CBSE Class 10 Science HOTs Acids Bases And Salts Set 05

More About Salts

Question. The name of the salt used to remove permanent hardness of water is
(a) Sodium hydrogen carbonate (\( NaHCO_3 \))
(b) Sodium chloride (\( NaCl \))
(c) Sodium carbonate decahydrate (\( Na_2CO_3 \cdot 10H_2O \))
(d) Calcium sulphate hemihydrate (\( CaSO_4 \cdot \frac{1}{2} H_2O \))
Answer: (c) Sodium carbonate decahydrate (\( Na_2CO_3 \cdot 10H_2O \))

Question. Study the following table and choose the correct option.
Salt | Parent Acid | Parent Base | Nature of Salt
(a) Sodium chloride | HCl | NaOH | Basic
(b) Sodium carbonate | \( H_2CO_3 \) | NaOH | Neutral
(c) Sodium sulphate | \( H_2SO_4 \) | NaOH | Acidic
(d) Sodium acetate | \( CH_3COOH \) | NaOH | Basic
Answer: (d) Sodium acetate | \( CH_3COOH \) | NaOH | Basic

Question. Study the experimental set up shown in given figure and choose the correct option from the following.
P | Q | Change observed in calcium hydroxide solution
(a) \( K_2CO_3 \) | \( Cl_2 \) gas | No change
(b) \( KHCO_3 \) | \( CO_2 \) gas | No change
(c) \( KHCO_3 \) | \( H_2 \) gas | Turns milky
(d) \( K_2CO_3 \) | \( CO_2 \) gas | Turns milky
Answer: (d) \( K_2CO_3 \) | \( CO_2 \) gas | Turns milky

Question. Which of the following salts do not have the water of crystallisation?
(i) Bleaching Powder (ii) Plaster of Paris
(iii) Washing soda (iv) Baking soda
(a) (ii) and (iv)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (i) and (iv)
Answer: (d) (i) and (iv)

Question. Assertion (A) : Sodium hydrogen carbonate is used as an ingredient in antacids.
Reason (R) : \( NaHCO_3 \) is a mild non-corrosive basic salt.
(a) Both (A) and (R) are true and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A).

Question. Absence of tartaric acid in baking powder makes the taste of the cake bitter due to the presence of
(a) sodium hydrogen carbonate
(b) sodium carbonate
(c) sodium metabisulphite
(d) sodium sulphate.
Answer: (b) sodium carbonate

Question. Salt ‘A’ commonly used in food products, is a reactant to produce salt ‘B’, used in the kitchen for making tasty, crispy pakoras. Salt ‘B’ on heating converts into another salt ‘C’, which is used in the manufacturing of glass. Salts ‘A’, ‘B’ and ‘C’ respectively are
(a) \( NaHCO_3, NaCl, Na_2CO_3 \)
(b) \( Na_2CO_3, NaHCO_3, NaCl \)
(c) \( Na_2CO_3, NaCl, NaHCO_3 \)
(d) \( NaCl, NaHCO_3, Na_2CO_3 \)
Answer: (d) \( NaCl, NaHCO_3, Na_2CO_3 \)

Question. An aqueous solution of a salt shows an orange red colour when a drop of universal indicator is added to it. This salt is made up of
(a) a strong acid and a strong base
(b) a weak acid and a weak base
(c) a strong acid and a weak base
(d) a weak acid and a strong base.
Answer: (c) a strong acid and a weak base

Question. Given below is a reaction showing Chlor-alkali process
\( 2NaCl_{(aq)} + 2H_2O_{(l)} \rightarrow 2NaOH_{(aq)} + Cl_{2(g)} + H_{2(g)} \)
(A) (B) (C)
The products A, B and C are produced respectively
(a) at the anode, at the cathode, near the cathode
(b) near the cathode, at the anode, at the cathode
(c) at the cathode, near the cathode, at the anode
(d) at the anode, near the cathode, at the cathode.
Answer: (d) at the anode, near the cathode, at the cathode.

Question. In the activity shown in the diagram, if the climate is humid, the role of the calcium chloride taken in the guard tube is to
(a) absorb the evolved gas
(b) warm up the gas
(c) dry the gas
(d) absorb chloride ions from the evolved gas.
Answer: (c) dry the gas

Read the passage given below and answer the questions 50-53 :
One day Kamal saw that her mother was roasting peanuts in a pan (kadahi) in the kitchen and she had taken ordinary salt to roast the peanuts. She was surprised to observe that in spite of very high temperature, the salt does not melt and the peanuts also get roasted.

