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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