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

Ans: (b)
Both turmeric and litmus are natural indicators that change color when they come into contact with acids or bases. Turmeric turns red in alkaline solutions, while litmus turns red in acidic solutions and blue in alkaline solutions, making 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 made up of a mineral called hydroxyapatite, which is a form of calcium phosphate. This compound helps strengthen teeth and makes them resistant to decay.

Ans: (c)
An aqueous solution of sodium chloride (table salt) in distilled water is neutral, meaning it has a pH of 7. This is because sodium chloride does not produce any hydrogen ions (H⁺) or hydroxide ions (OH^-) in solution, keeping the acidity and alkalinity balanced.

Ans: (d)
When solid calcium oxide reacts with water, it releases heat (making it exothermic) and forms calcium hydroxide, which is a strong base. This means the pH of the resulting solution is greater than 7, indicating it is alkaline.

Ans: (c)
The juice of tamarind contains tartaric acid, which is a natural organic acid. This acid makes the solution acidic, which causes blue litmus paper to turn red, indicating the presence of acidity.

Ans: (b)
The answer is (b) Al₂O₃ (aluminum oxide). Aluminum oxide is an amphoteric oxide, meaning it can react with both acids like hydrochloric acid (HCl) and bases like potassium hydroxide (KOH). These reactions produce a salt and water, showing its ability to neutralize both acidic and basic substances.

Ans: (a)
In the given reaction, heating calcium carbonate (CaCO₃) results in its decomposition to form calcium oxide (CaO), commonly known as quick lime, and carbon dioxide (CO₂). This process is known as calcination, commonly used in the production of cement and lime. Hence, the product 'X' is 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)
(c) Salts having same cations but different anions belong to the same family of salts. e.g. sodium chloride (NaCl) and Washing Soda/sodium carbonate (Na₂CO₃) both have Na⁺ as cation.
OR
(c) A scale for measuring hydrogen ion (H⁺) concentration in a solution is called pH scale.
Potassium Sulphate / K₂SO₄
pH = 7

Ans: (i) HCl gas was evolved
(ii) (I) No change in colour
(II) Wet blue litmus turns red
HCl gas is acidic in nature

Ans: (a) (i) The molecules of water of crystallisation in ferrous sulphate crystals get evaporated on heating.
(ii) Green → White
(iii) Seven / (FeSO₄ × 7H₂O) (I) CuSO₄ × 5H₂O (II) Na₂CO₃ × 10H₂O
(iv) On heating gypsum (CaSO₄ × 2H₂O) at 373 K it loses water molecules

Two uses of plaster of Paris:
(i) Making toys / material for decoration
(ii) Supporting fractured bones
OR
(b) (i) X-Tartaric acid Y-Baking soda Z- Baking powder Y- NaHCO₃
(ii)
Sodium salt of acid CO₂ released during heating makes the cake soft and spongy
(iii) Magnesium hydroxide; Mg(OH)₂

Ans: Plaster of Paris; Calcium sulphate hemihydrate
Prepared from gypsum (CaSO₄ × 2H₂O) by heating it at 373K

Ans: (a) In the test tube containing magnesium.
(b) All three metals react with HCl because they are more reactive than hydrogen.
(c) (i) Because HNO₃ is a strong oxidizing agent and oxidizes the H₂ produced to water.
Ultimate products are water, oxides of nitrogen.
OR
(c) (ii) Displacement Reaction
If metal X displaces metal Y from its salt solution it is more reactive than Y or 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)
Sol: There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators. Clove oil and vanilla essence are olfactory indicators.

Ans: (b)
Sol: Sodium hydroxide, potassium hydroxide, calcium hydroxide, etc. are soluble in water, therefore they are termed as alkalies while bases like copper hydroxide, ferric hydroxide, aluminium hydroxide, etc. do not dissolve in water, therefore they are not termed as alkalies.

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

Ans: (b)
Sol: Baking soda, or sodium hydrogen carbonate (NaHCO₃) is used to delay the curdling of milk.

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 colour of the solution formed when copper oxide is treated with hydrochloric acid is a bluish-green colour. This is because copper oxide reacts with hydrochloric acid to produce copper chloride and water. The copper chloride formed 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.
(ii) The salt formed when sulphuric acid is added to sodium hydroxide is sodium sulphate (Na₂SO₄). Sodium sulphate is a neutral salt, and its aqueous solution will have a pH of 7.
(iii) Dry hydrochloric acid (HCl) gas does not change the colour of dry blue litmus paper because the colour change in blue litmus paper is only observed in the presence of hydrogen ions (H⁺) or hydronium ions (H₃O⁺). HCl can produce these ions only in aqueous solution, where it dissociates to release H⁺ ions. In the absence of water, HCl gas does not produce H⁺ ions and, therefore, does not cause a colour change in dry blue litmus paper.

