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Q 1.
Why is it advised not to use copper or brass vessels to store pickles or curd?
Solution
Acids
present in pickles or curd will react with metals like copper or zinc and
produces poisonous salts which are harmful to health.
Q 2.
What are Concentrated and dilute acids? How should we dilute an acid?
Solution
Concentrated
acid contains a very small amount of water or no water whereas dilute acids
have far more amount of water than its own mass. The dilution of a concentrated
acid should always be done by adding concentrated acid to water gradually with
stirring and not by adding water to concentrated acid.
Q 3.
Name the gas evolved when sodium hydrogen carbonate is made to react with
dilute hydrochloric acid. How will you test the gas?
Solution
Carbon
dioxide is evolved when sodium hydrogen carbonate is made to react with dil.
Hydrochloric acid. Test for the gas: Pass the gas through lime water. If it
turns lime water milky, then the gas is carbon dioxide.
Q 4.
(i) What change will you observe in the colour of red litmus paper when it is
dipped into a solution of sodium sulphate? Give a reason to explain your
observation. (ii) A bottle filled with concentrated sulphuric acid up to brim
is left open in the atmosphere by mistake. Will there be any change in the
level of liquid? Explain your answer with reason.
Solution
(i)
It will not undergo any colour change because the solution of Na2SO4 (sodium
sulphate) in water is almost neutral. (ii) Concentrated sulphuric acid is
highly hygroscopic. It absorbs moisture from the air and gets diluted. Since
the volume increases, the acid starts flowing out of the bottle.
Q 5.
What can you say about the pH of salts?
Solution
Salts
of strong acids and bases are neutral with a pH value of 7. Salts of a strong
acid and weak base are acidic with pH value less than 7 and those of strong
base and weak acid are basic in nature, with pH value more than 7.
Q 6.
The growth of plants depends to a large extent on the availability of proper pH
conditions. Elaborate on the effect of soil pH on plant growth and ways to
treat acidic and basic soil.
Solution
Most
of the plants grow best when the pH of the soil is close to 7. If the soil is
too acidic or too basic, the plants grow badly or do not grow at all. If
the soil is too acidic, then it is treated with substances like quicklime,
slaked lime or chalk. These substances are basic in nature and hence react with
the excess acid present in soil and reduce its acidity. If the soil is too
basic, then its basicity is reduced by adding decaying organic matter (manure
or compost) which contains acidic materials.
Q 7.
Classify the following into acids and bases:(a) Gastric juice(b) Milk of
magnesia(c) Lemon juice(d) Ammonium hydroxide
Solution
(a)
Gastric juices - Acid (b) Milk of magnesia - Base (c) Lemon juice - Acid (d)
Ammonium hydroxide - Base
Q 8.
Classify the following into acids, bases or neutral solution on the basis of
their pH: (a) A, pH = 3.2 (b) B ,pH = 4.4 (c) C, pH = 7.2 (d) D, pH = 12
Solution
(a)
A, pH = 3.2 - Acidic (b) B, pH = 4.4 - Acidic (c) C, pH = 7.2 - Neutral (d) D,
pH = 12 - Basic
Q 9.
What is the role of acids and bases in our digestive system?
Solution
Hydrochloric
acid produced in our stomach helps in digestion of food but if it becomes
excessive, the pH falls, and pain and irritation occurs. To get rid of
this, antacids like milk of magnesia (Mg(OH)2) is generally used to
adjust the pH in the stomach.
Q 10.
While diluting an acid, why is it recommended that the acid should be added to
water and not water to the acid?
Solution
Dilution
of concentrated acid is an exothermic process. If water is added to
concentrated acid, the heat generated may cause the mixture to splash out and
cause burns. When the acid is added to water slowly with constant stirring, the
mixture will not splash out.
Q 11.
(i) Why are some salts called hydrated salts? (ii) Give two examples of white
coloured hydrated salts with their chemical formula.
Solution
(i)
Salts which contain water of crystallization are called as hydrated salts. (ii)
Examples of white coloured hydrated salts are: (a)
CaSO4.2H2O - Gypsum (b)
Na2CO3.10H2O - Washing soda
Q 12.
Glucose has hydrogen in it, therefore can we classify it as an acid?
Solution
A
common feature of all acids is that they dissolve in water to produce hydrogen
ions. Since glucose has hydrogen but on dissolving in water, it does not
produce hydrogen ions, therefore it is not an acid.
Q 13.
Classify acids on the basis of their origin. Give an example of each kind.
