CBSE Class 10 Science Acids bases and salts Sure Shot Questions Set 05

These 22 sure shot questions for Class 10 Science Chapter 2, Acids, Bases and Salts, come with step-by-step answers and teacher's notes. They cover the board-style questions on Plaster of Paris, bleaching powder, baking soda, washing soda, the chlor-alkali process, neutralisation, pH and the test for hydrogen and carbon dioxide.

Short Answer Questions

Question 1. Explain the following giving equation in each:
(i) Baking soda is heated.
(ii) Washing soda is heated.
(iii) Gypsum is heated at 373 K.
Answer:
(i) Baking soda decomposes to give sodium carbonate, water and carbon dioxide: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑.
(ii) Washing soda loses its water of crystallisation and becomes a white powder (monohydrate), which on further heating gives anhydrous sodium carbonate: Na₂CO₃·10H₂O → Na₂CO₃·H₂O + 9H₂O; Na₂CO₃·H₂O → Na₂CO₃ + H₂O.
(iii) Gypsum loses water of crystallisation and forms calcium sulphate hemihydrate, which is Plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
In simple words: Heating takes away the loosely held water or carbon dioxide from these salts.

Teacher's Note:
1. Gypsum has 2 molecules of water and Plaster of Paris has half a molecule.
2. Do not forget to balance the water molecules released.

Question 2. Write the chemical name for Plaster of Paris. Write the chemical equation of its preparation. Why should Plaster of Paris be stored in a dry place?
Answer:
Chemical name: calcium sulphate hemihydrate, formula CaSO₄·½H₂O.
Preparation: gypsum is heated at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
It must be stored in a dry (moisture-proof) place because in the presence of moisture it absorbs water and sets into a hard solid mass of gypsum, CaSO₄·2H₂O.
In simple words: POP turns back into hard gypsum if it gets wet, so keep it dry.

Teacher's Note:
1. POP has half a molecule of water per formula unit, so write ½H₂O.
2. Quote the reverse reaction with water for the reason.

Question 3. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer:
Dissolving an acid or a base in water is a highly exothermic process.
If water is added to a concentrated acid, the large heat evolved can make the mixture splash out and cause burns, and the glass container may break due to local heating.
So the acid is added slowly to water with constant stirring, so that the heat spreads in the large amount of water.
In simple words: Add acid to water slowly; never add water to concentrated acid.

Teacher's Note:
1. This is a very frequently asked safety question.
2. Use the word exothermic in your answer.

Question 4. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Answer:
Hydrogen gas is usually liberated when an acid reacts with a metal.
Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑.
Test: pass the gas through soap solution to form bubbles and bring a burning candle near them. The gas burns with a pop sound, showing it is hydrogen.
In simple words: Metal + acid gives a salt and hydrogen; hydrogen burns with a pop.

Teacher's Note:
1. Mention the pop sound for full marks.
2. Hydrogen gas burns with a pop sound.

Question 5. Define water of crystallisation with two examples. How will you prove their existence in the examples given by you?
Answer:
Water of crystallisation is the fixed number of water molecules chemically combined with one formula unit of a salt in its crystals.
Examples: washing soda, Na₂CO₃·10H₂O (10 molecules); copper sulphate crystals, CuSO₄·5H₂O (5 molecules).
Proof: on heating the crystals, they lose water of crystallisation. Blue CuSO₄·5H₂O turns white, and droplets of water appear on the cooler part of the test tube. Adding water to the white salt restores the blue colour.
In simple words: Water of crystallisation is the water built into the crystals of a salt.

Teacher's Note:
1. Copper sulphate is the easiest example for the test.
2. Remember the number of water molecules: copper sulphate 5, washing soda 10, gypsum 2.

Question 6. Illustrate any three chemical properties of acids with examples.
Answer:
1. Reaction with metals: Acid + Metal → Salt + Hydrogen. Example: Fe + H₂SO₄ → FeSO₄ + H₂↑
2. Reaction with metal oxides: Acid + Metal oxide → Salt + Water. Example: CuO + 2HCl → CuCl₂ + H₂O
3. Reaction with bases (neutralisation): Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O
In simple words: Acids react with metals, metal oxides and bases (also with carbonates).

Teacher's Note:
1. Write the word equation before the example.
2. Balance every equation.

