Q1: You have been provided with three test tubes. One of them contains distilled water and the
other two contain an acidic solution and a basic solution, respectively. If you are given only
red litmus paper, how will you identify the contents of each test tube?
Answer: The colour of red litmus paper changes to blue indicates that the chemical is a base. If there
is no change in colour, then it is either acidic or neutral. We can easily identify a base.
1. Mark the three test tubes as A, B, and C.
2. Take a drop of the solution A and put it on the red litmus paper. Repeat the same process
iwth solution B and C.
3. If any of them changes colour to blue, then it is a base.And remaining two are acid and
neutral.
4. Now take a drop from the identified base and mix it with a drop taken from the remaining two
solutions separately.
5. Check the drops of the mixtures on red litmus paper.
6. If the colour of red litmus turns blue, then the second solution is neutral. If it does not change
colour, then the second solution is acid. This is because acidic and basic solutions neutralize
each other.
Q2: Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Curd and sour substances are acidic by nature. e.g. curd contains lactic acid. When these
substances are kept in metal containers like brass and copper, metal react with acid and liberate
hydrogen gas and other harmful substances, which may spoil these substances.
Q3: Why vinegar is used in pickling?
Answer: Vinegar contains an organic acid called acetic acid (CH 3COOH) which prevents the growth of
micro organisms. Therefore it is used in pickling.
Q4: Why phenolphthalein is considered acid base indicator?
Answer: It is because phenolphthalein turns pink in basic solution whereas it remains unaffected in
acid or neutral solutions. It is able to differentiate between acid (colouless) and base (pink colour).
Q5: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an
example. How will you test for the presence of this gas?
Answer: Hydrogen gas is liberated when an acid reacts with a metal.
Take few pieces of zinc (Zn) granules in a delivery tube. Add 5ml of hydrochloric acid (HCl). White
fumes will rise. Pass the gas to a soap solution or collect it in a balloon (as shown in the following
video). The gas collected is Hydrogen gas. Since H2 gas has lower density as compared to air, the
balloon will float in air.
Zn (s) + 2HCl (aq) → ZnCl2(s) + H2 (g)
You can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near
the balloon (or soap bubbles).
Q6: Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas
evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if
one of the compounds formed is calcium chloride.
Answer: Since CO2 gas is liberated and the salt obtain has metal Ca, it implies the reactant metal
compound shall be a carbonate i.e. CaCO3
The chemical equation is:
CaCO3 (s) + 2HCl (aq) → CaCl2 (s) + CO2 (g) + H2O (l)
Calcium Carbonate + Hydrochloric Acid → Calcium Chloride + Carbon Dioxide + Water
Q7: Why do HCl, HNO3 , etc., show acidic characters in aqueous solutions while solutions of
compounds like alcohol and glucose do not show acidic character?
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Answer: As aqueous solution, HCl and HNO3 dissociate into H and Hydronium (H3O ) ions.
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HCl (aq) → H + Cl
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H + H2O → H3O
Although glucose and alcohol contain hydrogen, as aqueous solution, they do not dissociate into ions
and do not exhibit acidic character.
Q8: Why does an aqueous solution of an acid conduct electricity?
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Answer: The aqueous solution of an acid contains H and Hydronium (H3O ) ions. Since ions carry
extra charge and are able to conduct electricity.
Q9: Why should acids be handled with care?
Answer: Acids have burning effect on our skin. They are corrosive to living beings. Proper care must
be taken while handling acids in laboratory.
Q10: Why does dry HCl gas not change the colour of dry litmus paper
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Answer: Dry HCl gas does not form H ions. It does not show its acidic character. When in the
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aqueous solution, an acid ionizes to form H ions. In this case, neither HCl is in an aqueous form nor
the litmus paper is wet, therefore, the colour of the litmus paper does not change.
Q11: While diluting an acid, why is it recommended that the acid should be added to water and
not water to the acid?
Answer: When diluting, the acid should always be added slowly to water and in small amounts.
Adding water to an acid is an exothermic process, it can cause uncontrolled boiling and splashing.
That's why it is recommended that the acid should be added to water and not water to the acid.
Q12: What is an amphoteric substance?
Answer: A substance is said to be amphoteric if it can behave either as an acid or as a
base. Water is the most common amphoteric substance.
Q13: On the basis of origin, how acids are classified?
Answer: On the basis of origin, acids are classified as:
1. Organic acids
2. Inorganic or Mineral acids
Q14: What are organic acids? Give two examples
Answer: Acids obtained from living beings (plants and animals) are called organic acid.
e.g. citric acid present in fruits like lemon, oxalic acid in tomato etc.
Q15: What are mineral acids? Give two examples
Answer: In general acids obtained from minerals or non-living things are called mineral or inorganic
acids. Examples are:
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
Sulphuric acid (H2SO4)
Nitric Acid (HNO3)
Q16: What is the chemical name of Bleaching powder? How it is prepared? List its uses.
Answer:
Chemical Name of Bleaching Powder: Calcium Oxychloride
Chemical Formula: CaOCl2
Preparation: It is prepared by passing chlorine gas through dry slaked lime.
Ca(OH)2+ Cl2 → CaOCl2 + H2O
Slaked Lime + Chlorine → Bleaching Powder + Water
Uses:
1. As bleaching agent in textile industry.
2. As disinfectant in water purification.
3. In paper industry to bleach wood pulp.
Q17: Which acid is used in Cola to give it a biting sharp taste?
Answer: Phosphoric acid.
Q18: What is gypsum?
Answer: Plaster of Paris reacts with water to give a hard mass called gypsum.
(CaSO4)2. H2O (s) + 3 H2O (l) → 2CaSO4.2H2O (s)
Plaster for Paris + Water → Gypsum
Q19 Define Dilution.
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Answer: Mixing an acid or base with water results in decrease in the concentration of ions (H 3O /OH )
per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
Q20: Tooth enamel is one of the hardest substance in our body. How does it undergo damage
due to the eating of chocolates and sweets? What should we do to prevent it?
or
Q: Why does tooth decay start when the pH of the mouth is lower than 5.5?
Answer: Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium
phosphate is the hardest substance in the body. It does not dissolve in water, but is corroded when the pH in
the mouth is below 5.5. Bacteria present in the mouth produce acids by degradation of sugar (due to
chocolates and sweets) and food particles remaining in the mouth after eating. The best way to prevent this is
to clean the mouth after eating food.
Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent
tooth decay.