CBSE Class 10 Chemistry Acids Base and Salts Worksheet Set B

Explanation: Dilute Hydrochloric acid turns pH paper red. It is acetic in nature.  

Short Answer Questions

Question. Explain why, an aqueous solution of sodium sulphate is neutral while an aqueous solution of sodium carbonate is basic in nature. 
Ans. • Aqueous solution of sodium sulphate gets hydrolysed to form sodium hydroxide and sulphuric acid which are strong base and strong acid respectively. So its aqueous solution is neutral.
• When sodium carbonate is dissolved in water, it gets hydrolysed to some extent and forms sodium hydroxide and carbonic acid. Now sodium hydroxide is a strong base which is fully ionised and gives a large amount of hydroxide ions [OH– (aq)]. On the other hand, carbonic acid is a weak acid which is only slightly ionised and hence gives a small amount of hydrogen ions [H+ (aq)]. The mixture contains more of hydroxide ions than hydrogen ions, so it is basic in nature.

Question. Which three chemical substances are obtained when electricity is passed through an aqueous solution of brine? Write one industrial use of each. 
Ans. When electricity is passed through a concentrated solution of sodium chloride (called brine), it decomposes to form sodium hydroxide, chlorine and hydrogen gas.
2NaCl (aq) + 2H₂O Electricity⎯⎯⎯→ 2NaOH (aq) + Cl₂↑ + H₂↑
(i) Use of Sodium hydroxide (NaOH). It is used for making soaps and detergents.
(ii) Use of Chlorine (Cl₂). It is used in the production of bleaching powder.
(iii) Use of hydrogen gas (H2). It is used to make ammonia for fertilisers; used as a fuel or margarine.

Question. When soap is scrubbed on a stain of curry on a white cloth, why does it become reddish brown, and turns yellow again when the cloth is washed with plenty of water? 
Ans. Curry contains turmeric which acts as an acid-base indicator. Turmeric contains a yellow dye, which turns red in basic solutions when soap is scrubbed on a stain of curry on a white cloth. This is due to the fact that soap solution is basic in nature which changes the colour of turmeric in the curry stain to red-brown. This stain turns to yellow again when the cloth is rinsed with plenty of water because then the basic soap gets removed with water.

Question. Name the gas which is usually produced when dil. sulphuric acid reacts with a metal. Illustrate it with an example. How will you test the evolution of this gas?
Ans. Metals react with dil. sulphuric acid to give metal sulphates and hydrogen gas.
For example, Zn (s) + H2SO4 (aq) ⎯⎯⎯→ ZnSO4 (aq) + H2≠ (Hydrogen gas) Test for hydrogen gas: Take about 5 ml of dilute sulphuric acid in a test tube and add a few pieces of zinc
granules to it. Hydrogen gas is evolved which forms bubbles in the soap solution.
Bring a burning candle near hydrogen gas-filled bubble. It burns with a pop sound.

Question. Write a chemical equation to describe how baking soda is produced on a large scale. Also write chemical name of the products obtained. 
Ans. The chemical name of baking soda is sodium bicarbonate (NaHCO3) or sodium hydrogen carbonate.
Sodium hydrogen carbonate is produced on a large scale by reacting a cold and concentrated solution of sodium chloride (called brine) with ammonia and carbon dioxide.
NaCl + NH₃ + H₂O + CO₂ ⎯⎯⎯→ NaHCO₃ + NH₄Cl
Baking soda Ammonium chloride

Question. (a) Write the name given to bases that are highly soluble in water? Give an example. 
(b) How is tooth decay related to pH? How can it be prevented?
(c) Why does bee sting cause pain and irritation? Rubbing of baking soda on the sting area gives relief. How?
Ans. (a) Water soluble bases are called alkalis, e.g., NaOH, KOH.
(b) Tooth decay starts when the pH of the mouth is lower than 5.5. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. This can be prevented by cleaning the mouth and teeth using tooth pastes which are generally basic and can neutralize the excess acid.
(c) When a honey bee stings a person, it injects an acidic liquid into the skin which causes immense pain and irritation. Rubbing a mild base like baking soda solution on the stung area of the skin gives relief. This is because, being a base, baking soda neutralises the acidic liquid injected by bee sting and
cancels its effect.

