH⁻) ratio:

• When an acid or base is diluted, the concentration of ions (H₃O⁺ and OH⁻) per unit volume decreases.
• This results in a reduced (H₃O⁺/OH⁻) ratio.

(c) If the concentration of H₃O⁺ ions increases in a solution:

• The pH will decrease.
• For pH paper in the range of 0-5, the probable colours will be red or orange, indicating acidic conditions.

Ans: The important products of the Chlor-alkali process are sodium hydroxide, chlorine, and hydrogen.

• Sodium hydroxide (NaOH): It is used in the manufacture of soaps and detergents.
• Chlorine (Cl₂): It is used as a germicide and disinfectant for sterilization of drinking water and swimming pools.
• Hydrogen (H₂): It is used in the manufacture of ammonia, which is used for the preparation of various fertilizers.

Ans:  Washing soda is prepared by recrystallising sodium carbonate. The chemical equation for its preparation is:

Na₂CO₃(s) 10H₂O(l) → Na₂CO₃.10H₂O(s)

Washing soda is classified as a basic salt. It is effective in removing:

• Permanently hard water by eliminating calcium and magnesium ions.
• Temporary hardness caused by these ions.

Ans: (i) Only one half of the water molecule is shown in the formula of plaster of Paris (CaSO₄.½H₂O) because:

• One molecule of water is shared by two molecules of calcium sulfate (CaSO₄).
• This means the effective water of crystallisation for each CaSO₄ unit is half a molecule.

(ii) Sodium hydrogen carbonate (NaHCO₃) is used as an antacid because:

• It is a weak base.
• It reacts with excess acid in the stomach, neutralising it and providing relief from hyperacidity.

(iii) On strong heating, blue-coloured copper sulfate crystals turn white because:

• These crystals are hydrated copper sulfate (CuSO₄.5H₂O).
• When heated, they lose their water of crystallisation and become anhydrous copper sulfate (CuSO₄), which is white.

Ans: (i) The formula of 'G' is Cl₂ and the formula of 'C' is CaOCl₂.
(ii) The chemical equations involved are:

• At the anode during electrolysis of brine: 
  2NaCl(aq) + 2H₂O(l) → Cl₂(g) + H₂(g) + 2NaOH(aq)
• When chlorine gas (G) is passed through slaked lime:
  Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)

(iii) The common name of compound 'C' is bleaching powder. Its chemical name is calcium hypochlorite.

Ans:  

Ans: (a)  The chemical name of 'X' is Sodium Carbonate, commonly known as washing soda. Its chemical formula is Na₂CO₃. 

• Na₂CO₃ + CaCl₂ → CaCO₃ + 2NaCl
• Na₂CO₃ + MgSO₄ → MgCO₃ + Na₂SO₄

Ans: 

• The water of crystallisation gives the crystals of the salts their shape and colour.
• For example, the presence of water of crystallisation in copper sulphate crystals imparts them blue colour and crystalline shape.
• Heat a few crystals of CuSO₄. 5H₂O which are blue in colour in a dry boiling tube.
• On heating, the blue copper sulphate crystals turn white and a powdery substance is formed.
• Tiny droplets of water are seen in the boiling tube.
• Cool the boiling tube and add 2-3 drops of water on the white copper sulphate powder.
• The crystals of copper sulphate again become blue in colour.
• CuSO_4.5H₂O → CuSO₄.5H₂O

Ans: The colour change will be observed in test tube A containing dilute HCl. The blue litmus paper will turn red in test tube A. This is because HCl is an acid and it turns blue litmus paper red.

Ans: Out of HCI and CH₃COOH, CH₃COOH is a weak acid because it dissociates partially in the solution. This can be proved with the help of the following example. If 1 M HCI and 1 M CH₃COOH are taken in the beaker as shown in the figure, greater deflection is observed in the case of HCI. This shows that more ions are produced by HCI in the solution, which produces more current.

