Saturday, 28 December 2019

Acids Bases and Salts

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Acids Bases and Salts
















Question 1
A solution turns red litmus blue, its pH is likely to be

(a) 1
(b) 4
(c) 5
(d) 10

Answer:
(d) 10
Question 2
A solution reacts with crushed-egg shells to give a gas that turns lime water milky. The solution contains
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl

Answer:
(b) HCl
Question 3
10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HC1. If we take 20 mL of the same solution of NaOH, the amount of HC1 solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL

Answer:
(d) 16 mL
Question 4
Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

Answer:
(c) Antacid
Question 5
Write word equations and then balanced equations for the reaction taking place when
(a) dilute sulphuric acid reacts with zinc granules
(b) dilute hydrochloric acid reacts with magnesium ribbon
(c) dilute sulphuric acid reacts with aluminium powder
(d) dilute hydrochloric acid reacts with iron filing

Answer:
(a) Zinc + dilute sulphuric acid → Zinc sulphate + Hydrogen
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
(b) Magnesium ribbon + dil. Hydrochloric acid → Magnesium chloride + Hydrogen
Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)
(c) Aluminium powder + dil. Sulphuric acid > Aluminium sulphate + Hydrogen
2Al (s) + 3H2SO4 (aq) → Al2 (SO4)3 (aq) + 3H2 (g)
(d) Iron filings + Dilute hydrochloric acid > Ferric chloride + Hydrogen
2Fe (s) + 6HCl (aq) → 2FeCl3 (aq) + 3H2 (g)
Question 6
Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Answer:
Though compounds like alcohol and glucose contain hydrogen but they do not ionise in the solution to produce H+ ions on passing current through them.
(i) Take solutions of alcohols and glucose.
(ii) Fix two nails on a cork, and place the cork in 100 mL beaker.
(iii) Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in the given Figure.

(iv) Now pour alcohol in the beaker and switch on the current.
(v) The bulb does not glow.
(vi) Repeat the experiment with glucose. The bulb does not glow in this case also.
(vii) This means no ions or H+ ions are present in the solution.
This shows that alcohols and glucose are not acids.
Question 7
Why does distilled water not conduct electricity, whereas rainwater does?

Answer:
Distilled water does not conduct electricity because it does not contain any ionic compound (like acids, bases or salts) dissolved in it.
Rainwater, while falling to the earth through the atmosphere, dissolves an acidic gas carbon dioxide from the air and forms carbonic acid (H2CO3). Carbonic acid provides hydrogen ions, H+ (aq) and carbonate ions, CO(aq)32to rainwater. Hence, due to the presence of carbonic acid which provides ions to rainwater, the rainwater conducts electricity.
Question 8
Why do acids not show acidic behaviour in the absence of water?

Answer:
The acidic behaviour of acids is due to the presence of hydrogen ions, [H+ (aq) ions], in them. The acid produces hydrogen ions only in the presence of water. So in the absence of water, an acid will not form hydrogen ions and hence will not show its acidic behaviour.
Question 9
Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively. Which solution is
(a) Neutral
(b) Strongly alkaline
(c) Strongly acidic
(d) Weakly acidic
(e) Weakly alkaline
Arrange the pH in increasing order of hydrogen ion concentration.

Answer:
(a) D
(b) C
(c) B
(d) A
(e) E
Increasing order of hydrogen ion concentration
11 < 9 < 7 < 4 < 1
i. e., C < E < D < A < B
Question 10
Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?

Answer:
Fizzing will occur more vigorously in test tube A. Hydrochloric acid (HCl) is a strong acid whereas acetic acid (CH3COOH) is a weak acid. Being strong acid, the hydrochloric acid solution contains a much greater amount of hydrogen ions in it due to which the fizzing will occur more vigorously in test tube A (containing hydrochloric acid). The fizzing is due to the evolution of hydrogen gas which is formed by the action of acid on the magnesium metal of magnesium ribbon.
Question 11
fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Answer:
pH of milk falls below 6 as it turns into curd due to the formation of lactic acid during this process. Lactic acid present in it reduces its pH value.
Question 12
A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?

Answer:
(a) Milk is made slightly alkaline so that it may not get sour easily due to the formation of lactic acid in it.
(b) The alkaline milk takes a longer time to set into curd because the lactic acid being formed has to first neutralise the alkali present in it.
Question 13
Plaster of Paris should be stored in a moisture proof container. Explain why?

Answer:
Plaster of Paris should be stored in a moisture proof container because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration. This will make the plaster of Paris useless after sometime.
Question 14
What is a neutralisation reaction? Give two examples.

Answer:
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
Examples:
Question 15
Give two important uses of washing soda and baking soda.

Answer:
Uses of washing soda :
(i) Washing soda is used in glass, soap and paper industries.
(ii) It is used for removing permanent hardness of water.
Uses of baking soda :
(i) Baking soda is used as an antacid in medicines to remove acidity of the stomach.
(ii) Baking soda is used for making baking powder (used in making cakes, bread, etc.).