Thursday, June 30, 2011

NCERT solution Class X Science Acids, Bases and Salts

Class X Self Evaluation Tests For Science :Acids, Bases and Salts
Q.1: A solution turns red litmus blue; its pH is likely to be a) 1 b) 4 c) 5 d) 10 
Answer: (d)

Q.2: A solution reacts with crushed egg-cells to give a gas that turns lime water milky. The solution contains -
a) NaCl b) HCl c) LiCl d) KCl
Answer: (b)
Q.3: 10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of a given solution of HCl. If we take 20 ml of the same solution of NaOH, the amount of HCL (the same solution as before) required to neutralize it will be - a) 4 ml b) 8 ml c) 12 ml d) 16 ml
Answer: (d)
Q.4: Which one of the following types of medicoines is used fr treating indigestion?
a) Antibiotic b) Analgesic c) Antacid d) Antiseptic
Answer: (c)
Q.6: Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Answer: The nails are fitted in a cork and are kept in a 100 ml beaker. Then these are connected to the two terminals of a 6 volt battery along with a bulb and a switch. Now some dilute HCl is poured in the beaker and current is allowed to pass through the circuit. The same experiment is carried out using glucose and alcohol solutions separately in place of HCl. 
The bulb glows in HCl solution only. The reason is that HCl solution dissociates into H+ and Cl– ions which help to conduct electricity while glucose and alcohol solutions do not dissociate into ions and so, do not conduct electricity. That is why we find the bulb glowing in HCl solution only.
The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. Due to this reason compounds such as alcohols and glucose although contain hydrogen, but they are not categorized as acids.
Q.7: Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water which is a pure form of water, is neither acidic nor basic in nature. So distilled water does not dissociate into ions. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.
Rain water being an impure form of water, contains many ionic species. These ions are responsible for electrical conductivity of rain water.
Q.8: Why do acids not show acidic behavior in the absence of water?
Answer: In absence of water acids do not dissociate into ions and hence, they do not show acidic behaviour.
Q.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.
Ans: (a) D (b) C (c) B (d) A (e) E.   pH in increasing order of H+ ions concentration:
C (11) < E (9) < D (7) < A (4) < B (1).

Q.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?
Ans: In test tube A, fizzing occur more vigorously. It is because HCl is a stronger acid than acetic acid and therefore, produces H2 gas at a faster rate due to which fizzing occurs.
Q.11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans: Curd is acidic in nature which means that the pH of fresh milk will start decreasing from 6.
Q.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?
(a) By adding small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline so that he can keep it for longer time as the milk in alkaline condition, does not set curd easily.
(b) Due to the addition of baking soda this milk becomes slightly basic because of which acids produced in this milk due to bacterial actions are neutralized. Hence, this milk takes longer time to set as curd.
Q.13: Plaster of Paris should be stored ion a moisture-proof container. Explain why.
Answer: Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.
CaSO4.½H2O + 1½H2O --> CaSO4.2H2O
Q.14: What is a neutralization reaction? Give two examples.
Ans: A reaction in which an acid and a base react to give a salt and water is known as a ‘neutralization reaction’. For example,
NaOH (aq) + HCl (aq) --> NaCl (aq) + H2O (liq)
Ca(OH)2 (aq) + H2SO4 (aq) --> CaSO4 (s) + H2O
Q.15: Give two important uses of washing soda and baking soda.
Uses of Washing Soda:
1. Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
2. It is used in the manufacture of sodium compounds such as Borax.
Uses of Baking Soda:
1. Sodium hydrogen carbonate is also ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
2. It is also used in soda-acid fire extinguishers.
3. It is used as baking powder in confectionary items like cake, pastry etc.

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