Question. State the number of water molecules present in crystals of washing soda and plaster of paris. What are these water molecules called as? 
Ans. • Washing soda crystals contain 10 molecules of water (Na₂CO₃ . 10H₂O).
• Plaster of Paris crystals contain ½ molecule of water (CaSO₄ . ½ H₂O or 2CaSO₄ . H₂O).
The water molecules, which form part of the structure of a crystal, are called water of crystallization.

Question. Write the chemical name of Plaster of Paris. Write the chemical equation of its preparation. Why should Plaster of Paris be stored in a dry place?
Ans. Chemical name of Plaster of Paris is calcium sulphate hemihydrate and its formula is CaSO₄. 1/2 H₂O.
Preparation:
CaSO₄.2H₂O Heat⎯⎯⎯→ CaSO₄ . 1/2 H₂O + 1, 1/2 H₂O
 Gypsum Plaster of Paris
thus it become useless.
Following reaction take place in this process:
CaSO₄. 1/2 H₂O + 1, 1/2 H₂O ⎯⎯⎯→ CaSO₄.2H₂O
Plaster of paris Moisture Gypsum

Long Answer Questions

Question. (a) Write the chemical name and chemical formula of washing soda.
(b) How is it obtained from sodium chloride? Give equations of the reactions.
(c) Why it is called a basic salt? Give its any one use. 
Ans. (a) Chemical name of washing soda: Sodium carbonate decahydrate Chemical formula of washing soda: Na2CO₃ . 10 H₂O
(b) Washing soda is produced by Solvay process by the reaction of ammonical brine with CO₂ gas
NaCl + NH₃ + CO₂ + H₂O ⎯⎯⎯→ NaHCO₃ + NH4 Cl
NaHCO₃ on heating produces Na2CO₃ (soda ash) which on recrystallisation produces washing soda
2NaHCO₃ Heat⎯⎯⎯→ Na2CO₃ + CO₂ ↑ + H2O
Na2CO₃ + 10H₂O ⎯⎯⎯→ Na2CO₃.10H₂O
(Washing soda)
(c) Washing soda is a basic salt because it is a salt of a strong base (NaOH) and weak acid (H₂CO₃) with pH value more than 7.
Use: Washing soda is used for softening hard water.

Question. (a) Identify the compound of calcium which is yellowish white powder and is used for disinfecting drinking water. Write its chemical name and formula. How is it manufactured? Write the chemical equation for the reaction involved. Also list two other uses of the compound.
(b) Write the balanced chemical equation of chlor-alkali process. 
Ans. (a) • Compound is bleaching powder.
• Chemical name: Calcium oxychloride
• Formula: CaOCl₂
• It is produced by the action of chlorine on dry slaked lime Ca(OH)₂.
Ca(OH)₂ + Cl2 ⎯⎯⎯→ CaOCl₂ + H₂O
Uses: (i) It is used to bleach wood pulp in paper factories.
(ii) It is used as an oxidizing agent in many chemical industries.
(b) When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. This process is called chlor-alkali process.
2NaCl (aq) + 2H₂O (l) ⎯⎯⎯→ 2NaOH (aq) + Cl2 (g) + H₂ (g)

Question. (a) Study the following chemical equation: 
CaSO4 . 2H₂O 373k ⎯⎯⎯→ CaSO4 . 1/2 H2O + 3/2 H2O
Name the reactant and the product and mention one use of the product.
(b) The following salts are formed by the reaction of an acid with a base:
(i) Sodium chloride (ii) Ammonium nitrate
Identify the acid and the base and tabulate your answer in the format given below:


Ans. (a) CaSO₄ . 2H₂O ^373k ⎯⎯⎯→ CaSO₄. 1/2 H₂O + 3/2 H₂O
Reactant: Gypsum
Product: Plaster of Paris and water
Use: Plaster of Paris is used in hospitals for setting fractured bones in the right position to ensure correct healing. 

