CBSE Class 10 Science Acids Bases and Salts Assignment Set 08

This second assignment for Class 10 Science Chapter 2, Acids, Bases and Salts, gives 22 practice questions with marks, complete answers, simple-words summaries and teacher's notes. It focuses on indicators, observations in test tube reactions, the pH scale, the uses of baking soda and bleaching powder, antacids and the manufacture of common salt based chemicals.

Very Short Answer Questions (1 and 2 marks)

Question 1. What is an alkali? What do you understand by pH of a solution?
Answer:
Alkali: a base that is soluble in water, for example NaOH and KOH.
pH is a number that expresses the acidity or basicity of a solution; it is the negative logarithm of the hydrogen ion concentration, pH = −log[H⁺]. Neutral solutions have pH = 7.
In simple words: An alkali is a water-soluble base; pH measures acidity.

Teacher's Note:
1. All alkalis are bases but not all bases are alkalis.
2. pH range is 0 to 14.

Question 2. Define strong acid and strong base. Write the definition of weak acid and weak base.
Answer:
Strong acid: an acid that ionises almost completely in water, giving a large number of H⁺ ions, for example HCl.
Strong base: a base that ionises almost completely in water, giving a large number of OH⁻ ions, for example NaOH.
Weak acid: an acid that ionises only partially in water, giving few H⁺ ions, for example CH₃COOH.
Weak base: a base that ionises only partially in water, giving few OH⁻ ions, for example NH₄OH.
In simple words: Strong ones ionise completely; weak ones only partly.

Teacher's Note:
1. Give one example of each.
2. Strength is about ionisation, not concentration.

Question 3. A student collected common names and formulae of some substances but he forgot to note which formula is for which compound. Help him to match the correct formula:
(i) Caustic soda (ii) Slaked lime (iii) Baking soda (iv) Lime. Formulae: NaHCO₃, CaO, NaOH, Ca(OH)₂.
Answer:
(i) Caustic soda: NaOH
(ii) Slaked lime: Ca(OH)₂
(iii) Baking soda: NaHCO₃
(iv) Lime (quick lime): CaO
In simple words: Learn the common names with their formulae.

Teacher's Note:
1. Quick lime is CaO; slaked lime is Ca(OH)₂.
2. Caustic soda is sodium hydroxide.

Question 4. Salts are formed by the neutralisation reaction between an acid and a base. Complete the following table by filling the missing data: (1) Ammonium chloride, NH₄Cl, parent base ......, parent acid ......; (2) Copper sulphate, formula ......, parent base Cu(OH)₂, parent acid ......
Answer:
(1) Ammonium chloride, NH₄Cl: parent base NH₄OH (ammonium hydroxide), parent acid HCl (hydrochloric acid).
(2) Copper sulphate, CuSO₄: parent base Cu(OH)₂, parent acid H₂SO₄ (sulphuric acid).
In simple words: A salt is named from the base and acid that form it.

Teacher's Note:
1. Write the acid and base for each salt.
2. The cation comes from the base and the anion from the acid.

Question 5. What is observed when 2 mL of dilute hydrochloric acid is added to 1 g of sodium carbonate taken in a clean and dry test tube? Write chemical equation for the reaction involved.
Answer:
A brisk effervescence is observed because carbon dioxide gas is evolved.
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
In simple words: Sodium carbonate fizzes with HCl.

Teacher's Note:
1. The gas turns lime water milky.
2. Write the general word equation first and then the example.

Question 6. List two observations when a pinch of sodium hydrogen carbonate is added to acetic acid in a test tube. Write a chemical equation for the reaction that occurs.
Answer:
Observations: (1) brisk effervescence appears; (2) a colourless, odourless gas (carbon dioxide) is released.
Equation: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
In simple words: Baking soda fizzes with vinegar.

Teacher's Note:
1. Name the salt formed: sodium acetate.
2. Equation must be balanced.