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 V by adding water to Plaster of Paris, which will convert it back to gypsum. The reaction is as follows:
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 color with a universal indicator.
(c) If ZCO₃ is added to a blue litmus solution and Z is potassium, the solution will turn red. This is because ZCO₃ is potassium carbonate, which is a basic salt. It will neutralize the acidity of the blue litmus solution, causing it to turn red.

Ans: Compound X is sodium chloride (NaCl).
Gas Y liberated at the cathode is hydrogen gas (H₂).
Gas Z liberated at the anode is chlorine gas (Cl₂).
When chlorine gas (Z) is treated with dry slaked lime (Ca(OH)₂), it forms bleaching powder (CaOCl₂ or B).

Ans: (c)
Sol: Orange is a natural source of citric acid while 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 solution may boil very violently, splashing concentrated acid. If you add acid to water, the solution that forms is very dilute and 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 determine whether a substance is acidic or basic. It changes color in the presence of acids or bases, with blue indicating a basic solution and red indicating an acidic solution.

Ans: Hydrochloric acid is a stronger acid than acetic acid because it dissociate completely in aqueous solution to give H⁺ ions. H₂SO₄ (sulphuric acid) is an example of strong 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 phenol- phthalein 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 helps break down food during digestion. It also creates an acidic environment that kills harmful bacteria, making it essential for proper digestion and maintaining stomach health.

Ans:
 
(ii) The litmus paper does not show any change in color when tested with dry blue litmus paper. However, when tested with moist blue litmus paper, it turns red. This indicates that hydrogen chloride (HCl) gas does not exhibit acidic behavior in the absence of water but shows acidic behavior in the presence of water.
(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: (i) Dilution of a concentrated acid is a highly exothermic reaction. To prevent potential hazards, the dilution process should be carried out carefully. The recommended method is to take a beaker with a sufficient amount of water and slowly add the concentrated acid to it while stirring. It is important to add acid into water and not the other way around to reduce the risk of splashing or glass breakage due to localized heating.
(ii) Dilution of an acid or base leads to a decrease in the concentration of ions (H₃O⁺ and OH^-) per unit volume. Therefore, the (H3O⁺/OH^-) ratio is also reduced.
(iii) If the concentration of H₃O⁺ ions increases in a solution, the pH will decrease. As for the probable colors of pH paper, pH range 0-5 to 2-0 typically corresponds to acidic conditions. The probable colors of pH paper in this range can be red or orange, indicating acidity.

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 recrystallization of sodium carbonate. The chemical equation for its preparation is:
Na₂CO₃(s) + 10H₂O(l) → Na₂CO₃.10H₂O(s)
Washing soda is a basic salt. It is used to remove the permanent hardness of water. It can remove the calcium and magnesium ions that cause temporary hardness of water.

Ans: (i) Only one half of the water molecule is shown in the formula of plaster of Paris (CaSO₄.¹/₂H₂O) because one molecule of water is being shared by two molecules of calcium sulfate (CaSO₄). So the effective water of crystallization for one CaSO₄ unit comes to half a molecule of water.
(ii) Sodium hydrogen carbonate (NaHCO₃) is used as an antacid because it is a weak base and can react with the excess acid produced in the stomach due to hyperacidity, neutralizing it and providing relief.
(iii) Blue-colored copper sulfate crystals are hydrated copper sulfate (CuSO₄.5H₂O). On strong heating, these crystals lose their water of crystallization and turn into anhydrous copper sulfate (CuSO₄), which is white in color.

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: Onion juice is an of factory indicator. Olfactory indicators give one type of odour in acidic medium and a different odour in basic medium. As the liquid 'X' turns blue litmus red, hence it is an acidic solution.
(a) Acids react with active metals such as zinc, magnesium, etc. and evolve hydrogen gas. The observation with zinc granules will be the evolution of hydrogen gas. The chemical equation for the reaction is: Zn(s) + dil.H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
(b) Acids react with metal carbonates to give carbon dioxide with brisk effervescence. The observation with solid sodium carbonate will be the release of carbon dioxide gas. The chemical equation for the reaction is:
2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + H₂O(l) + CO₂(g)