Solution
On
the basis of their origin, acids can be classified as: 1.Organic acids: Acids
which are obtained from plants are called organic acids. They contain carbon
atom also along with hydrogen atom. Example: Tartaric acid is present in tamarind.
2.Inorganic acids: Acids which are usually obtained from minerals of the earth
are known as inorganic acids. They do not contain carbon except carbonic acid(H2CO3).
Example: Sulphuric acid(H2SO4)
Q 14.
What is the pH of human blood?
Solution
The
pH of human blood is 7.4.
Q 15.
What are the indicators? Give an example.
Solution
Indicators
are weak organic compounds [acids or bases] which change colour in accordance
with the pH of the solution. For example, Blue litmus paper turns red in
acidic solutions and re litmus paper turns blue in basic solutions.
Q 16.
With the help of equations show the amphoteric nature of zinc oxide.
Solution
Zinc
oxide is amphoteric because it reacts with both acids and bases to form salts.
ZnO + 2HCl → ZnCl2
+ H2O
Zinc chloride 2NaOH +
ZnO → Na2ZnO2 +
H2O
Sodium zincate
Q 17.
A milkman added a small amount of baking soda to fresh milk. (a) Why does he
shift the pH of fresh milk to slightly alkaline? (b) Why does this milk take a
longer time to set as a curd?
Solution
(i)
Fresh milk is acidic and gets spoiled easily. In presence of baking soda, milk
becomes alkaline and can be stored for a longer time. (ii) When milk sets to
curd, pH decreases. The alkali does not allow it to become more acidic easily.
So it takes more time.
Q 18.
The pH of the mouth of a person is lower than 5.5. What changes will occur in
his mouth? How these changes can be controlled? Write any two measures.
Solution
When
the pH of the mouth of a person is lower than 5.5 tooth decay starts. Tooth
enamel made up of calcium phosphate is corroded. Measures: (i) Clean the mouth
after eating food. (ii) Use of toothpaste (generally basic in nature).
Q 19.
"Dry HCl gas does not change the colour of dry blue litmus paper".
Give reasons.
Solution
In
dry state, HCl gas does not give any H+ ions. It does not
behave as an acid. That is why the colour of litmus paper does not change.
Q 20.
HCl and HNO3 show acidic characters in aqueous solution while
alcohol and glucose solutions do not. Give reasons.
Solution
HCl
and HNO3 produce H+ ions in aqueous solution.
Hence, they show an acidic character. Alcohol and Glucose do not produce H+ ions
in aqueous solution, hence, they do not show acidic characters.
Q 21.
What are acids? Give physical properties of acids.
Solution
Acids
are substances that dissolve in water to produce hydronium ions (H3O+).
Physical properties of acids: a) Acids have a sour taste. b) They are highly
corrosive in nature and cause painful burns on the skin. c) They change the
colour of the indicators. d) Some acids are solids and some are liquids at room
temperature. e)They are electrolytes, i.e., they conduct electricity in the
aqueous state.
Q 22.
(a) State, in brief, the method of preparation of bleaching powder. Give a
balanced chemical equation for the reaction involved. (b) Bleaching powder is
used for disinfecting drinking water. Give reason.
Solution
(a)
When Chlorine gas is passed through dry slaked lime, it forms bleaching powder.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
(b) Bleaching powder is used for disinfecting drinking water due to the
Chlorine liberated which kills the germs.
Q 23.
Why is it advised to add concentrated acid to water and not water to the acid?
Solution
If
water is added to concentrated acid, the heat generated may cause the mixture
to splash out and cause burns. When the acid is added to water slowly with
constant stirring, the mixture will not splash out.
Q 24.
What is the chemical formula of Gypsum? How many molecules of water of
crystallisation are present in its one molecule?
Solution
The
chemical formula of Gypsum is CaSO4.2H2O. In its one
molecule, two molecules of water of crystallization are present.
Q 25.
Explain how pH change in the lake water can endanger the lives of aquatic
animals (like fish). What can be done to lessen the danger to the lives of
aquatic animals in the lake?
Solution
Sometimes,
the pH of lake water becomes lower because of too much acid rain. The high
acidity of lake water can kill aquatic animals like fish since they can survive
within a narrow range of pH change. Calcium carbonate is added to acidic lake
water to neutralise the acid that comes from acid rain and this prevents the
fish in the lake from being killed.
Q 26.
What are strong acids and weak acids? Give two example for each.
Solution
Acids
that give a high concentration of H+ ions on ionisation
in aqueous solution are said to be strong acids and acids which give less H+ ions on
ionisation in aqueous solution are said to be weak acids. Examples of
Strong acids: Hydrochloric acid and nitric acid Examples of Weak acids: Acetic
acid and formic acid
Q 27.
