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Science Answer the questions.[Che-3M]

Acids, Bases and Salts · CBSE STD 10 (CBSE - English Medium). 19 questions.

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Answer the questions.[Che-3M]

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Question 13 Marks
Write the name and formula of one salt each which contains:
  1. Two molecules of water of crystallisation.
  2. Tive molecules of water of crystallisation.
  3. Ten molecules of water of crystallisation.
Answer
  1.  Gypsum (calcium sulphate crystals) contains two molecules of crystallisation in one formula unit and hence is written as CaSO4.2H2O. It is also called calcium sulphate dehydrate.
  2. Copper sulphate crystals contain five molecules of crystallisation in one formula unit. The chemical formula is CuSO4.5H2O (copper sulphate pent hydrate).
  3. Washing soda crystals (sodium carbonate crystals) contain 10 molecules of crystallisation in one formula unit. The chemical formula is Na2CO3.10H2O (sodium carbonate decahydrate).
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Question 23 Marks
Write the formulae of sodium chloride and sodium carbonate. Explain why an aqueous solution of sodium chloride is neutral but an aqueous solution of sodium carbonate is basic (or alkaline). Write chemical equations of the reactions involved.
Answer
Sodium chloride – NaCl.

Sodium carbonate – Na CO.

The aqueous solution of sodium chloride is neutral because it is formed from a strong acid and a strong base.

The aqueous solution of sodium carbonate is basic because it gets hydrolysed to some extent and forms sodium hydroxide which is a strong base and carbonic acid which is a weak acid.

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Question 33 Marks
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer
When a concentrated acid is added to water for preparing a dilute acid, then the heat is evolved gradually and easily absorbed by the large amount of water (to which the acid is being added) however if water is added to concentrated acid, then large amount of heat is evolved at once. This heat changes some of the water to steam explosively which can splash the acid on our face or clothes and causeacid burns. Even the glass container may break due to the excessive heating.
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Question 43 Marks
Which gas is usually liberated when an acid reacts with a metal? How will you test for the presence of this gas?
Answer
Hydrogen gas is liberated when an acid reacts with a metal. When reaction between an acid and a metal occurs, we observe formation of gas bubbles. When these gas bubbles are passed through soap solution, gas filled soap bubbles rise into the air. When a burning candle is brought near a gas-filled soap bubble, the gas present in the soap-bubble burns with a ‘pop’ sound. Only hydrogen gas burns making a ‘pop’ sound. This shows that hydrogen gas is evolved in the process.
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Question 53 Marks
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.
  1. What are X, Y and Z?
  2. Write equations for the reactions which take place:
  1. When limestone reacts with dilute HCl.
  2. When gas X reacts with lime water to form white precipitate Y.
  3. When excess of gas X dissolves white precipitate Y to form a soluble compound Z.
Answer
  1. X is carbon dioxide; Y is calcium carbonate; Z is calcium hydrogen carbonate.
  1.  

  1. $\text{CaCO}_3(\text{S})+2\text{HCl(aq)}\rightarrow2\text{NaCl(aq)}+\text{H}_2\text{O(l)}+\text{CO}_2\text{g}$

  2. $\text{Ca(OH)}_2\text{(aq)}+\text{CO}_2\text{g}\rightarrow\text{CaCO}_3\text{(S)}+\text{H}_2\text{O}(\text{l})$

  3. $\text{CaCo}_3\text{(S)}+\text{H}_2\text{O}\text{l}+\text{CO}_2\text{(g)}\rightarrow\text{Ca}(\text{HCO}_3)_2\text{(aq)}$

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Question 63 Marks
What is washing soda? State two properties and two uses of washing soda.
Answer
Washing soda is:
sodium carbonate decahydrate. Properties:
  1. It is transparent crystalline solid.
  2. It is soluble in water.
Uses:
  1. It is used for removing permanent hardness of water.
  2. It is used in the manufacture of glass, soap and paper.
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Question 73 Marks
What is meant by strong bases and weak bases? Classify the following into strong bases and weak bases:
NH4OH, Ca(OH)2, NaOH, KOH, Mg(OH)2
Answer
Strong base: A base which completely ionises in water and produces a large amount of hydroxide ions.

Strong bases: NaOH, KOH

Weak base: A base which is partially ionised in water and produces a small amount of hydroxide ions.

