Saturday, June 21, 2014

SOLVED CBSE TREND SETTER QUESTIONS CLASS - X (Acid Base and Salt)

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Question:  How does baking powder differ from baking soda?
Answer:  Baking soda is a single compound which has sodium hydrogen carbonate which is  alkaline (basic) in nature
Baking powder is a mixture of sodium hydrogencarbonate and a mild edible(eatable) acid such as tartaric acid. When baking powder mixes with water, then the sodium hydrogencarbonate reacts with tartaric acid to evolve carbon dioxide gas which gets trapped in the wet dough and bubbles out slowly making the cake to rise and hence 'soft and spongy'.The equation which takes place can be shown as:
NaHCO+ H+  ----> Na+ +  CO+ H2O
Question:  What will happen if heating is not controlled while preparing Plaster of Paris from gypsum?
Answer:  It may be noted that the temperature should be controlled carefully. It should not be allowed to rise above 152°C (425K) because, if whole of the water is lost then anhydrous calcium sulphate is produced, which is called dead burnt plaster and it does not have the properties of Plaster of Paris.

Question:   Explain how pH change in the river water can endanger the lives of aquatic animals.
Answer:  Living organisms can survive only in a narrow range of pH change. When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers becomes difficult.

 Question:  What are the two main ways in which common salt (sodium chloride) occurs in nature? How is common salt obtained from sea water? Explain
Answer:  Salt exists in two forms in nature i.e. in sea water and in the form of Halite a mineral rock of common salt. Common salt can be prepared by evaporation of sea water. Sea water is collected in division of land and it is allowed to evaporate under sunlight. Water is evaporated leaving behind mixtures of salt, form which common salt is separated by dissolving it in suitable reagent and then, recrystallizing salt from the solution.
Question: Why aqueous solution of sodium carbonate is basic in nature?

Answer: Sodium bicarbonate is an amphoteric compound. Aqueous solutions are mildly alkaline due to the formation of carbonic acid and hydroxide ion:
NaHCO− 3 + H2O → H2CO3 + OH−  
Question: Why is an aqueous solution of ammonium chloride acidic in nature?
Answer: A basic component is the one that produces hydroxide (OH-) ions when dissolved in water. In aqueous solution of ammonium chloride, ammonium ions (NH4+) first associate with H2O and form ammonia and hydroxide ions.
NH4Cl + H2O = NH4+ + HCl ( equation 1)
NH4+ + H2O = NH3 + OH- (equation 3)
Since ammonium ions produce hydroxide ions, NH4+ are considered to be the basic components.
Question: Dry ammonia has no action on litmus paper but a solution of ammonia in water turns red litmus paper blue. Why is it so?

Answer: In dry state Ammonia contains no hydroxyl ions. On dissolving in water, it forms NH4OH which dissociates to give NH4 + and OH-ions. Thus solution becomes basic and turns red litmus paper blue.
NH3 (g) + H2O (l) ---------------> NH4OH (aq)
NH4OH (aq) --------------> NH4+ (aq) + OH- (aq)

Question:  What happens when (a) Solid sodium hydrogen carbonate is heated, (b) Chlorine gas is passed through dry slaked lime, (c) Gypsum is heated to a temperature of 373 K?
Answer: (a) When Solid sodium hydrogen carbonate is heated  from sodium chloride Na2CO3.10H2O (washing soda).
2NaHCO3 --------Heat---------->  Na2CO3 +H2O+CO2
(b) when Chlorine gas is passed through dry slaked lime, Bleaching powder is produced. Bleaching powderis represented as CaOCl2, though the actual composition is quite complex.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
(c) if Gypsum is heated to a temperature of 373 K it forms plaster of Paris, CaSO4 ½ H2O
CaSO4 2H2O --- -373 K ------>  CaSO4 ½ H2O + 3/2 H2O
Question: Give the Names of Raw Materials that are required for the manufacture of  washing soda by solvay process. Write the reactions involved in the   process.
 Answer:  Common salt : NaCl.Sodium chloride.  Ammonia : NH3 .Ammonia
Lime Stone : CaCO3.Lime stone (Calcium carbonate)
 H2O + NH3 + NaCl + CO2  -----------> NaHCO3 + NH4Cl
2NaHCO3 ---------- Heat------>  Na2CO3+ CO2+H2O
CaCO3 ----------- Heat > CO2+CaO
 CaO + H2O --------->  Ca (OH)2 
Ca(OH)2+ 2NH4Cl ------------> CaCl2 +2NH3+2H2O
 Question:  What is efflorescence? Name one compound which shows efflorescence? support your answer with reaction?
Answer:  The processes of loosing water of crystallization, when the substance is exposed to air for long time to form anhydrous or monohydric  substance is called as efflorescence.  
Na2CO3 10 H2O, Sodium carbonate deca hydrate is one example. 
 Na2CO3 , 10 H2O -------------->Na2CO3 H2O + 9H2O
Question:   why is calcium sulphate hemihydrate called ‘Plaster of Paris’ ?

Answer: Plaster of Paris gets its name from large gypsum deposits in Montmartre in Paris. We know that plaster of Paris, which is chemically calcium sulphate hemihydrate is obtained by heating gypsum. So the origin of the name 'Plaster of Paris' is the fact that the source material gypsum is found in large deposits at Montmartre in Paris
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