Answer the questions.[Che-5M]

Question 15 Marks

Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

Answer

Two nails are fitted on a cork and are kept it in a 100 mL beaker. The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute HCl is poured in the beaker and the current is switched on. The same experiment is then performed with glucose solution and alcohol solution.
Observations:
It will be observed that the bulb glows in the HCl solution and does not glow in the glucose solution.

Result:
HCl dissociates into H⁺ and Cl⁻ ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity.
Conclusion:
From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids.
That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.

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Question 25 Marks

Complete and balance the following chemical equations:

$\text{Al}_2\text{O}_3+\text{HCl}\xrightarrow{ \ \ \ \ \ \ \ \ \ \ \ \ }$
$\text{K}_2\text{O}+\text{H}_2\text{O}\xrightarrow{ \ \ \ \ \ \ \ \ \ \ \ \ }$
$\text{Fe}+\text{H}_2\text{O}\xrightarrow{ \ \ \ \ \ \ \ \ \ \ \ \ }$

An element ‘X’ displaces iron from the aqueous solution of iron sulphate. List your observations if the element ‘X’ is treated with the aqueous solutions of copper sulphate, zinc sulphate and silver nitrate. Based on the observations arrange X, Zn, Cu and Ag in increasing order of their reactivities.

Answer

$\text{Al}_2\text{O}_3+6\text{HCl}\rightarrow2\text{AlCl}_3+3\text{H}_2\text{O}$
$\text{K}_2\text{O}+\text{H}_2\text{O}\rightarrow2\text{KOH}$
$3\text{Fe}+4\text{H}_2\text{O}\rightarrow\text{Fe}_3\text{O}_4+4\text{H}_2$

$\text{X}+\text{CuSO}_4\rightarrow$Displacement reaction

$\text{X}+\text{ZnSO}_4\rightarrow$Displacement reaction

$\text{X}+\text{AgNO}_3\rightarrow$Displacement reaction

$\text{X}>\text{Zn}>\text{Cu}>\text{Ag}$

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Question 35 Marks

Write word equations and then balanced equations for the reaction taking place when–

Dilute sulphuric acid reacts with zinc granules.
Dilute hydrochloric acid reacts with magnesium ribbon.
Dilute sulphuric acid reacts with aluminium powder.
Dilute hydrochloric acid reacts with iron filings.

Answer

Zinc + Sulphuric acid Zinc sulphate + Hydrogen

Zn + H₂SO₄ → ZnSO₄ + H₂

Magnesium + Hydrochloric acid magnesium chloride + Hydrogen gas

Mg + 2HCl → MgCl₂ + H₂

Aluminum + Sulphuric acid Aluminum sulphate + Hydrogen gas

2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂

Iron + Hydrochloric acid Iron chloride + Hydrogen

Fe + 2HCl → FeCl₂ + H₂

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Question 45 Marks

Write the chemical formula of ammonium chloride. Explain why an aqueous solution of ammonium chloride is acidic in nature? Illustrate your answer with the help of a chemical equation.

Answer

The chemical formula of ammonium chloride is NH₄Cl. Since, ammonium chloride is the salt of a strong acid HCl and a weak base NH₄OH, so an aqueous solution of ammonium chloride is acidic in nature. When dissolved in water, it gets hydrolysed to some extent to form HCl and NH₄OH. HCl being a strong acid is fully ionised and gives a large amount of hydrogen ions whereas NH₄OH is only slightly ionised. So, NH₄Cl contains more of hydrogen ions than hydroxide ions and is hence acidic in nature.

$\text{NH}_4\text{Cl}\text{(s)}+\text{H}_2\text{O}\text{(l)}\xrightarrow{\text{hydrolysis}}\text{NH}_4\text{OH}\text{(aq)}+\text{HCl(aq)}$

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Question 55 Marks

With the help of labelled diagrams, describe an activity to show that acids produce ions only in aqueous solutions.

Answer

Activity: Take about 1g solid NaCl in a clean and dry test tube and add some concentrated sulphuric acid to it. Fit a rubber cork with a small delivery tube in the mouth of the test tube. Concentrated sulphuric acid reacts with sodium chloride to form hydrogen chloride gas. The hydrogen chloride gas starts coming out of the open end of the glass tube.