Weak bases: NH4OH, Ca(OH)2, Mg(OH)2

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Question 83 Marks
What is baking soda? Write the chemical name of baking soda. Give the important uses of baking soda. How does baking soda differ chemically from washing soda?
Answer
Baking soda is a substance added to food for its faster cooking. Its chemical name is sodium hydrogen carbonate.
Uses:
  1. It is used as an antacid to remove acidity of stomach.
  2. It is used in fire extinguishers. Baking soda is sodium hydrogencarbonate whereas washing soda is sodium carbonate decahydrate.
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Question 93 Marks
What happens when an acid reacts with a base? Explain by taking the example of hydrochloric acid and sodium hydroxide. Give equation of the chemical reaction which takes place. What is the special name of such a reaction?
Answer
When an acid reacts with a base, then a salt and water are formed. When hydrochloric acid reacts with sodium hydroxide solution, then a neutralisation reaction takes place to form sodium chloride and water.
$\text{NaOH}\text{(aq)}+\text{HCl}\text{(aq)}\rightarrow\text{NaCl}\text{(aq)}+\text{H}_2\text{O}(\text{l})$
Such a reaction is termed as neutralisation reaction.
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Question 103 Marks
The metal salt A is blue in colour. When salt A is heated strongly over a burner, then a substance B is eliminated and a white powder C is left behind. When a few drops of a liquid D are added to powder C, it becomes blue again. What could be A, B, C and D?
Answer
Copper sulphate crystals (A) are blue and have the chemical formula CuSO4.5H2O. When copper sulphate crystals are heated strongly, they lose all the water of crystallisation (B) and form anhydrous copper sulphate (C), which is white.
$\ \ \ \ \text{A}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{C}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{B}\\\text{CuSO}_4.5\text{H}_2\text{O}\rightarrow\text{CuSO}_4+5\text{H}_2\text{O}$
Anhydrous copper sulphate turns blue on addition of a few drops of water (D) because it gets hydrated again.
$\text{CuSO}_4+5\text{H}_2\text{O}\rightarrow\text{CuSO}_4.\text{H}_2\text{O}$
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Question 113 Marks
State some of the uses of mineral acids in industry.
Answer
Uses of mineral acids in industry:
  1. Sulphuric acid is used in the manufacture of fertilizers, paints, dyes, detergents etc.
  2. Nitric acid is used for making fertilizers, explosives, dyes and plastics.
  3. Hydrochloric acid is used for removing oxide film from steel objects, in textile, food and leather industries.
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Question 123 Marks
On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green.
  1. Predict the new compound formed which imparts a blue-green colour to solution.
  2. Write a balanced chemical equation of the reaction which takes place.
  3. On the basis of the above reaction, what can you say about the nature of copper oxide?
Answer
  1. Copper (II) chloride CuCl2
  2. CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)
  3. Copper oxide is basic in nature
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Question 133 Marks
Name three products of the chlor-alkali process. State two uses of each of these products.
Answer
Uses of Sodium hydroxide:
  1. It is used for making soaps and detergents.
  2. It is used in the manufacture of paper.
Uses of chlorine:
  1. It is used in the production of bleaching powder.
  2. It is used in the production of hydrochloric acid.
Uses of hydrogen:
  1. It is used in the production of hydrochloric acid.
  2. It is used in the hydrogenation of oils.
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Question 143 Marks
Hydrochloric acid reacts with a metal X to form a gas Y which bums with a ‘pop’ sound. Sodium hydroxide solution also reacts with the same metal X (on heating) to form the same gas Y.
  1. Name X and Y.
  2. Write the chemical equation of the reaction of metal X with.
  1. Hydrochloric acid, and
  2. Sodium hydroxide solution.
Answer
  1. X is the metal zinc (Zn) and Y is hydrogen gas. Zinc reacts with hydrochloric acid and forms zinc chloride salt with the liberation of hydrogen gas (Y), which burns with a 'pop' sound. Zinc displaces hydrogen from hydrochloric acid.
    Hence, X is zinc and Y is hydrogen.
  2. The chemical equation is as follows.
  1. Zn + 2HCl →ZnCl2 + H2 ↑
  2. 2NaOH + Zn →Na2ZnO2 + H2
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Question 153 Marks
Here are some results of solutions tested with universal indicator paper:
  1. Sulphuric acid: Red
  2. Metal polish: Dark blue
  3. Washing-up liquid:
  4. Yellow Milk of magnesia:
  5. Light blue Oven cleaner:
  6. Purple Car battery acid: Pink
Arrange the solutions in order of their increasing pH values (starting with the one with the lowest pH).
Answer
 Sulphuric acid < car battery acid < washing up liquid < milk of magnesia < metal polish < oven Cleaner since:
  1. Red: pH = 1
  2. Pink: pH = 3-4
  3. Yellow: pH = 5-6
  4. Light blue: pH = 9
  5. Dark blue: pH = 10
  6. Purple: pH = 11
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Question 163 Marks
Give one example each of a salt which gives an aqueous solution having:
  1. pH less than 7
  2. pH equal to 7
  3. pH more than 7
Answer
  1. Ammonium chloride, NH4Cl
  2. Sodium chloride, NaCl
  3. Sodium carbonate, Na2CO3
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Question 173 Marks
Consider the following salts:
Na2CO3, NaCl, NH4Cl, CH3COONa, K2SO4, (NH4) SO4 Which of these salts will give:
  1. Acidic solutions?
  2. Neutral solutions?
  3. Basic solutions (or alkaline solutions)?
Answer
a.Acidic solutions.NH4Cl, (NH4)2SO4
b.Neutral solutions.NaCl, K2SO4
c.Basic solutions
(or alkaline solutions).		Na2CO3, CH3COONa
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Question 183 Marks
A white shirt has a yellow stain of curry. When soap is rubbed on this shirt during washing, the yellow stain turns reddish-brown. On rinsing the shirt with plenty of water, the reddish-brown stain turns yellow again.
  1. Name the natural indicator present in curry stain.
  2. Explain the changes in colour of this indicator which take place during washing and rinsing the shirt.
  3. What is the nature of soap (acidic/ basic) as shown by the indicator present in curry stain?
Answer
  1. Turmeric.
  2. The yellow stain of curry turns reddish-brown when soap is scrubbed on it because of 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 yellow again when the cloth is rinsed with water because then the basic soap gets removed with water.
  3. Basic.
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Question 193 Marks
A compound X which is prepared from gypsum has the property of hardening when mixed with a proper quantity of water.
  1. Identify the compound X,
  2. Write the chemical equation for its preparation.
  3. For what purpose is it used in hospitals?
Answer

  1. Compound X is plaster of Paris, or calcium sulphate half hydrate $(\text{CaSO}_4.\frac{1}{2}\text{H}_2\text{O})$

  2. $\text{CuSO}_4.2\text{H}_2\text{O}\rightarrow\text{CaSO}_4.\frac{1}{2}\text{H}_2\text{O}+\frac{3}{2}\text{H}_2\text{O}$

  3. Plaster of Paris is used in hospitals for setting fractured bones in the right position to ensure correct healing. Casts made of plaster of Paris help keep the fractured bone straight.

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