Thursday, July 19, 2012

CBSE Class 10 - Science - CH2 - Acids, Bases and Salts
MCQs, NCERT Solutions, Q & A

Q1: The most commonly used indicator in laboratory is
(a) Methyl Orange
(b) Litmus
(c) Phenolphtalein
(d) Universal Indicator

Answer:  Universal Indicator

Q2: Olfactory indicators are:
(a) Clove
(b) Turmeric
(c) Soap
(d) Rose Petals

Answer: (a) Clove

Q3: An element common to all acids is
(a) Chlorine
(b) Nitrogen
(c) Oxygen
(d) Hydrogen

Answer: (d) Hydrogen

Q4: Metal carbonate on reaction with dilute acid release
(a) CO2
(b) CO
(c) H2O
(d) H2

Answer:  (a) CO2

Q5 (Teachers FA manual): ln general, salts
(a) are ionic compounds
(b) contain hydrogen ions
(c) contain hydroxide ions
(d) turn litmus red

Answer:  (a) are ionic compounds

Q6: On passing excess of CO2 gas in an aqueous solution of calcium carbonate, milkiness of the solution
(a) persists
(b) fades
(c) deepens
(d) disappears

Answer: (b) fades

Q7(FA manual): When magnesium and hydrochloric acid react, they produce
(a) Oxygen and magnesium chloride
(b) Chlorine and magnesium oxide
(c) Hydrogen and magnesium chloride
(d) Hydrogen and magnesium oxide

Answer: (c) Hydrogen and magnesium chloride

Q8:  Dissolution of acid or base in water is 
(a) Exothermic
(b) Endothermic
(c) Violent
(d) None of these

Answer: (a) Exothermic

Q9: Water contains more H+ ions than OH- ions. In this case, water is
(a) neutral
(b) basic
(c) acidic
(d) cannot say.

Answer:  (c) acidic

Q10: When an acid reacts with a base what compounds are formed?
(a) water only
(b) metal oxides only
(c) a salt only
(d) a salt and water

Answer: (d) a salt and water

Q11: Which of the following is a property of an acid?(a) slippery feel
(b) non-reactive
(c) sour taste
(d) strong color

Answer: (c) sour taste

Q12: On diluting an acid concentration of H+ per unit volume
(a) increases
(b) decreases
(c) remains unaffected
(d) depends on type of acid used.

Answer: (b) decreases

Q13: What is pH?
(a) the positive logarithm of the hydroxide ion concentration
(b) the positive logarithm of the hydrogen ion concentration
(c) the negative logarithm of the hydroxide ion concentration
(d) the negative logarithm of the hydrogen ion concentration

Answer: (d) the negative logarithm of the hydrogen ion concentration

Q14: Tartaric acid is the constituent of
(a) bleaching powder
(b) baking powder
(c) washing powder
(d) plaster of paris

Answer:  (b) baking powder

Q15: A solution turns blue litmus red. Its pH value is likely to be
(a) 4
(b) 7
(c) 9
(d) 12

Answer:  (a) 4

Q16: What is the name of  H2SO3?
(a) Sulphuric Acid
(b) Sulphurous Acid
(c) Sulphite Acid
(d) Hydrogen Sulphide.

Answer: (b) Sulphurous Acid

Q17: Which gas is produced by reaction of base with metal ?
(a) Carbon dioxide
(b) Dioxygen
(c) Dihydrogen
(d) Dinitrogen

Answer: (c) Dihydrogen

Q18: Acid + Metal-oxide  → ?
(a) Base + Water
(b) Salt + Water
(c) Base + Salt
(d) Metal + Salt

Answer: (b) Salt + Water

Q19: Which of the following is a weak acid?
(a) Hydrochloric Acid
(b) Nitric Acid
(c) Acetic Acid
(d) Sulphuric Acid

Answer:   (c) Acetic Acid

Q20: An indicator is what type of compound?
(a) reducing agent
(b) strong base or acid
(c) weak base or acid
(d) salt

Answer: (c) weak base or acid

Q21: Which of the following is strong acid ?
(a) Acetic acid
(b) Citric acid
(c) Nitric acid
(d) Oxalic acid

Answer: (c) Nitric acid

Q22: Name the organic acid present in tomato
(a) Tartaric Acid
(b) Malic Acid
(c) Lactic Acid
(d) Oxalic Acid

Answer: (d) Oxalic Acid

Q23: Fill in the blanks.
(a) Turmeric, clove, rose petals are ______  _____ .
(b) Acids ______ _______  bases to form ________ and water.
(c) The term pH was coined by _______ .
(d) Turmeric turns _____ _______ in basic medium.
(e)  ________ ________ is used in softening water.
(f) String acids show ______ ionisation while weak acids show _______ ionisation.
(g) Calcium Sulphate hemihydrate is popularly known as _____ ___ _____.
(h) Acids contain the hydrogen (H+) as the only ______ _______.
(i) Bases contain the Oxide (O2-) or hydroxide (OH-) radical as the only _______ radical.