Ans: (a) pH scale is used to measure the strength of acids and bases.
(b) The pH value ranges from 0 to 14.
(c) Lowest value of pH scale indicates the highly acidic solution. Highest value of pH scale indicates the highly basic solution.

Ans: Sodium hydrogen carbonate (NaHCO₃) is basic in nature as on hydrolysis it gives a mixture of strong base (NaOH) and weak acid (H₂CO₃). Sodium hydrogen carbonate is converted to washing soda in the following way:
(i) Thermal decomposition of NaHCO₃:
2 NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(g)
(ii) Recrystallization of sodium carbonate:
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O (solid)
Thus, sodium hydrogen carbonate is converted into washing soda through thermal decomposition and recrystallization.

Ans: Solution 'X' is an acidic solution with a pH range of 3-4. Solution 'Y' is a basic solution with a pH of 9.

Ans: (a) The increasing order of pH values for solutions X, Y, and Z is Y < X < Z.
(b) Solution Z will change the color of phenolphthalein because it is a basic solution that changes the color of phenolphthalein from colorless to pink.

Ans:

• Sodium hydrogen carbonate (NaHCO₃) is considered an alkaline salt because it has a pH value greater than 7.
• This occurs as it is a salt formed from a strong base and a weak acid.
• Two important uses of sodium hydrogen carbonate are:
• It serves as an antacid in medicines, helping to neutralise excess stomach acid.
• It is used as a food additive in various products, enhancing taste and texture.

Ans:  

• Sodium chloride is obtained by the neutralization of sodium hydroxide (base) with hydrochloric acid (acid). 
•  It is a neutral salt. 
•  When sodium chloride is found in the form of solid deposits with impurities, it is called rock salt. 
• Rock salt is formed by the evaporation of salty water from inland lakes. 

Ans: 

• The white powder added while baking cakes to make them soft and spongy is baking powder.
• Its main ingredients are sodium hydrogen carbonate and a mild edible acid like tartaric acid or citric acid.
• Sodium hydrogen carbonate decomposes to give out carbon dioxide, which causes the cake to rise and makes it soft and spongy.
• The function of tartaric acid or citric acid is to neutralize sodium carbonate formed during heating, which can otherwise make the cake bitter.
• The reaction taking place when the powder is heated is : 2 NaHCO₃ (heat) → Na₂CO₃ + H₂O + CO₂

Ans: (a) The electrolysis of brine is called the 'Chlor-alkali process' because it produces chlorine (chlor) and sodium hydroxide (alkali). The chemical equation involved in this process is: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
(b) Two observations when hydrated copper sulphate crystals are heated in a dry test-tube are:
(i) The color of copper sulphate crystals becomes white after heating.
(ii) Water droplets are noticed at the mouth side of the boiling tube, which are obtained from the water of crystallization.

Ans: (A) Observation: The moist blue litmus paper will turn red. 
Inference: The gas liberated is acidic in nature. 
(B) Observation: Wet red litmus paper will remain red. 
Inference: The gas liberated is acidic in nature.

Ans: 

• It is observed that active metals like zinc react with strong bases like sodium hydroxide (NaOH) to liberate hydrogen gas (H₂) and the corresponding salt.
• The equation for the chemical reaction is: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)
• The evolution of gas is confirmed by the bubble formation in soap solution.
• The test to detect hydrogen gas (H₂) is as follows:
• When a burning matchstick is kept on the mouth of the test tube, a "pop" sound is heard, which confirms the presence of hydrogen gas.
• When the same metal (zinc) reacts with a dilute solution of a strong acid, hydrogen gas (H₂) will also be evolved.

Ans: Sodium bicarbonate (NaHCO₃), but the pH is approximately 8-9, not 14.
Sodium bicarbonate (NaHCO₃) is indeed used in cooking (e.g., pakoras) to release CO₂, making the batter light and crispy:

• Preparation (Solvay process):
  NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
  However, NaHCO₃ is a weak base (from a strong base, NaOH, and weak acid, H₂CO₃), and its aqueous solution has a pH of about 8-9, not 14. A pH of 14 indicates a strong base like NaOH, which is not used for pakoras due to its caustic nature.
• Reaction in cooking:
  2NaHCO₃ (heat) → Na₂CO₃ + H₂O + CO₂

Ans: 

• X must be a compound which forms water with acids.
• This means it must be a base, which reacts with acids to form salt and water.
• This base also reacts with zinc metal and releases hydrogen gas.
• So, it must be NaOH (sodium hydroxide).