Now, hold a ‘dry’ blue litmus paper in HCl gas. There is no change in colour of the ‘dry’ blue litmus paper. This shows that HCl gas does not behave as an acid in the absence of water. However, when we hold a ‘moist’ blue litmus paper in HCl gas, we will see that the ‘moist’ blue litmus paper turns red. This indicates that HCl gas shows acidic behavior in the presence of water as hydrogen ions are formed. This proves that acids produce ions only in aqueous solutions or in presence of water.

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Question 65 Marks

What does a soda-acid type fire extinguisher contain? How does it work?
Explain the working of a soda- acid fire extinguisher with the help of a labelled diagram.

Answer

Working: A soda-acid type fire extinguisher contains a solution of sodium hydrogencarbonate and sulphuric acid in separate containers in separate containers inside them. When the knob of the fire extinguisher is pressed, then sulphuric acid mixes with sodium hydrogencarbonate solution to produce carbon dioxide gas which forms a blanket around the burning substance and cuts off the supply of air to burning substance; this stops the process of burning and fire gets extinguished.

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Question 75 Marks

The pH values of five solutions A, B, C, D and E are given below:

Which solution is:

Weakly alkaline.
Neutral.
Strongly acidic.
Strongly alkaline, and.
Weakly acidic?

Answer

Solution D, with a pH value of 11, is weakly alkaline.
Solution C, with a pH value of 7, is neutral.
Solution A, with a pH value of 1, is strongly acidic.A.
Solution E, with a pH value of 13, is strongly alkaline.
Solution B, with a pH value of 5, is weakly acidic.

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Question 85 Marks

State two uses each of the following compounds:

Sodium hydroxide.
Chlorine.
Hydrogen.
Hydrochloric acid.

Answer

Sodium hydroxide:

It is used for making soaps and detergents.
It is used in the manufacture of paper.

Chlorine:

It is used in the production of bleaching powder.
It is used in the production of hydrochloric acid.

Hydrogen:

It is used in the production of hydrochloric acid.
It is used in the hydrogenation of oils.

Hydrochloric acid:

It is used in medicines and cosmetics.
It is used in textile/ dyeing and tanning industries.

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Question 95 Marks

Solution A turns universal indicator blue to purple whereas solution B turns universal indicator orange to red.

What will be the action of solution A on litmus?
What will be action of solution B on litmus?
Name any two substances which can give solutions like A.
Name any two substances which can give solutions like B.
What sort of reaction takes place when solution A reacts with solution B?

Answer

Solution A turns universal indicator blue to purple so it is basic in nature and will turn litmus blue.
Solution B turns universal indicator orange to red so it is acidic in nature and will turn litmus red.
Milk of magnesia and sodium hydroxide solution are bases like solution A.
Lemon juice and hydrochloric acid are acids like solution B.
Neutralisation reaction.

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Question 105 Marks

How would you distinguish between baking powder and washing soda by heating?

Answer

Baking soda is sodium hydrogen carbonate (NaHCO₃) which forms sodium carbonate (Na₂CO₃), water and carbon dioxide gas while heating.

$2\text{NaHCO}_3\rightarrow\text{Na}_2\text{CO}_3+\text{H}_2\text{O + CO}_2\uparrow$

Presence of CO₂ can be detected with the help of lime water which turns milky after reacting with carbon dioxide gas

Washing soda is sodium carbonate deca-hydrate (Na₂CO₃.10H₂O) which means there are 10 water molecules present with crystal hence named as water of crystallization

On heating washing soda, it becomes anhydrous and presence of water of crystallization can be detected with the help of anhydrous CuSO₄ which shows colour change from white to blue with moisture.

Na₂CO₃.10H₂O → Na₂CO₃ + 10H₂O

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Question 115 Marks

Describe how washing soda is produced starting from sodium chloride (common salt). Write equations of all the reactions involved.