Question. The chemical name of common salt is
(a) potassium chloride
(b) sodium carbonate
(c) sodium hydrogen carbonate
(d) sodium chloride.
Answer: (d) sodium chloride.

Question. Common salt is
(a) a covalent compound
(b) an ionic compound formed by sharing electrons
(c) an ionic compound formed by the transfer of electrons
(d) soluble in organic solvents.
Answer: (c) an ionic compound formed by the transfer of electrons

Question. Common salt is not a raw material for
(a) bleaching powder
(b) plaster of paris
(c) baking soda
(d) caustic soda.
Answer: (b) plaster of paris

Question. Common salt is used as a raw material in the preparation of washing soda in which the number of molecules of water of crystallisation is
(a) 10
(b) 7
(c) 5
(d) 2.
Answer: (a) 10

Question. Baking soda is a mixture of
(a) sodium carbonate and acetic acid
(b) sodium carbonate and tartaric acid
(c) sodium hydrogen carbonate and tartaric acid
(d) sodium hydrogen carbonate and acetic acid.
Answer: (c) sodium hydrogen carbonate and tartaric acid

Question. The chemical formula for plaster of Paris is
(a) \( CaSO_4 \cdot 2H_2O \)
(b) \( CaSO_4 \cdot H_2O \)
(c) \( CaSO_4 \cdot \frac{1}{2} H_2O \)
(d) \( 2CaSO_4 \cdot H_2O \)
Answer: (c) \( CaSO_4 \cdot \frac{1}{2} H_2O \)

SA I 

Question. On heating X at 373 K, it loses water molecules and becomes Y. Y is a substance which doctors use for supporting fractured bones in the right position.
(i) Identify X and Y.
(ii) How can X be reobtained from Y ?
Answer: (i) X is Gypsum (\( CaSO_4 \cdot 2H_2O \)) and Y is Plaster of Paris (\( CaSO_4 \cdot \frac{1}{2}H_2O \)).
(ii) X can be reobtained by mixing Y with water.
\( CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O \)

Question. ‘‘Sodium hydrogen carbonate is a basic salt’’. Justify this statement. How is it converted into washing soda?
Answer: \( NaHCO_3 \) is a salt of a strong base (NaOH) and a weak acid (\( H_2CO_3 \)), hence it is basic. On heating, it decomposes to form sodium carbonate, which on recrystallisation gives washing soda.
\( 2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2 \)
\( Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O \)

Question. 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: Solution 'X' is acidic (pH around 3-4) as it turns pH paper orange. Solution 'Y' is basic (pH around 9-10) as it turns pH paper bluish. pH of X < 7 and pH of Y > 7.

Question. On adding a few drops of universal indicator in three colourless solutions X, Y and Z taken separately in three test tubes, a student observed the changes in colour as green in X, red in Y and blue in Z.
(a) Arrange X, Y and Z in increasing order of their pH values.
(b) Which one of the three X, Y and Z, will change the colour of phenolphthalein ? Why ?
Answer: (a) Increasing order of pH: Y (red, < 7) < X (green, 7) < Z (blue, > 7).
(b) Solution Z will change phenolphthalein to pink because Z is basic in nature.

Question. Why is sodium hydrogen carbonate an alkaline salt ? List its two important uses.
Answer: It is formed from a strong base and a weak acid. Uses: (i) as an antacid, (ii) in soda-acid fire extinguishers.

Question. Write the chemical formula of bleaching powder. How is bleaching powder prepared? For what purpose is it used in drinking water?
Answer: Formula: \( CaOCl_2 \). It is prepared by the action of chlorine gas on dry slaked lime.
\( Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O \)
It is used in drinking water to kill germs/disinfectant.