Question. (a) A student dropped a few pieces of marble in dilute hydrochloric acid contained in a test tube. The evolved gas was passed through lime water. What change would be observed in lime water? Write balanced chemical equations for both the changes observed.
(b) State the chemical property in each case on which the following uses of baking soda are based:
(i) as an antacid
(ii) as a constituent of baking powder. 
Ans. (a) When a piece of marble (CaCO₃) is dropped in dil HCl, CO₂ gas will be evolved which turns lime water milky.
CaCO₃ (s) + 2HCl (l) ⎯⎯⎯→ CaCl₂ (s) + H₂O + CO₂
(Limestone) (dil)
Ca(OH)₂ + CO₂ ⎯⎯⎯→ CaCO₃ ↓ + H₂O
(Lime water) (White ppt)
Formation of CaCO₃ white ppt makes the lime water milky.
(b) (i) Being alkaline, sodium hydrogen carbonate (baking soda) neutralises the excess acid present in the stomach and relieves indigestion.
(ii) Baking powder is a mixture of baking soda and a mild edible acid such as tartaric acid. When baking powder mixes with water (present in dough made for baking cake or bread), then sodium hydrogen carbonate reacts with tartaric acid to evolve CO₂ gas.
NaHCO₃ (aq) + H+ (aq) ⎯⎯⎯→ Na+ (aq) + CO₂ (g) + H₂O (l) 
 Baking soda Hydrogen ions
  from tartaric acid

Question. (a) What is the importance of pH in everyday life?
(b) How are sodium hydroxide and Cl₂ (Chlorene) gas produced from common salt. What is this process called?
Ans. (a) (i) Living organisms can survive only in a narrow range of pH change. Acidic rain water when flows into the rivers, it lowers the pH value of river water and makes the survival of acquatic life in such river water difficult. Plants require a specific pH range for their healthy growth.
(ii) Our stomach and intestines work in a specific pH range. Stomach acts in slightly acidic medium while small intestine digests the food in slightly alkaline medium.
(iii) Tooth decay starts when the pH of the mouth is lower than 5.5.
(b) When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. This process is called Chlor-alkali process because the products formed are chlorine and alkali NaOH.
2NaCl (aq) + 2H₂O (l) ⎯⎯→ 2NaOH (aq) + Cl2(g) + H₂ (g)
Cl2 gas is given off at the anode and H2 gas at the cathode while sodium hydroxide solution is formed near the cathode.

 

a. III

b. I

c. II

d. IV

Explanation: Reaction will be rapid in both test tubes

Zn + 2HCl →ZnCl₂ + H₂

Na₂CO₃ + 2HCl →2NaCl + H₂O + CO₂

d. NaZnO₂ +  H₂

  

Answer :   i. Dry HCl gas does not produce free H+ ions, hence, it does not change the color of the dry blue litmus paper.

ii. Alcohol and glucose contain hydrogen but they do not ionize in the solution to produce H⁺ ions and can not conduct electricity.

iii. The volume of the solution increases when a solution is diluted, but the number of ions remains the same, so the concentration of H₃O⁺ per unit volume decreases.

Answer :  The pH of a solution is inversely proportional to its hydrogen ion concentration. This means that an aqueous solution having a higher concentration of hydrogen ion will have lower pH and vice-versa.

Solution 'A’ which is a soluble acidic oxide will supply a large number of hydrogen (H⁺ ) ions and thus, its pH will be less than 7.

Solution 'B' which is a soluble base will supply a lesser number of hydrogen ( H⁺) ions and thus, its pH will be more than 7. 

Compound 'Y' is used as antacid, hence compound 'Y' is sodium bicarbonate.

Compound 'X' is ammonium chloride.

Compound 'Y' decomposes to form another solid 'Z'. Compound 'Z' is sodium carbonate.

The chemical equations for the reactions are as follows:

Answer :  a. An aqueous solution of an acid conducts electricity because in water an acid (HCl) dissociates to give ions. Since the current is carried out by the movement of ions, an aqueous solution of acid conducts electricity. 

b. During dilution, more of acid dissociates into ions. Thus concentration of [H₃O]⁺ ions will increase on dilution.

c. Even on increasing [H₃O]⁺ ions, the number of ions per unit volume decreases.

Therefore ph will increases on dilution.

d. i. CO₂ gas will evolves accompanied by brick effervescence.