Thursday, May 10, 2012

chemistry adda: CBSE X Chemical Reaction and Equation Extra marks ...

1-mark Questions/ Answers
1. What happens when magnesium ribbon burns in air?
Ans. When magnesium ribbon burns in air, it combines with the oxygen to form magnesium oxide.
 2Mg(s) + O2(g) ® 2MgO(s)
2. Name the gas evolved when zinc reacts with dil. HCl.
Ans. Hydrogen gas is evolved.
3. What is a chemical equation?
Ans. A chemical equation is a symbolic notation that uses formulae instead of words to represent a chemical equation.
4. On what chemical law, balancing of chemical equation is based?
Ans. Balancing of a chemical equation is based on the law of conservation of mass.
5. Represent decomposition of ferrous sulphate with the help of balanced chemical equation.
Ans. 2FeSO4(s) ® Fe2O3(s) + SO2(g) + SO3(g)
6. When carbon dioxide is passed through lime water, it turns milky, why?
Ans. Lime water (calcium hydroxide) combines with carbon dioxide to form a suspension of calcium carbonate which makes lime water milky. Ca(OH)2 + CO2 ® CaCO3 + H2O
7. A zinc rod is left for nearly 20 minutes in a copper sulphate solution. What change would you observe in zinc rod?               
Ans. Zinc rod will change into zinc sulphate.
8. What type of reaction is this:  Na2SO4           +          BaCl2   ®         BaSO4 +          2NaCl
Ans. It is a double displacement reaction.
9. Identify the compound oxidized in the following reaction. H2S (g) + Cl2  ®  S(s) + 2HCl (g)
Ans. H2S is oxidized.
10. What is rust? 
Ans. Rust is mainly hydrated iron (III) oxide, Fe2O3.xH2O.
11. How does the food become rancid?
Ans. Food becomes rancid when fat and oils present in the food are oxidized.
2-marks Questions/ Answers
Q.1. An iron knife kept dipped in a blue copper sulphate solution turns the blue solution light green. Why?
Ans. As we know iron is more reactive than copper. So, it displaces Cu from CuSO4 solution and forms ferrous sulphate which is of Light Green Colour.
CuSO4 (aq) + Fe (s)              FeSO4(aq)                  +               Cu(s)
 Blue colour                            light green colour
Q.2. A copper coin is kept in a solution of silver nitrate for some time. What will happen to the coin and the colour of the solution?
Ans. We know that copper is more reactive than silver, so it will displace silver from its salt solution:
 Cu(s)   +          2AgNO3(aq)             Cu(NO3)2(aq)    +          2Ag(s)
So the solution will turn blue due to the formation of copper nitrate.
Q.3. What do you understand by precipitation reaction? Explain with suitable examples.
Ans.  Precipitate. When two reactants react and product formed remains insoluble and settles as a solid it is called a precipitate. Such reactions in which precipitate is formed are called precipitation reactions. For example,
(i) when aqueous solution of sodium sulphate is mixed with the aqueous solution or barium chloride, barium sulphate comes in the form of white precipitate
Na2SO4(aq)      +          BaCl2(aq)                 BaSO4()          +          2NaCl(aq)
(ii) When aqueous solution of sodium chloride is mixed with the aqueous solution of silver nitrate, silver chloride comes in the form of white precipitate.
Q.4. What is lime-water test for the detection of carbon dioxide?
Ans. When carbon dioxide gas is passed through lime water ,it turns milky due to the formation of milky suspension (precipitate) of calcium carbonate. Carbon dioxide is produced by the action of dilute HCl on sodium carbonate.
Na2CO3(s)        +          2HCl(aq)                    2NaCl   +          H2O(l)   +          CO2
Carbon dioxide gas produced in this reaction is passed through lime water that changes to milky colour due to the formation of calcium carbonate.
 Ca(OH)2(aq)     +          CO2(g)                      Caco3   +          CO2(g)
3 Marks Questions/ Answers
Q.5. What is corrosion? State the conditions necessary for rusting of iron. How rusting is harmful?
Ans:  Corrosion. The process of eating away of the metal by the action of atmospheric reagents changing the metal into its compound is called corrosion.
Rusting of Iron. When iron and iron objects are exposed to atmosphere, they are attacked by air and moisture (water) of the atmosphere and a brown and orange coloured layer is formed on the surface. It is called rust which is mainly hydrated iron (iii) oxide Fe2O3.xH2O.
Harmful Effect of Rusting. Hydrated iron (iii) oxide is brittle substance and moves away from the surface thus the object is damaged. The objects get holes, cavities and rough surface.
Conditions necessary for rusting :
(i) Open surfaces of the metal.   (ii) Presence of air (Oxygen).  (iii) Presence of moisture (water).
Q.6. What is rancidity? Write the common method to prevent it.
Ans. When food item are kept unprotected for some time, they give some unpleasant smell and taste and become rancid. This process is called rancidity. Actually, the micro organisms oxidise the fat and oils present in them. So oxidation of food items need to be prevented to protect them.
Common methods to Prevent Rancidity of Food item:
(i) Keeping the food at low temperature             (ii) Keeping food item in air tight containers
(iii) By filling nitrogen in the food storage bags.
5 Marks Questions/ Answers
1. a. Why cannot a chemical change be normally reversed?
b. why is it always essential to balance a chemical equation?
c. what happens when CO2 gas is passed through lime water and why does it disappear on passing excess CO2?
d. Can rusting of iron takes place in distilled water?
Ans:  (a) In a chemical change some bonds are broken and some bonds are formed. The products are quite different from the reactants. Therefore it normally can’t be reversed.
(b) A chemical equation has to be balanced to satisfy the law of conservation of mass.
(c) On passing CO2 gas through lime water, it turns milky due to formation of insoluble calcium carbonate which dissolves on passing excess CO2 due to formation of soluble calcium bicarbonate.
Ca(OH)2 + CO2(g) -------------- CaCO3(s) + H2O(l)
CaCO3(s) + H2O(l) + CO2 (g) --------------- Ca(HCO3)2(soluble)
(d)  No, rusting of iron cannot take place in distilled water because it neither contains dissolved oxygen nor CO2 both are essential for rusting of iron.
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