SA II

Question. Consider the following salts: (i) YCl (ii) \( NH_4X \) (iii) \( ZCO_3 \)
(a) What would be the pH of the salt solution if in YCl, Y is sodium? Give reason for your answer.
(b) If in salt \( NH_4X \), X is nitrate, then its solution will give what colour with universal indicator? Why?
(c) What would be the change in colour in blue litmus solution if \( ZCO_3 \) is added to it and Z is potassium?
Answer: (a) pH will be 7 because NaCl is a neutral salt of strong acid and strong base.
(b) It will give yellow/orange colour (acidic) because ammonium nitrate is a salt of a weak base and strong acid.
(c) No change in colour because \( K_2CO_3 \) is a basic salt and bases do not change blue litmus colour.

Question. The industrial process used for the manufacture of caustic soda involves electrolysis of an aqueous solution of compound ‘X’. In this process, two gases ‘Y’ and ‘Z’ are liberated. ‘Y’ is liberated at cathode and ‘Z’, which is liberated at anode, on treatment with dry slaked lime forms a compound ‘B’. Name X, Y, Z and B.
Answer: X: Sodium chloride (NaCl), Y: Hydrogen gas (\( H_2 \)), Z: Chlorine gas (\( Cl_2 \)), B: Bleaching powder (\( CaOCl_2 \)).

Question. List the important products of the Chlor-alkali process. Write one important use of each.
Answer: 1. \( NaOH \): used for making soaps.
2. \( Cl_2 \): used in PVC manufacturing/water treatment.
3. \( H_2 \): used as fuel/margarine.

Question. How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?
Answer: Recrystallisation of sodium carbonate yields washing soda.
\( Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O \)
It is a basic salt. It removes permanent hardness of water.

Question. Give reasons for the following :
(i) Only one half of water molecule is shown in the formula of plaster of Paris.
(ii) Sodium hydrogen carbonate is used as an antacid.
(iii) On strong heating, blue coloured copper sulphate crystals turn white.
Answer: (i) Because two formula units of \( CaSO_4 \) share one molecule of water.
(ii) It is a mild basic salt that neutralises excess acid in the stomach.
(iii) Heating removes the water of crystallisation from hydrated copper sulphate, leaving behind white anhydrous salt.

Question. During electrolysis of brine, a gas ‘G’ is liberated at anode. When this gas ‘G’ is passed through slaked lime, a compound ‘C’ is formed, which is used for disinfecting drinking water.
(i) Write formula of ‘G’ and ‘C’.
(ii) State the chemical equations involved.
(iii) What is common name of compound ‘C’? Give its chemical name.
Answer: (i) G: \( Cl_2 \), C: \( CaOCl_2 \).
(ii) \( 2NaCl + 2H_2O \rightarrow 2NaOH + H_2 + Cl_2 \); \( Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O \).
(iii) Common name: Bleaching powder. Chemical name: Calcium oxychloride.

Question. 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: Acid: HCl, Base: NaOH. It is a neutral salt. Large crystals of common salt are called rock salt. It is formed by the drying up of ancient seas.

Question. A white powder is added while baking cakes to make it soft and spongy. Name its main ingredients. Explain the function of each ingredient. Write the chemical reaction taking place when the powder is heated during baking.
Answer: Ingredients: Baking soda (\( NaHCO_3 \)) and tartaric acid. \( NaHCO_3 \) releases \( CO_2 \) which makes cake rise. Tartaric acid neutralises sodium carbonate formed to prevent bitter taste.
\( 2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2 \)

Question. (a) Why is electrolysis of brine called ‘Chlor-alkali process’ ? Write the chemical equation involved in this process.
(b) A few crystals of hydrated copper sulphate are heated in a dry test-tube. Enlist any two observations.
Answer: (a) Because products are chlorine (chlor) and sodium hydroxide (an alkali).
\( 2NaCl + 2H_2O \rightarrow 2NaOH + H_2 + Cl_2 \)
(b) (i) Blue crystals turn white. (ii) Water droplets appear on the sides of the tube.

Question. The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses.
Answer: The salt is Baking soda (\( NaHCO_3 \)).
\( NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3 \).
Uses: (i) In baking, (ii) as an antacid.

Question. 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) Hydrated salt contains water of crystallisation; anhydrous does not.
(ii) Washing soda is \( Na_2CO_3 \cdot 10H_2O \); soda ash is anhydrous \( Na_2CO_3 \).
(iii) Baking soda is \( NaHCO_3 \); baking powder is a mixture of baking soda and a mild edible acid.