Answer

Production of washing soda: Washing soda is produced from sodium chloride (or common salt) in the following three steps:

A cold and concentrated solution of sodium chloride (called brine) is reacted with ammonia and carbon dioxide to obtain sodium hydrogen carbonate:

$\underset{\substack{\text { Sodium chloride } \\ \text { (Common Salt) }}}{ NaCl }+\underset{\text { Ammonia }}{ NH _3}+\underset{\text { Water }}{ H _2 O }+\underset{\substack{\text { Carbon } \\ \text { dioxide }}}{ CO _2}\rightarrow \underset{\substack{\text { Sodium hydrogen } \\ Carbonate}}{ NH _4 Cl }+\underset{\substack{\text { Ammonium } \\ \text { Chloride}}}{ NH _4 Cl }$

Sodium hydrogen carbonate formed is only slightly soluble in water, so it precipitates out as a solid.

Sodium hydrogen carbonate is separated by filtration, dried and heated. On heating, sodium hydrogen carbonate decomposes to form sodium carbonate:

$\underset{\substack{\text { Sodium chloride } \\ \text { Carbonate }}}{ 2NaCHO_3} \xrightarrow{\text { Heat }} \underset{\substack{\text { Sodium carbonate } \\ \text { (Sodaash) }}}{ Na_2 CO _3}\underset{\substack{\text { Carbonate } \\ \text { dioxide }}}{ CO_2 }+\underset{ \text {Water}}{ H _2 O }$

The anhydrous sodium carbonate obtained here is called soda ash.

Anhydrous sodium carbonate (soda ash) is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystallisation:

$\underset{\substack{\text { Anhydroussdluumcarbonate } \\ \text { (Sodaas) }}}{ Na _2 CO _3}+ \ \ \underset{\text { Water }}{1 CH _2 O } \ \ \rightarrow \underset{\substack{\text { Sodumcarbomatedecahydrate }\\ \text { (Washimgsoda) }}}{ Na _2 CO _3, 1 CH _2 O }$

An aqueous solution of washing soda is alkaline because it turns red litmus to blue.
Washing soda has detergent properties because it can remove dirt and grease from dirty clothes.

It is used as cleansing agent for domestic purposes.
It is used for removing permanent hardness of water.

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Question 125 Marks

Consider the following substances :

NaCl, Ca(OH)₂, NaHCO₃, NH₃, Na₂CO, H₂O, Cl₂, CO₂, CaSO₄.2H₂O, 2CaSO₄.H₂O, CaOCl₂

Which two substances combine to form bleaching powder?
Which four substances are utilised in the production of washing soda?
Which compound represents plaster of Paris?
Which compound is a part of baking powder?
Which compound is used as an antacid?

Answer

Ca(OH)₂ and Cl₂
NaCl, NH₃, H₂O and CO₂
2CaSO₄.H₂O
NaHCO₃
NaHCO₃

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Question 135 Marks

Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Answer

Activity.
Take solutions of glucose, alcohol, hydrochloric acid and sulphuric acid. Fix two nails on a cork, and place the cork in a 100 ml beaker. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch. Now pour some dilute HCl in the beaker and switch on the current. The bulb starts glowing. This shows that HCl solution taken in the beaker conducts electricity. If we replace hydrochloric acid with sulphuric acid and perform the experiment, the bulb would glow again. This shows that an aqueous solution of an acid conducts electricity due to the presence of charged particles called ions in it.

Now, if we take glucose solution in the beaker and switch on the current, the bulb would not glow. If we repeat the experiment by taking alcohol solution in the beaker, the bulb would not glow again. This shows that due to the absence of ions, glucose and alcohol solutions do not conduct electricity. From this activity, we conclude that the hydrogen containing compounds such as glucose and alcohol are not categorised as acids because they do not dissociate (or ionise) in water to produce hydrogen ions [H(aq) ions].

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Question 145 Marks

In the following schematic diagram for the preparation of hydrogen gas as shown in Figure, what would happen if following changes are made?

In place of zinc granules, same amount of zinc dust is taken in the test tube.
Instead of dilute sulphuric acid, dilute hydrochloric acid is taken.
In place of zinc, copper turnings are taken.
Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated.