Question 7. Blue litmus solution is added to two test tubes A and B containing dilute HCl and NaOH solution respectively. In which test tube a colour change will be observed? State the colour change and give its reason.
Answer:
A colour change is observed in test tube A.
Blue litmus turns red in dilute HCl because it is an acid and gives H⁺ ions. In test tube B, NaOH is a base and blue litmus stays blue.
In simple words: Blue litmus turns red only in acid.

Teacher's Note:
1. Base does not change the colour of blue litmus.
2. Say which tube has which solution.

Question 8. How will you prove that a given salt is a carbonate of a metal?
Answer:
Take a little of the salt in a test tube and add dilute hydrochloric acid. Brisk effervescence shows a gas is evolved.
Pass the gas through lime water. If the lime water turns milky, the gas is carbon dioxide, so the salt is a carbonate (or hydrogencarbonate): Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.
In simple words: Acid gives fizz of CO₂ which turns lime water milky.

Teacher's Note:
1. Do the test with a freshly prepared lime water.
2. Write the equation for one carbonate.

Question 9. Solutions X, Y and Z have pH value of 8, 9 and 10 respectively. Arrange them in increasing order of basic character giving reasons.
Answer:
The order of increasing basic character is X < Y < Z.
A higher pH above 7 means a lower H⁺ concentration and a more basic solution; Z (pH 10) is the most basic and X (pH 8) is the least basic.
In simple words: Higher pH means more basic.

Teacher's Note:
1. Every pH unit is a tenfold change in H⁺ concentration.
2. State the order clearly.

Question 10. Explain how an antacid works.
Answer:
The stomach produces hydrochloric acid for digestion. When too much acid is produced (indigestion), it causes pain and irritation.
An antacid, such as milk of magnesia, is a mild base that reacts with the excess hydrochloric acid and neutralises it: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. This gives relief.
In simple words: An antacid is a mild base that neutralises extra stomach acid.

Teacher's Note:
1. Name one antacid with the equation.
2. Antacids do not remove all acid; they bring pH to a safe level.

Short Answer Questions (3 marks)

Question 11. A few crystals of copper sulphate are heated in a dry boiling tube:
(i) What is the colour before and after heating?
(ii) What is the reason for the colour change?
(iii) Can its original colour be restored? How?
Answer:
(i) Before heating the colour is blue; after heating it becomes white.
(ii) On heating, the crystals lose their water of crystallisation and form anhydrous copper sulphate.
(iii) Yes. On adding a few drops of water to the white powder, the blue colour is restored.
In simple words: Blue copper sulphate crystals lose water on heating and turn white; water restores the blue colour.

Teacher's Note:
1. Water droplets appear on the cooler part of the tube.
2. Anhydrous CuSO₄ is a test for moisture.

Question 12. A teacher provided acetic acid, water, lemon juice, aqueous solution of sodium hydrogen carbonate and sodium hydroxide to students in the school laboratory to determine the pH values of these substances using pH papers. One of the students reported the pH values of the given substances as 3, 12, 4, 8 and 14 respectively. Which one of these values is not correct? Write its correct value stating the reason.
Answer:
The value for water, 12, is not correct.
Water is neutral, so its correct pH is 7.
The other values are acceptable: acetic acid 3 and lemon juice 4 are acidic, sodium hydrogencarbonate solution 8 is mildly basic and sodium hydroxide solution 14 is strongly basic.
In simple words: Pure water has pH 7.

Teacher's Note:
1. Compare each substance with the acidic, neutral and basic ranges.
2. Check the order in which the substances are listed.

Question 13. A metal compound X reacts with dil. H₂SO₄ to produce effervescence. The gas evolved extinguishes a burning candle. If one of the compound formed is calcium sulphate, then what is X and the gas evolved? Also, write a balanced chemical equation for the reaction which occurred.
Answer:
X is calcium carbonate, CaCO₃, and the gas evolved is carbon dioxide, which extinguishes a burning candle.
CaCO₃ + H₂SO₄(dil.) → CaSO₄ + H₂O + CO₂↑
In simple words: A carbonate with an acid gives CO₂ with effervescence.