(a) Olfactory indicators
(b) reacts with, salt 
(c) Sørensen
(d) deep brown
(e) Washing Soda
(f) more, less
(g) Plaster of Paris
(h) positive radical
(i) negative

Q24: List the organic acid present in the following objects: 
(a) Citrus Fruits (lemon, orange)
(b) Sting of bees and ants
(c) Rancid butter
(d) Tamarind, grapes, apples
(e) Apples
(f) Gastric juice
(g) Vinegar
(h) Fats
(i) Urine
(j) Sour milk, curd

(a) Citrus Fruits (lemon, orange): Citric acid
(b) Sting of bees and ants: Formic acid
(c) Rancid butter: Butryic acid
(d) Tamarind, grapes, apples : Tartaric acid
(e) Apples: Malic acid
(f) Gastric juice:  Hydrochloric acid
(g) Vinegar: Acetic acid
(h) Fats: stearic Acid
(i) Urine: Uric Acid
(j) Sour milk, curd: Lactic acid

Q25: State true () or false ()
(a) Heat is evolved during ionisation.
(b) Solution of glucose in water produces ions.
(c) Dry ammonia has no effect on litmus paper.
(d) Higher is the concentration of H(aq), lower is the pH value.
(e) The concentration of aqueous solutions of acid, base and salt are generally expressed in molarity.
(f) pH of a solution can vary from 0-14.
(g) HC1 is a weak acid.
(h) NaOH is a strong base.
(i) Curd is alkaline in comparison to fresh milk.
(j) pH = + log10[(H3O-]
(k) Distilled water does not conduct electricity.
(l) Analgesics helps to relieve acidity.

(a) Heat is evolved during ionisation. - True(✓)
(b) Solution of glucose in water produces ions. False (✗)
(c) Dry ammonia has no effect on litmus paper. (True)
(d) Higher is the concentration of H(aq), lower is the pH value. True(✓)
(e) The concentration of aqueous solutions of acid, base and salt are generally expressed in molarity.True(✓)
(f) pH of a solution can vary from 0-14.True(✓)
(g) HC1 is a weak acid. False (✗)
(h) NaOH is a strong base.True(✓)
(i) Curd is alkaline in comparison to fresh milk.False (✗)
(j) pH = + log10[(H3O-] False (✗)
(k) Distilled water does not conduct electricity. True(✓)
(l) Analgesics helps to relieve acidity. False (✗)

Q26: Identify which of the following chemical formulas is acid, base or salt. Write their corresponding chemical names also.
(a) HCl
(b) NaOH
(c) NaCl
(d) Ca(NO3)2
(e) Ca(OH)2
(f) HNO3
(g) NH4OH
(h) (NH4)2SO4
(i) CuO
(j) H2SO4
(k) CuCO3
(l) H2CO3
(m) MgO
(n)  Mg3(PO4)2

(a) HCl - Hydrochloric Acid
(b) NaOH - Sodium Hydroxide - Base
(c) NaCl - Sodium Chloride - Salt
(d) Ca(NO3)2 - Calcium Nitrate - Salt
(e) Ca(OH)2 -  Calcium Hydroxide
(f) HNO3 - Nitric Acid
(g) NH4OH - Ammonium hydroxide - Base
(h) (NH4)2SO4 - Ammonium sulphate - Salt
(i) CuO - Copper Oxide
(j) H2SO4 - Sulphuric Acid
(k) CuCO3 -  Copper Carbonate
(l) H2CO3 -  Carbonic acid
(m) MgO - Magnesium Oxide
(n)  Mg3(PO4)2 - Magnesium Phosphate

Q27: 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. 

Q28: 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.

Q29: Why vinegar is used in pickling?

Answer: Vinegar contains an organic acid called acetic acid (CH3COOH) which prevents the growth of micro organisms. Therefore it is used in pickling.

Q30: 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).

Q31(NCERT): 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).

Q32(NCERT): 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

Q33(NCERT): Why do HCl, HNO3 , etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Answer: As aqueous solution, HCl and HNO3 dissociate into H+ and Hydronium (H3O+) ions. 
 HCl (aq) →  H+ + Cl-
 H+ + H2O →  H3O+

Although glucose and alcohol contain hydrogen, as aqueous solution, they do not dissociate into ions and do not exhibit acidic character.

Q34(NCERT): Why does an aqueous solution of an acid conduct electricity?

Answer: The aqueous solution of an acid contains H+ and Hydronium (H3O+) ions. Since ions carry extra charge and are able to conduct electricity.

Q35: 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.

Q36: Why does dry HCl gas not change the colour of dry litmus paper

Answer: Dry HCl gas does not form H+ions. It does not show its acidic character. When in the 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.

Q37: 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.

Q38: 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.

Q39: 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
Q40:  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.

Q41: 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:
  • Sulphuric acid (H2SO4)
  • Nitric Acid (HNO3)
Q42: What is the chemical name of Bleaching powder? How it is prepared? List its uses.

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

  1. As bleaching agent in textile industry.
  2. As disinfectant in water purification.
  3. In paper industry to bleach wood pulp.
Q43: Which acid is used in Cola to give it a biting sharp taste?

Answer: Phosphoric acid.

Q44: 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

Q45 Define Dilution.

Answer: Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.

Q46(CBSE 2010): 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?
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.