Question 7. Identify the acid and the base from which sodium chloride is obtained. Which type of salt is it? When is it called rock salt? How is rock salt formed?
Answer:
Sodium chloride is obtained from hydrochloric acid and sodium hydroxide: HCl + NaOH → NaCl + H₂O.
It is a neutral salt (salt of a strong acid and a strong base).
Large crystals of common salt, usually brown because of impurities, are called rock salt. It is found in underground deposits formed when ancient seas dried up long ago.
In simple words: Common salt comes from HCl and NaOH; rock salt is its large natural crystals.

Teacher's Note:
1. Rock salt is mined like coal.
2. Give the equation of formation.

Question 8. A sanitary worker uses a white chemical having strong smell of chlorine gas to disinfect the water tank:
(i) Identify the chemical compound, write its chemical formula.
(ii) Give chemical equations for its preparation.
(iii) Write its two uses other than disinfection.
Answer:
(i) The compound is bleaching powder (calcium oxychloride), CaOCl₂.
(ii) It is made by passing chlorine over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
(iii) It is used for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories, and as an oxidising agent in chemical industries.
In simple words: Bleaching powder smells of chlorine, is made from slaked lime and chlorine, and is used for bleaching and disinfecting.

Teacher's Note:
1. Dry slaked lime is used, not lime water.
2. Give two uses other than the one asked.

Question 9. 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid?
Answer:
Zinc reacts with sodium hydroxide to give sodium zincate and hydrogen: Zn + 2NaOH → Na₂ZnO₂ + H₂↑.
Test: the gas forms soap bubbles. A burning splinter brought near them burns with a pop sound, which shows hydrogen.
With dilute strong acid, the same gas, hydrogen, is evolved: Zn + 2HCl → ZnCl₂ + H₂↑.
In simple words: Zinc gives hydrogen with both a base and an acid.

Teacher's Note:
1. Sodium zincate is the salt formed with NaOH.
2. Hydrogen gas burns with a pop sound.

Question 10. Answer the following questions:
(i) State the number of water molecules present in crystals of washing soda and Plaster of Paris. What are these water molecules called as?
(ii) Write the chemical name and formula of gypsum. What happens when gypsum is heated to 373 K. Write chemical equation for the reaction.
Answer:
(i) Washing soda, Na₂CO₃·10H₂O, has 10 molecules of water; Plaster of Paris, CaSO₄·½H₂O, has half a molecule per formula unit. These water molecules are called water of crystallisation.
(ii) Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. When heated at 373 K it loses water and forms Plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
In simple words: Washing soda has 10 and POP has half a water molecule; gypsum gives POP on heating.

Teacher's Note:
1. Learn all water-of-crystallisation numbers together.
2. Remember the number of water molecules: copper sulphate 5, washing soda 10, gypsum 2.

Question 11. Two solutions X and Y are tested with universal indicator. Solution X turns orange whereas solution Y turns red. Which of the solutions is a stronger acid? State the meaning of strong acids and weak acids. Give example of each.
Answer:
Red colour corresponds to a lower pH than orange, so solution Y is the stronger acid.
Strong acids ionise completely in water, for example HCl and H₂SO₄.
Weak acids ionise only partially in water, for example acetic acid, CH₃COOH, and carbonic acid, H₂CO₃.
In simple words: Red means stronger acid; strong acids ionise fully, weak acids only partly.

Teacher's Note:
1. Lower pH means more H⁺ and a stronger acid.
2. Give one example each.

Question 12. Explain why an aqueous solution of sodium sulphate is neutral while an aqueous solution of sodium carbonate is basic in nature.
Answer:
Sodium sulphate is the salt of a strong acid (H₂SO₄) and a strong base (NaOH), so its solution is neutral (pH = 7).
Sodium carbonate is the salt of a weak acid (carbonic acid) and a strong base (NaOH), so its solution is basic (pH greater than 7).
In simple words: The strong partner decides the nature of the salt solution.

Teacher's Note:
1. Strong acid + weak base gives an acidic salt solution.
2. Weak acid + strong base gives a basic salt solution.

Question 13. (i) State the chemical properties on which the following uses of baking soda are based:
(a) As an antacid (b) As a soda acid fire extinguisher (c) To make bread and cake soft and spongy.
(ii) How is washing soda obtained from baking soda? Write balanced chemical equation.
Answer:
(i)(a) It is mildly basic, so it neutralises excess acid in the stomach.
(b) It reacts with acid (sulphuric acid) to release carbon dioxide, which smothers the fire: NaHCO₃ + H₂SO₄ → NaHSO₄ + H₂O + CO₂↑ (or 2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂).
(c) On heating it gives carbon dioxide, which makes bread and cake soft and spongy.
(ii) Baking soda is heated to give sodium carbonate: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Recrystallisation of sodium carbonate gives washing soda: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
In simple words: Baking soda is basic, gives CO₂ with acids and on heating, and can be changed to washing soda.