Ans:  

• When a base reacts with a non-metallic oxide, it forms a salt and water.
• For example, when calcium hydroxide (a base) reacts with carbon dioxide (a non-metallic oxide), it produces calcium carbonate (salt) and water.
• The reaction can be represented as follows: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

Ans: The dissociation of the given substances to produce ions in their solutions are as follows:
(i) Hydrochloric acid (HCI): HCI → H⁺ + Cl^-
(ii) Nitric acid (HNO₃): HNO₃ → H⁺ + NO₃^-
(iii) Sulphuric acid (H₂SO₄): H₂SO₄ → 2H⁺ + SO₄^2-
(iv) Sodium hydroxide (NaOH): NaOH → Na⁺ + OH^-
(v) Potassium hydroxide (KOH): KOH → K⁺ + OH^-
(vi) Magnesium hydroxide (Mg(OH)₂): Mg(OH)₂ → Mg²⁺ + 2OH^-

Ans:  An antacid works by neutralising excess stomach acid, which can lead to discomfort and irritation. Here's how it functions:

• Antacids contain a base, such as magnesium hydroxide (found in Milk of Magnesia).
• When taken, the base reacts with the excess hydrochloric acid in the stomach.
• This reaction produces salt and water, alleviating symptoms of indigestion.
• The resulting salt, magnesium chloride, is neutral and helps relieve pain.
  The reaction can be represented as follows:
  Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Ans: 
(i) Phenolphthalein in soap solution turns pink.
(ii) Chlorine gas (Cl₂) is the by-product of the chlor-alkali process used to manufacture bleaching powder.
(iii) One indicator that shows various levels of H⁺ ion concentration is litmus.

Ans:  

• When excess carbon dioxide is passed through lime water, the following occurs:
• The lime water initially turns milky due to the formation of calcium carbonate (CaCO₃).
• With further addition of carbon dioxide, the solution becomes colourless as calcium bicarbonate (Ca(HCO₃)₂) is formed, which is soluble in water.
• The chemical reactions involved are:
  Ca(OH)₂ + CO₂ → CaCO₃(s) + H₂O
  CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂

Ans: 
(i) Combination reaction.
(ii) Double displacement reaction.
(iii) Decomposition reaction.

Ans: (a) 

• Indicators which work on the basis of odour are called olfactory indicators.
• Onion and vanilla extracts can be used as olfactory indicators.

Ans: (a)  To reduce the acidity of soil, the following materials can be added: 

• Lime, (CaO) can be added to neutralize acidity.
• Chalk, (CaCO3) can also be added to neutralise acidity.

(b) The nature of gas is basic because it turns phenolphthalein pink. The following reaction takes place in aqueous solution, 

Ans: 
(a) Soil B has a pH of 4.5, making it acidic. It should be treated with powdered chalk, which is basic, to raise its pH.

(i) During an ant's sting: Formic acid (HCOOH).

(ii) During a nettle sting: Formic acid (HCOOH).

• The effects of these stings can be neutralised by rubbing the area with a mild base, such as baking soda (NaHCO₃).

• It may reduce the amount of oxygen dissolved in the water.
• Both acidic and basic water can harm the skin of aquatic animals, such as fish.

Ans: 
(i)

• A stain of curry on a white cloth turns reddish-brown when soap is scrubbed on it because:
• Turmeric in the curry reacts with sodium hydroxide in the soap, forming a red compound.
• When the cloth is rinsed with plenty of water, the sodium hydroxide becomes very dilute, stopping the reaction and returning the stain to its original yellow colour.