The products obtained on electrolysis of concentrated aqueous solution of a
substance 'X' are NaOH, Cl2 and H2. (a) Name the
substance 'X'. (b) What is the special name of this process and why? (c)
Which gas is liberated at anode? (d) List one commercial use of NaOH.
Solution
(a)
Substance 'X' = Sodium chloride (b) Name of the process = Chlor-alkali
This name is due to the formation of NaOH (alkali) and Cl2 (chlorine).
(c) Gas liberated at anode = Cl2 (d) It is used in making soaps
and detergents.
Q 28.
Why a large amount of heat is evolved when a concentrated acid is mixed with
water?
Solution
The
process of mixing concentrated acid with water is highly exothermic i.e. heat
producing reaction hence a large amount of heat is evolved when a concentrated
acid is mixed with water.
Q 29.
You might have seen lemon or tamarind juice being used to clean the tarnished
surface of copper vessels. Explain why these sour substances are effective in
cleaning the vessels?
Solution
Lemon
juice contains citric acid and tamarind contains tartaric acid. These acids
react with a basic layer of copper carbonate on the surface to form soluble
salts which are easily removed and surface shines.
Q 30.
What is meant by the term pH of a solution? The pH of rain water collected from
two cities A and B was found to be 6 and 5 respectively. The water in which city
is more acidic?
Solution
pH
of a solution is a measure of the H+ ion concentration in a
solution. Lesser the pH, more acidic is the solution. Thus rainwater of city B
is more acidic.
Q 31.
You are provided with three test tubes A, B, C which contain distilled water,
acidic and basic solutions. If you are given blue litmus paper only, how will
you identify the nature of the solutions in three test tubes?
Solution
(i) Solution A: Test the solution with blue litmus paper. There will be no
change in the colour of blue litmus paper. (ii) Solution B: Test the
solution with blue litmus paper. The colour of blue litmus paper will change to
red.(iii) Solution C: Test the solution with a piece of red litmus paper
(formed in step (ii)). The colour of red litmus paper will change back to
blue.Again test solution A with a piece of red litmus paper, there will be no
change in the colour.Hence, solution A (distilled water) is neutral since it
does not bring out any change in the colour of litmus paper. Solution B is acidic
since it turns blue litmus paper red.Solution C is basic since it turns red
litmus blue.
Q 32.
What happens to the orange colour of Methyl orange when treated with a base and
an acid?
Solution
Methyl
orange turns yellow in a basic solution and Reddish pink in an acidic solution.
Q 33.
Why does a curry stain on washing turns red?
Solution
Turmeric
is a natural indicator that turns red when treated with a base. Since soaps are
basic in nature, they convert the yellow colour of turmeric to red.
Q 34.
A student prepared solutions of (i) an acid and (ii) a base in two separate
beakers. She forgot to label the solutions and litmus paper is not available in
the laboratory. Since both the solutions are colourless, how will she
distinguish between the two?
Solution
In
the absence of litmus paper, reagent phenolphthalein can be used. The
solution which develops pink colour with phenolphthalein is a base and the
other is acid.
Q 35.
What is the name given to the aqueous sodium chloride?
Solution
The
name given to aqueous solution of sodium chloride is Brine.
Q 36.
What happens during a bee sting? What is its remedy?
Solution
When
a honey-bee stings a person, it injects an acidic liquid into the skin which
causes immense pain and irritation. We can get relief by rubbing a mild base
like baking soda solution on the stung area of the skin.
Q 37.
Name the acids found in: (a) Vinegar (b) Tamarind (c) Lemon (d) Nettle sting
Solution
(a)
Vinegar contains Acetic acid. (b) Tamarind has Tartaric acid. (c) Lemon
contains Citric acid. (d) Nettle sting contains Methanoic acid.
Q 38.
A knife, which is used to cut fruit, was immediately dipped into water
containing drops of blue litmus solution. If the colour of the solution is
changed to red, what inference can be drawn about the nature of the fruit and
why?
Solution
The
fruit may be a citrus fruit or its juice may contain acid and the traces of
acid on the knife turned the blue litmus red.
Q 39.
Fresh milk has a pH of 6. How do you think the pH will change as it turns into
curd? Explain
Solution
The
pH of milk decreases from '6' as it turns into curd. The curd is more acidic
than milk.
Q 40.
Why are acids never stored in metal containers? What types of containers are
used to store acids?
Solution
Acids
are never stored in metal containers because they gradually corrode and eat up
the metal container. Containers made of glass and ceramics are used to store
acids.
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