Ans: (a) The chemical name of 'X' is Sodium Carbonate, commonly known as washing soda. Its chemical formula is Na₂CO₃.
(b) The equation involved in the preparation of Sodium Carbonate is:
2NaCl + H₂O + CO₂ → Na₂CO₃ + 2HCl
(c) When Sodium Carbonate is treated with water containing calcium (Ca) or magnesium (Mg) salts, it forms insoluble precipitates. For example:
Na₂CO₃ + CaCl₂ → CaCO₃ + 2NaCl
Na₂CO₃+ MgSO₄ → MgCO₃ + Na₂SO₄

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 an alkaline salt because its pH value is greater than 7. It is a salt of a strong base and a weak acid. Two important uses of sodium hydrogen carbonate are:
(i) It is used as an antacid in medicines.
(ii) It is used as an additive in food and drinks.

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: 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: The salt used to make tasty and crispy pakoras is sodium bicarbonate (NaHCO₃). On a large scale, sodium bicarbonate is prepared as follows:
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Two uses of sodium bicarbonate are as follows:
(i) It is used as an antacid in medicines.
(ii) It is used as an additive in food and drinks.

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
From this reaction, we can infer that non-metallic oxides are generally acidic in nature.

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 neutralizing excess stomach acid, which can cause indigestion, pain, and irritation. It contains a base, such as magnesium hydroxide (Milk of Magnesia), which reacts with the excess hydrochloric acid in the stomach to form a salt and water. The reaction can be represented as follows:
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
The formed salt, magnesium chloride, is neutral and helps alleviate the symptoms of indigestion.

Ans: 
(i) Pink
(ii) Chlorine
(iii) Universal indicator

Ans:
Ca(OH)₂ + CO₂ → CaCO₃(s) + H₂O
CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂
Lime water turns milky due to the formation of calcium carbonate. It becomes colourless when excess of CO₂ is passed due to formation of Ca(HCO₃)₂ which is soluble in water.

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.
(b) H₂SO₄ and HNO₃ are strong acids out of the four acids given. 

Ans: 
(a) 
(i) Lime, (CaO) can be added to neutralize acidity.
(ii) Chalk, (CaCO₃) can also be added to neutralise acidity.
It is because CaO and CaCO₃ are basic in nature which neutralize acid present in soil.
(b) The nature of gas is basic because it turns phenolphthalein pink. The following reaction takes place in aqueous solution,
Example:  NH₃ + H₂O → NH₄⁺ + OH^-

Ans: 
(a) Soil ‘B’ is acidic. It should be treated with powdered chalk which is basic so as to adjust its pH.
(b)
(i) Formic acid, (HCOOH).
(ii) Formic acid, (HCOOH).
The effect of these stings neutralized by rubbing the skin with mild base like baking soda (NaHCO₃).
(c) If pH of river water changes, amount of oxygen dissolved in water may decrease. Acidic and basic water is harmful for skin of aquatic animals like fish.

Ans: 
(i) Turmeric reacts with sodium hydroxide present in soap to form red coloured compound. It turns yellow again because sodium hydroxide becomes very dilute on adding lot of water and reaction stops.
(ii) Curd contains lactic acid which reacts with copper or brass vessels and taste changes. Curd should be kept in glass, steel or ceramic container which does not react with lactic acid present in it.

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) While diluting concentrated sulphuric acid, water is not added to the acid, rather acid is added to water.
Take some water in a beaker. Add cone, sulphuric acid in small lots to water. Stir the mixture with glass rod constantly. Cool the beaker under water if it becomes hot.

Ans: Chemical name of bleaching powder is calcium oxychloride.
Formula: CaOCl₂ 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) The acid is to be added slowly in water to prevent the mixture to be from being splashed. The  reaction is highly exothermic, therefore, constant cooling should be done.
(ii) The process is called 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 contains ions which conduct electricity, distilled water does not contain ions.
(b) Dry HCl does not form ions but HCl gives H⁺ and Cl^-.
(c) Baking soda does not allow milk to change to lactic acid which makes milk sour.
(d) Adding water to acid is highly exothermic and and acid may splash. Therefore, acid is added to water very slowly with continuous stirring.
(e) Ammonia dissolves in water forms OH^-. Therefore, it is basic.
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.
(b) Solution A is alkaline in nature.
Solution B is acidic in nature.
Solution A has a higher pH value.
(c) Salts of weak acid and strong base give a solution which have pH more than 7.
i.e. basic salt
Sodium acetate is an example of such salts.
Salts of strong acid and weak base give a solution which has pH less than 7.
i.e. Acidic salt. Ammonium chloride is an example of such salts.