(ii) 

• Curd should not be stored in copper or brass vessels because:
• Curd contains lactic acid, which reacts with these metals, altering the taste.
• To prevent this, curd should be kept in glass, steel, or ceramic containers that do not react with lactic acid.

Ans: 
(a) A is basic, ‘B’ is acidic ‘C’ is neutral and ‘D’ is strongly acidic.
(b) Hyperacidity is Caused by excess of hydrochloric acid in stomach. Antacid is basic in nature. It neutralises excess of acid and releases CO₂ gas which gives relief from pain caused by hyperacidity.

Ans: 
(a) Hydrogen gas will be formed, e.g.
Mg (s) + 2HCl (dil) → MgCl₂ (aq) + H₂ (s)
(b) Sodium chloride and water will be formed, e.g.
NaOH + HCl → NaCl + H₂O (Neutralization reaction)
(c) Crushed egg shells are made up of CaCO₃ which reacts with dil HCl to give brisk effervescence due to CO₂, e.g.
CaCO₃ (s) + 2HCl → CaCl₂ + H₂O + CO₂

Ans: 

• Water which is present in the crystals of a compound is called water of crystallisation.
• It is defined as the fixed number of water molecules present in one formula unit of a salt.
• Copper sulphate crystals are blue in colour.
• On heating solid copper sulphate crystals, they lose water of crystallisation and become white.
• On adding a few drops of water to the white powder, blue colour of copper sulphate is restored.
• This shows that copper sulphate crystals contain water of crystallisation.

Ans: 
(a) (i) Solution A has the highest concentration of H⁺ ions.
(ii) Solution C has the lowest concentration of H⁺ ions.
(b)  To dilute concentrated sulphuric acid:

• Always add the acid to water, not the other way around.
• Start by pouring some water into a beaker.
• Gradually add concentrated sulphuric acid in small amounts to the water.
• Stir the mixture continuously with a glass rod.
• If the beaker gets hot, cool it by placing it in cold water.

Ans: Chemical name of bleaching powder is calcium oxychloride.
Formula: CaOCl₂ . On exposure to air, it absorbs moisture.
Uses:
(i) As an oxidising agent in chemical industries.
(ii) For disinfecting drinking water to make it free from germs. 

Ans: 
(a) (i) The pH value will be greater than 7.
(ii) The pH value will be less that than 7.
(b) (i) When mixing, the acid should be added slowly to water. This prevents splashing and allows for constant cooling since the reaction is highly exothermic.

(ii) This process is known as dilution.

(c) (i) Sulphurous acid is formed.
SO₂ + H₂O → H₂SO₃ (Sulphurous acid)

Lime water turned turns milky due to formation of calcium sulphite.

Ans: 
(a)  (i) It is weakly basic in nature and neutralises hyperacidity.
(ii) It liberates CO₂ with H₂SO₄ reacts with it, which extinguisher fire.
(iii) It liberates CO₂ on heating which makes bread and cake soft and sponge.
(b) Baking soda on heating gives sodium carbonate which on crystallisation from aqueous solution gives washing soda, e.g.
Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O

Ans: 

(a) Tap water conducts electricity because it contains ions that facilitate electrical flow, while distilled water lacks these ions. 

(b) Dry hydrogen chloride (HCl) gas does not turn blue litmus red because it does not form ions. In contrast, dilute hydrochloric acid releases H and Cl- ions, which are responsible for its acidic properties.

(c) A milkman adds a small amount of baking soda to fresh milk to prevent it from souring. Baking soda inhibits the formation of lactic acid, which causes milk to spoil. 

(d)  When diluting an acid, it is crucial to add acid to water rather than the other way around. This is because adding water to acid can cause a violent reaction and splashing due to the heat generated. 

(e) Ammonia is classified as a base despite not containing a hydroxyl group. When dissolved in water, it produces hydroxide ions (OH-), which gives it basic properties.  NH₃ + H₂O → NH₄⁺ + OH^-

Ans: (a)  A Universal indicator is a mixture of several indicators. It shows different colours at different concentrations of H⁺ ion concentrations.