Teacher's Note:
1. Effervescence and a gas that puts out a flame point to carbon dioxide.
2. Name both X and the gas.

Question 14. How is pH paper used to find the pH of a solution? The pH value of water is 7. What will be the pH value of:
(a) Aqueous solution of sodium hydroxide.
(b) Dil. HCl.
Answer:
A few drops of the solution are put on a strip of pH paper. The colour produced is compared with the colour chart supplied, which gives the pH value.
(a) Sodium hydroxide solution is basic, so its pH is more than 7 (about 13 to 14).
(b) Dilute HCl is acidic, so its pH is less than 7.
In simple words: Match the paper's colour with the chart.

Teacher's Note:
1. pH paper gives a rough value; a pH meter is more accurate.
2. State the numbers or ranges.

Question 15. What do you observe when you add few drops of acetic acid to a test tube containing:
(i) Phenolphthalein (ii) Distilled water (iii) Universal indicator (iv) Sodium hydrogen carbonate powder.
Answer:
(i) Phenolphthalein: no change, it stays colourless.
(ii) Distilled water: no change.
(iii) Universal indicator: turns orange (acidic).
(iv) Sodium hydrogencarbonate powder: brisk effervescence due to colourless, odourless carbon dioxide gas.
In simple words: Acetic acid is a weak acid: orange with universal indicator and fizzing with baking soda.

Teacher's Note:
1. Phenolphthalein is colourless in acids.
2. Acetic acid is a weak acid, so the colour is orange, not red.

Question 16. Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it.
(i) What will be the action of gas on (a) dry litmus paper? (b) moist litmus paper?
(ii) Write a balanced chemical equation for the reaction taking place.
Answer:
(i)(a) There is no action on dry litmus paper.
(b) The gas is sulphur dioxide. It dissolves in the moisture to form sulphurous acid, so moist blue litmus paper turns red.
(ii) S(s) + O₂(g) → SO₂(g); SO₂(g) + H₂O(l) → H₂SO₃(aq)
In simple words: Non-metal oxides like SO₂ are acidic; they need water to show it.

Teacher's Note:
1. Burning sulphur gives SO₂.
2. Write the answer in short, clear steps.

Question 17. A chemical compound X is prepared using sodium chloride as starting material. The compound X is used for faster cooking. It also finds use as an ingredient in medicine to treat indigestion.
(i) Identify the compound X.
(ii) Give an equation for the chemical reaction which takes place upon heating X during cooking.
(iii) Which quality of compound X makes it suitable for treating indigestion?
Answer:
(i) X is sodium hydrogencarbonate (baking soda), NaHCO₃.
(ii) 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑
(iii) It is a mild, non-corrosive base, so it neutralises the excess acid in the stomach.
In simple words: Baking soda is a mild base that gives CO₂ on heating.

Teacher's Note:
1. Made from brine, ammonia and CO₂.
2. Mention that it is mildly basic.

Question 18. A cloth strip dipped in onion juice is used for testing a liquid X. The liquid X changes its odour. Which type of an indicator is onion juice? The liquid X turns blue litmus red. List the observations the liquid X will show on reacting with the following:
(A) Zinc granules (B) Solid sodium carbonate
Answer:
Onion juice is an olfactory indicator (its smell changes in acidic or basic medium). Liquid X turns blue litmus red, so it is an acid.
(A) With zinc granules, hydrogen gas is evolved with brisk bubbling and the test tube becomes warm; the gas burns with a pop sound.
(B) With solid sodium carbonate, brisk effervescence of carbon dioxide is seen; the gas turns lime water milky.
In simple words: Onion is an olfactory indicator; acids react with zinc and carbonates.

Teacher's Note:
1. Name the gases evolved.
2. State a test for each gas.