Teacher's Note:
1. Use the keywords: mild base, carbon dioxide.
2. Write both steps of the conversion.

Question 14. For making cake, baking powder is taken. If at home your mother uses baking soda instead of baking powder in cake:
(i) How will it affect the taste of the cake and why?
(ii) How can baking soda be converted into baking powder?
(iii) What is the role of tartaric acid added to baking soda?
Answer:
(i) The cake will taste bitter. On heating, baking soda changes to sodium carbonate, which is bitter and basic: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
(ii) Baking soda becomes baking powder when mixed with a mild edible acid such as tartaric acid.
(iii) Tartaric acid neutralises the sodium carbonate formed, so the cake is not bitter. The carbon dioxide released makes the cake rise and become soft and spongy.
In simple words: Baking powder has an acid that removes the bitter taste of sodium carbonate.

Teacher's Note:
1. Bitter taste is a property of bases.
2. Name tartaric acid in the answer.

Long Answer Questions

Question 15. State reason for the following statements:
(i) Tap water conducts electricity whereas distilled water does not.
(ii) Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does.
(iii) During summer season, a milk man usually adds a very small amount of baking soda to fresh milk.
(iv) For a dilution of acid, acid is added into water and not water into acid.
(v) Ammonia is a base but does not contain hydroxyl group.
Answer:
(i) Tap water contains dissolved salts that give ions, which carry current. Distilled water has no ions.
(ii) Dry HCl gas has no H⁺ ions. In dilute hydrochloric acid, HCl ionises to give H⁺, which turns blue litmus red.
(iii) In summer, bacteria turn milk sour (lactic acid) and it curdles. Baking soda is basic and neutralises the acid, so the milk stays fresh for longer. Only a small amount is added so that milk does not become too basic.
(iv) Dissolving an acid in water is highly exothermic. If water is added to concentrated acid, the heat can make the mixture splash out and cause burns or crack the container. Adding acid slowly to water with stirring spreads the heat safely.
(v) Ammonia dissolves in water and gives hydroxide ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The OH⁻ ions come from water, so ammonia does not itself contain a hydroxyl group but behaves as a base.
In simple words: Ions carry current; acids need water to release H⁺; baking soda neutralises sour milk; acid goes into water for safety.

Teacher's Note:
1. Each part needs one clear reason in one or two sentences.
2. For part (v), remember that bases need not contain OH in their formula.

Question 16. Answer the following questions:
(i) What is the importance of pH in everyday life?
(ii) Why does tooth decay start when the pH of mouth is lower than 5.5?
(iii) What is the change in pH value of milk when it changes into curd? Explain.
Answer:
(i) Many body processes work within a narrow pH range, for example our stomach has pH around 1.2 for digestion, plants need a suitable soil pH, and acid rain with pH below 5.6 harms aquatic life.
(ii) Tooth enamel, calcium phosphate, is corroded when the pH in the mouth falls below 5.5. Bacteria in the mouth break down sugar and food particles to make acids, which attack the enamel and cause decay.
(iii) Milk has pH close to 6. When bacteria change it into curd, lactic acid is formed, so the pH decreases (to about 4.5 to 5).
In simple words: pH matters for digestion, teeth, soil and rain; curd has a lower pH than milk.

Teacher's Note:
1. Use the value 5.5 for teeth.
2. Milk to curd: pH falls, acidity rises.

Question 17. Describe an activity to show that the effect of base is nullified by addition of an acid and vice versa. What is the name given to this reaction? Define it.
Answer:
1. Take about 2 mL of dilute NaOH solution in a test tube and add two drops of phenolphthalein. The solution turns pink.
2. Add dilute HCl drop by drop with shaking. The pink colour disappears.
3. Now add a few drops of NaOH again. The pink colour returns.
The activity shows that the effect of a base is cancelled by an acid and vice versa: HCl + NaOH → NaCl + H₂O.
This reaction is called neutralisation. It is the reaction of an acid with a base to form salt and water.
In simple words: Adding acid removes the pink colour of a base with phenolphthalein, and adding base brings it back.

Teacher's Note:
1. Name the indicator and the colour changes.
2. Define neutralisation in one line.

