CBSE Class 10 Science Acids Bases and Salts Assignment Set 09

This assignment for Class 10 Science Chapter 2, Acids, Bases and Salts, has 12 questions of 2 to 5 marks with complete answers, simple-words summaries and teacher's notes. It practises the difference between acids and bases, the activity on conduction of electricity, the reaction of metals with bases, hydrated and anhydrous salts, indicators and the pH of solutions.

Short Answer Questions (2 marks)

Question 1. A solution X gives orange colour when a drop of it falls on pH paper, while another solution Y gives bluish colour when a drop of it falls on pH paper. What is the nature of both the solutions? Determine the pH of solutions X and Y.
Answer:
Orange colour on pH paper corresponds to pH about 3 to 4, so solution X is acidic.
Bluish colour corresponds to pH about 10 to 11, so solution Y is basic (alkaline).
In simple words: Orange is acidic and blue is basic on pH paper.

Teacher's Note:
1. Red means strongly acidic and violet strongly basic.
2. Green is neutral (pH 7).

Question 2. Answer the following questions:
(i) Give the constituents of baking powder.
(ii) Why cake or bread swells on adding baking powder? Write chemical equation.
Answer:
(i) Baking powder is a mixture of sodium hydrogencarbonate (baking soda) and a mild edible acid such as tartaric acid.
(ii) On heating or on mixing with water, it releases carbon dioxide, which gets trapped in the dough and makes the cake or bread rise (swell and become soft and spongy): 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑
In simple words: Baking powder gives out CO₂ gas bubbles, which make the cake fluffy.

Teacher's Note:
1. The role of tartaric acid is to neutralise the bitter sodium carbonate formed.
2. Write the equation with correct balancing.

Short Answer Questions (3 marks)

Teacher's Note:
1. Present the difference in a table.
2. Write the answer in short, clear steps.

Question 4. How can it be proved that the water of crystallisation makes a difference in the state and colour of the compounds?
Answer:
Take a few blue crystals of copper sulphate, CuSO₄·5H₂O, in a dry boiling tube and heat them. The crystals lose their water of crystallisation and a white powder of anhydrous copper sulphate is left; water droplets collect on the cooler parts of the tube: CuSO₄·5H₂O → CuSO₄ + 5H₂O.
Cool the tube and add 2 to 3 drops of water on the white powder. The blue colour of the crystals returns: CuSO₄ + 5H₂O → CuSO₄·5H₂O.
Thus the water of crystallisation gives the crystals their shape and blue colour.
In simple words: Heating removes the water and the colour goes; adding water brings it back.

Teacher's Note:
1. Draw the boiling tube set-up in the examination.
2. Write both equations.

Question 5. How will you show that acids conduct electricity?
Answer:
Take a beaker with two iron nails fixed in a cork and connect them to a 6 V battery through a bulb and a switch.
Pour dilute sulphuric acid (or hydrochloric acid) into the beaker and switch on the current. The bulb glows, which shows that the acid solution conducts electricity because it contains ions.
Repeat with glucose or alcohol solution: the bulb does not glow, as these do not give ions.
In simple words: A glowing bulb shows that the acid solution has ions.

Teacher's Note:
1. Draw the set-up in the exam.
2. Glucose and alcohol do not conduct.

Question 6. Describe an activity to show that reaction between bases and metals is different from reaction between acids and metals.
Answer:
Put a few pieces of granulated zinc in sodium hydroxide solution in a test tube and warm it. Zinc dissolves and hydrogen gas is evolved: Zn + 2NaOH → Na₂ZnO₂ + H₂. Sodium zincate is formed.
With dilute acid: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen is formed at once without heating.
Difference: acids react with most reactive metals; a base reacts with only a few metals such as zinc and aluminium, and usually needs warming. The salt formed is different (zincate, not chloride).
In simple words: Zinc gives hydrogen with both acid and a strong base.

Teacher's Note:
1. Test the gas with a burning match stick.
2. Mention warming for NaOH.

Question 7. Write one point of difference between each of the following:
(i) A hydrated salt and an anhydrous salt.
(ii) Washing soda and soda ash.
(iii) Baking soda and baking powder.
Answer:
(i) A hydrated salt contains water of crystallisation (for example CuSO₄·5H₂O); an anhydrous salt does not (for example CuSO₄).
(ii) Washing soda is hydrated sodium carbonate, Na₂CO₃·10H₂O; soda ash is anhydrous sodium carbonate, Na₂CO₃.
(iii) Baking soda is sodium hydrogencarbonate, NaHCO₃; baking powder is a mixture of baking soda and a mild edible acid such as tartaric acid.
In simple words: Each pair differs in water content or composition.