Question. Complete the following table :
Sample solution | Red litmus solution | Blue litmus solution | Phenolphthalein solution
Acetic acid | | |
Sodium hydroxide | | |
Baking soda | | |
Answer: Acetic acid: No change | Turns red | Colourless
Sodium hydroxide: Turns blue | No change | Pink
Baking soda: Turns blue | No change | Pink

Question. A white coloured powder is used by doctors for supporting fractured bones.
(a) Write chemical name and formula of the powder.
(b) When this white powder is mixed with water a hard solid mass is obtained. Write balanced chemical equation for this change.
Answer: (a) Calcium sulphate hemihydrate, \( CaSO_4 \cdot \frac{1}{2}H_2O \).
(b) \( CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O \)

Question. (a) Define an acid-base indicator. Mention one synthetic acid-base indicator.
(b) If someone in the family is suffering from a problem of acidity after overeating, which of the following substances would you suggest as a remedy?
Lemon juice, vinegar or baking soda solution.
Mention the property on the basis of which you will choose the remedy.
Answer: (a) Substances which change colour in acidic and basic media. Synthetic: Phenolphthalein.
(b) Baking soda solution because it is basic and neutralises the excess acid.

LA 

Question. A chemical compound ‘X’ is used in the soap and glass industry. It is prepared from brine.
(a) Write the chemical name, common name and chemical formula of ‘X’.
(b) Write the equation involved in its preparation.
(c) What happens when it is treated with water containing Ca or Mg salts ?
Answer: (a) Sodium carbonate decahydrate, Washing soda, \( Na_2CO_3 \cdot 10H_2O \).
(b) \( 2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2 \); \( Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O \).
(c) It removes hardness from water by forming insoluble precipitates.

Question. Define water of crystallisation. Give the chemical formula for two compounds as examples. How can it be proved that the water of crystallisation makes a difference in the state and colour of the compounds?
Answer: Fixed number of water molecules per formula unit. Examples: \( CuSO_4 \cdot 5H_2O \), \( FeSO_4 \cdot 7H_2O \). Proved by heating hydrated crystals: they lose colour and change state (crumble to powder).

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.
Answer: (a) Lime water turns milky.
\( CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2 \); \( Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \).
(b) (i) Mild alkalinity/neutralisation. (ii) Release of \( CO_2 \) on heating.

Question. (a) What are anhydrous and hydrated salts? Explain with a suitable example of each.
(b) How is plaster of Paris prepared? What reaction takes place when it sets to a hard mass?
Answer: (a) Hydrated salts have water of crystallisation (e.g., \( CuSO_4 \cdot 5H_2O \)). Anhydrous do not (e.g., \( NaCl \)).
(b) By heating gypsum at 373 K. It forms gypsum again upon mixing with water.

Question. (a) Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. Giving an activity illustrate how these two are interconvertible.
(b) Write chemical names and formulae of plaster of Paris and gypsum.
Answer: (a) Hydrated: \( CuSO_4 \cdot 5H_2O \) (blue); Anhydrous: \( CuSO_4 \) (white). Heating blue crystals makes them white; adding water makes them blue again.
(b) Plaster of Paris: Calcium sulphate hemihydrate, \( CaSO_4 \cdot \frac{1}{2}H_2O \). Gypsum: Calcium sulphate dihydrate, \( CaSO_4 \cdot 2H_2O \).

Question. How is sodium hydroxide produced? Write the balanced chemical equation also. Why is this process called as chlor-alkali process? In this process name the products given off at :
(a) anode
(b) cathode
Write one use of each of these products.
Answer: Produced by electrolysis of brine.
\( 2NaCl + 2H_2O \rightarrow 2NaOH + H_2 + Cl_2 \).
Called chlor-alkali because products are chlorine and an alkali (\( NaOH \)).
(a) Anode: \( Cl_2 \) (water treatment). (b) Cathode: \( H_2 \) (fuel). \( NaOH \) (soap making).

Question. What is water of crystallisation? Write the common name and chemical formula of a commercially important compound which has ten water molecules as water of crystallisation. How is this compound obtained? Write the chemical equation also. List any two uses of this compound.
Answer: Fixed number of water molecules in formula unit. Compound: Washing soda, \( Na_2CO_3 \cdot 10H_2O \). Obtained by recrystallising anhydrous sodium carbonate.
\( Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O \).
Uses: (i) Glass industry, (ii) removing permanent hardness.