Question 18. A student dropped few pieces of marble in dilute hydrochloric acid, contained in a test-tube. The evolved gas was then passed through lime water. What change would be observed in lime water? What will happen if excess of gas is passed through lime water? With the help of balanced chemical equations for all the changes explain the observations.
Answer:
Marble is calcium carbonate. With dilute HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑.
The gas is carbon dioxide. Lime water turns milky: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.
If excess CO₂ is passed, the milkiness disappears because soluble calcium hydrogencarbonate is formed: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂.
In simple words: CO₂ makes lime water milky, but excess CO₂ makes it clear again.

Teacher's Note:
1. Write three equations in sequence.
2. Milky means insoluble CaCO₃ is formed.

Question 19. How is sodium hydroxide manufactured in industries? Name the process. In this process a gas X is formed as byproduct. This gas reacts with lime water to give a compound Y, which is used as a bleaching agent in the chemical industry. Identify X and Y and write the chemical equation of the reaction involved.
Answer:
Sodium hydroxide is made by passing electricity through a concentrated solution of sodium chloride (brine): 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g).
This is called the chlor-alkali process. Chlorine is formed at the anode and hydrogen at the cathode.
X is chlorine gas. It reacts with slaked lime to give Y, bleaching powder (calcium oxychloride): Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
In simple words: Electrolysis of brine gives NaOH, Cl₂ and H₂; chlorine is used to make bleaching powder.

Teacher's Note:
1. Name the three products of the chlor-alkali process.
2. Mention the electrodes where the gases collect.

Question 20. Answer the following questions:
(i) Crystals of a substance changed their colour on heating in a closed test tube but regained it after sometime when they were allowed to cool down. Name the substance and write its formula and explain the phenomenon involved.
(ii) Name the compound whose one formula unit is associated with 10 water molecules. How is it prepared? Give equations of related reactions. Give two uses of the compound.
Answer:
(i) The substance is hydrated copper sulphate, CuSO₄·5H₂O. On heating, it loses its water of crystallisation and the blue crystals turn white. In a closed tube the water vapour cannot escape and on cooling it recombines with the salt, so the blue colour is regained.
(ii) The compound is washing soda (sodium carbonate decahydrate), Na₂CO₃·10H₂O. Baking soda is heated to give sodium carbonate: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑. Recrystallisation of sodium carbonate from water gives washing soda: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
Uses: it is used to remove permanent hardness of water and in the manufacture of glass (also for cleaning clothes).
In simple words: Copper sulphate crystals are blue when hydrated, and washing soda has 10 water molecules.

Teacher's Note:
1. Say that the water vapour in a closed tube recombines on cooling.
2. Learn both preparation equations together.

Question 21. When a piece of limestone reacts with dilute HCl, a gas X is produced. When gas X is passed through lime water then a white precipitate Y is formed. On passing excess of gas X, the white precipitate dissolves forming a soluble compound Z.
(i) What are X, Y and Z?
(ii) Write equations for the reactions which take place:
(a) When limestone reacts with dilute HCl.
(b) When gas X reacts with lime water to form white precipitate Y.
(c) When excess of gas X dissolves white precipitate Y to form a soluble compound Z.
Answer:
(i) X is carbon dioxide; Y is calcium carbonate; Z is calcium hydrogencarbonate.
(ii)(a) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
(b) Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(c) CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)
In simple words: CO₂ turns lime water milky, and excess CO₂ turns it clear again.

Teacher's Note:
1. Limestone is calcium carbonate.
2. Give states of matter for full marks.

Question 22. A substance X which is used as an antacid reacts with dilute hydrochloric acid to produce a gas Y which is used in one type of fire-extinguisher. Name the substance X and gas Y. Write a balanced equation for the chemical reaction which takes place. You have been provided with three test-tubes. One of these test-tubes contains distilled water and the other two contain an acidic and a basic solution respectively. If you are given only blue litmus paper, how will you identify the contents of each test-tube?
Answer:
X is sodium hydrogencarbonate and gas Y is carbon dioxide: NaHCO₃ + HCl → NaCl + H₂O + CO₂↑.
Dip blue litmus paper in each tube. The solution that turns it red is the acid. Dip the red litmus paper obtained in the other two tubes: the one that turns it back to blue is the base, and the tube with no change contains distilled water.
In simple words: Blue litmus becomes red in acid; that red litmus becomes blue again in base; water changes nothing.

Teacher's Note:
1. Use the red paper obtained from the first test.
2. Learn the colour of each indicator in acid and in base.