Teacher's Note:
1. Give the formulae for full marks.
2. Hydrated means with water.

Question 8. Answer the following questions:
(i) Define a universal indicator. Mention one of its use.
(ii) Solution A gives pink colour when a drop of phenolphthalein indicator is added to it. Solution B gives red colour when a drop of methyl orange is added to it. What type of solutions are A and B and which one of the solutions A and B will have a higher pH value?
(iii) Name one salt whose solution has pH more than 7 and one salt whose solution has pH less than 7.
Answer:
(i) A universal indicator is a mixture of several indicators that shows different colours at different pH values over the whole pH range. It is used to find the approximate pH of a solution.
(ii) A is basic (phenolphthalein is pink in a base) and B is acidic (methyl orange is red in an acid). A has the higher pH.
(iii) pH more than 7: sodium carbonate. pH less than 7: ammonium chloride.
In simple words: Universal indicator gives a colour for each pH; bases have higher pH than acids.

Teacher's Note:
1. Phenolphthalein is colourless in acids and pink in bases.
2. Methyl orange is red in acids and yellow in bases.

Question 9. How is plaster of paris chemically different from gypsum? How may they be interconverted? Write one use of plaster of paris.
Answer:
Plaster of Paris is CaSO₄·½H₂O (calcium sulphate hemihydrate), whereas gypsum is CaSO₄·2H₂O (calcium sulphate dihydrate).
Gypsum to POP: heat gypsum at 373 K, CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
POP to gypsum: add water, CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
Use: plastering fractured bones to keep them in place (and making toys and statues).
In simple words: Gypsum and POP differ in the amount of water of crystallisation.

Teacher's Note:
1. POP sets as gypsum.
2. Plaster of Paris has half a molecule of water: CaSO₄·½H₂O.

Question 10. (a) Three acidic solutions A, B and C have pH = 0, 3 and 5 respectively.
(i) Which solution has highest concentration of H⁺ ions?
(ii) Which solution has the lowest concentration of H⁺ ions?
(b) How concentrated sulphuric acid can be diluted? Describe the process.
Answer:
(a)(i) Solution A (pH 0), since the lower the pH, the higher the H⁺ concentration.
(ii) Solution C (pH 5) has the lowest H⁺ concentration.
(b) Dilution of an acid is highly exothermic. Take about 10 mL of water in a beaker, add concentrated sulphuric acid dropwise to the water with constant stirring, and let the heat spread through the water. Never add water to the acid.
In simple words: Lowest pH means most H⁺; always add acid to water.

Teacher's Note:
1. pH 0 is the most acidic solution here.
2. Use the words slowly and stirring.

Question 11. What is neutralisation reaction? Give two examples. Give two important uses of washing soda and baking soda.
Answer:
A reaction between an acid and a base to give salt and water is called a neutralisation reaction. Examples: NaOH + HCl → NaCl + H₂O; KOH + HNO₃ → KNO₃ + H₂O.
Washing soda: removing permanent hardness of water; glass, soap and paper industries.
Baking soda: baking powder (to make cakes soft and fluffy); antacid to cure acidity.
In simple words: Acid + base gives salt + water; washing soda and baking soda have many uses.

Teacher's Note:
1. Write two examples with equations.
2. Two uses each are enough.

Long Answer Questions (5 marks)

Question 12. (i) Explain why is hydrochloric acid a strong acid and acetic acid, a weak acid. How can it be verified?
(ii) Explain why aqueous solution of an acid conducts electricity.
(iii) You have four solutions A, B, C and D. The pH of solution A is 6, B is 9, C is 12 and D is 7.
(a) Identify the most acidic and most basic solutions.
(b) Arrange the above four solutions in the increasing order of H⁺ ion concentration.
(c) State the change in colour of pH paper on dipping in solution C and D.
Answer:
(i) HCl ionises almost completely in water, giving many H⁺ ions, so it is a strong acid. Acetic acid ionises only slightly, so it is a weak acid. It can be verified by testing both acids of the same concentration with universal indicator or pH paper: HCl gives a lower pH (red) than acetic acid (orange).
(ii) The acid gives ions (H⁺ and anions) in water, and these ions carry the current.
(iii)(a) Most acidic: A (pH 6). Most basic: C (pH 12).
(b) Increasing H⁺ concentration: C < B < D < A.
(c) In C the pH paper turns blue/violet; in D it turns green.
In simple words: Lower pH means more H⁺ ions.

Teacher's Note:
1. Arrange solutions by pH from high to low for increasing H⁺.
2. Neutral solution gives a green colour.