Answer

The reaction will become faster because zinc dust has more surface area.
Nearly same amount of hydrogen gas will be evolved.
No reaction will take place as copper is less reactive than hydrogen.
The reaction will take place and hydrogen gas will be evolved.

$\underset{\substack{\text { Sodium } \\ \text { hydroxide }}}{ Zn }+\underset{\substack{\text { Sodium } \\ \text { zincate }}}{2 NaOH } \xrightarrow{ Na _2 ZnO _2}+\underset{\text { Hydrogen }}{ H _2(g)}$

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Question 155 Marks

Identify the compound X on the basis of the reactions given below. Also, write the name and chemical formula of A, B and C.

Answer

Since in first reaction there is liberation of hydrogen gas on reaction with X. Metals liberate htdrogen gas on reaction with bases. Also in second reaction X react with acid to form water and B. Since acid react with base. So, X is a base. Most common base is Sodium hydroxide.

The compound X is sodium hydroxide.

Reaction:

$\text{NaOH}+\text{Zn}\rightarrow\text{Na}_2\text{ZnO}_2+\text{H}_2$

Hence, compound A is sodium zincate Na₂ZnO₂

Reaction:

$\text{NaOH}+\text{HCl}\rightarrow\text{NaCl}+\text{H}_2\text{O}$

Hence, compound B is sodium chloride NaCl

Reaction:

$\text{NaOH}+\text{CH}_3\text{COOH}\rightarrow\text{CH}_3\text{COONa}+\text{H}_2\text{O}$

Hence, compound C is sodium acetate.

Thus, X is NaOH.

A is sodium zincate (Na₂ZnO₂)

B is common salt (NaCl)

C is sodium acetate (CH₃COONa)

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Question 165 Marks

A metal carbonate X on reacting with an acid gives a gas which when passed through a solution Y gives the carbonate back. On the other hand, a gas G that is obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identity X, Y, G and Z.

Answer

Here X is calcium carbonate which is a metal carbonate which reacts with dilute HCl to form calcium carbonate (CaCO₃), water and carbon dioxide gas.

$\text{CaCO}_3+\text{dil. 2HCl}\rightarrow\text{CaCl}_2+\text{H}_2\text{O + CO}_2\uparrow$

When carbon dioxide gas is passed through lime water or calcium hydroxide (Ca(OH)₂), it gives the calcium carbonate back. So Y is calcium hydroxide.

$\text{Ca(OH)}_2+\text{CO}_2\rightarrow\text{CaCO}_3\downarrow+\text{H}_2\text{O}$

Brine is saturated solution of sodium chloride (NaCl). Electrolysis of brine forms hydrogen gas at cathode and chlorine gas at anode.

$2\text{NaCl}+2\text{H}_2\text{O}\xrightarrow{\text{Electrolysis}}2\text{NaOH + Cl}_2\text{(g) + H}_2\text{(g)}$

So G is chlorine gas which is passed through dry calcium hydroxide (Ca (OH)₂), produces bleaching powder, CaOCl₂(Z).

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Hence,

X = Calcium carbonate;

Y = Lime water;

G = Chlorine gas and

Z = Calcium oxychloride or bleaching powder

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Question 175 Marks

A group of students measured the pH of some substances they found in their homes. Their results are given in the following table:

Sabstance	pH	Sabstance	pH
Apples	3.0	Salt	7.0
Baking soda	8.5	Sugar	7.0
Black coffee	5.0	Toothpaste	9.0
Houshold ammonia	12.0	Vinegar	3.0
Lemon juice	2.5	Washing soda	11.5
Milk	6.0

What would the students have used to measure the pH?
Which solution is the most acidic?
Which solution is the most alkaline?
Which solutions are neutral?
Which solution can be used to treat wasp stings?
Which solution can be used to treat bee stings?

Answer

Universal indicator paper is used to measure the pH.
Lemon juice with pH = 2.5 is the most acidic.
Household ammonia with pH = 12 is the most alkaline.
Salt solution and sugar solution with pH = 7 are neutral.
Vinegar (acid) can be used to treat wasp stings since it injects an alkaline liquid into the skin.
Baking soda can be used to treat bee stings since it injects methanoic acid into the skin.

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