Long Answer Questions (5 marks)

Question 19. A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by-product of chlor alkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution.
Answer:
B is sodium hydroxide, NaOH. It is deliquescent, so it absorbs moisture from air and turns sticky, and it is a product of the chlor-alkali process.
When a base reacts with an acidic oxide, a neutralisation reaction occurs and salt and water are formed.
2NaOH + CO₂ → Na₂CO₃ + H₂O
In simple words: NaOH absorbs moisture; with a non-metal oxide such as CO₂ it gives a salt and water.

Teacher's Note:
1. Non-metal oxides are acidic in nature.
2. Write the equation with the 2 in front of NaOH.

Question 20. During the reaction of some metals with dilute hydrochloric acid, the following observations were made by a student:
(i) Silver does not show any change.
(ii) Some bubbles of a gas are seen when lead is reacted with the acid.
(iii) The reaction of sodium is found to be highly explosive.
(iv) The temperature of the reaction mixture rises when aluminium is added to the acid. Explain these observations giving appropriate reason.
Answer:
(i) Silver is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid and no reaction occurs.
(ii) Lead is only slightly more reactive than hydrogen, so it reacts slowly and only a few bubbles of hydrogen are formed: Pb + 2HCl → PbCl₂ + H₂↑.
(iii) Sodium is a very reactive metal, so it reacts violently with the acid, releasing hydrogen and much heat: 2Na + 2HCl → 2NaCl + H₂↑.
(iv) The reaction of aluminium with the acid is exothermic, so heat is released and the temperature rises: 2Al + 6HCl → 2AlCl₃ + 3H₂↑.
In simple words: More reactive metals react faster with acids; metals below hydrogen do not react.

Teacher's Note:
1. Link the speed of reaction to the position in the reactivity series.
2. Metals above hydrogen in the reactivity series liberate hydrogen from dilute acids.

Question 21. Write the name, method of preparation and uses of the following:
(i) CaOCl₂ (ii) CaSO₄·½H₂O (iii) NaHCO₃
Answer:
(i) CaOCl₂: calcium oxychloride, bleaching powder. Preparation: chlorine is passed over dry slaked lime, Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Uses: bleaching in textile and paper industries, disinfecting drinking water.
(ii) CaSO₄·½H₂O: calcium sulphate hemihydrate, Plaster of Paris. Preparation: heating gypsum at 373 K, CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Uses: setting fractured bones, making toys, statues and decorative material, smoothening surfaces.
(iii) NaHCO₃: sodium hydrogencarbonate, baking soda. Preparation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. Uses: in baking powder, as an antacid, in soda-acid fire extinguishers.
In simple words: Learn name, preparation and uses of the three common compounds together.

Teacher's Note:
1. Keep the equations balanced.
2. Write the answer in short, clear steps.

Question 22. Answer the following questions:
(i) What colour will be produced when we put a drop of distilled water on pH paper? What is the pH of distilled water?
(ii) The pH of a solution is less than 7. What does it indicate? What is the pH of 1 M HCl solution?
(iii) What is the pH of solution A which liberates CO₂ gas with a carbonate salt? Give the reason.
(iv) What is the pH of solution B which liberates NH₃ gas with an ammonium salt? Give reason.
(v) How do you increase or decrease the pH of pure water?
(vi) What is the action of NaHSO₄ solution on litmus and why?
(vii) If the pH value of a solution is 6, then solution will be of what type?
Answer:
(i) Green; the pH of distilled water is 7.
(ii) It indicates that the solution is acidic. The pH of 1 M HCl is 0.
(iii) The pH of A is less than 7, because only an acid liberates CO₂ from a carbonate.
(iv) The pH of B is more than 7, because only a base liberates NH₃ from an ammonium salt.
(v) Adding a few drops of alkali increases the pH of pure water; adding a few drops of acid decreases it.
(vi) It turns blue litmus red, because NaHSO₄ gives H⁺ ions in water.
(vii) Acidic (weakly).
In simple words: Acids have pH below 7 and bases above 7.

Teacher's Note:
1. Neutral water gives a green colour with universal indicator.
2. Remember that pH below 7 is acidic, 7 is neutral and above 7 is basic.