Give an example of a metal which
(i) is a liquid at room temperature.
(ii) can be easily cut with a knife.
(iii) is the best conductor of heat.
(iv) is a poor conductor of heat.
Answers:
(i) Metal that exists in liquid state at room
temperature → Mercury
(ii) Metal that can be easily cut with a
knife → Sodium
(iii) Metal that is the best conductor of
heat → Silver
(iv) Metals that are poor conductors of
heat → Mercury and lead
Question 2:
Explain the meanings of malleable and ductile
Answer:
Malleable: Substances that can be beaten
into thin sheets are called malleable. For example, most of the metals are
malleable.
Ductile: Substances that can be drawn into thin
wires are called ductile. For example, most of the metals are ductile.
Question 3:
Why is sodium kept immersed in kerosene oil?
Answers:
Sodium and potassium are very reactive metals
and and combine explosively with air as well as water. Hence, they catch
fire if kept in open. Therefore, to prevent accidental fires and accidents,
sodium is stored in kerosene oil.
Question 4:
Write equations for the reactions of
(i) iron with steam
(ii) calcium and potassium with water
Answer:
Question 5:
Samples of four metals A, B, C and D were taken
and added to the following solution one by one. The results obtained have been
tabulated as follows.
Metal
|
Iron (II) sulphate
|
Cooper (II) sulphate
|
Zinc sulphate
|
Silver nitrate
|
A.
|
No reaction
|
Displacement
|
||
B.
|
Displacement
|
No reaction
|
||
C.
|
No reaction
|
No reaction
|
No reaction
|
Displacement
|
D.
|
No reaction
|
No reaction
|
No reaction
|
No reaction
|
Use the Table above to answer the following questions about
metals A, B, C and D.
(i) Which is the most reactive metal?
(ii) What would you observe if B is added to a solution of
copper (II) sulphate?
(iii) Arrange the metals A, B, C and D in the order of
decreasing reactivity.
Answers
Explanation :
A + FeSO4 → No reaction, i.e., A is less
reactive than iron
A + CuSO4 → Displacement, i.e., A is more
reactive than copper
B + FeSO4 → Displacement, i.e., B is more
reactive than iron
B + ZnSO4 → No reaction, i.e., B is less
reactive than zinc
C + FeSO4 → No reaction, i.e., C is less
reactive than iron
C + CuSO4 → No reaction, i.e., C is less
reactive than copper
C + ZnSO4 → No reaction, i.e., C is less
reactive than zinc
C + AgNO3 → Displacement, i.e., C is more
reactive than silver
D + FeSO4/CuSO4/ZnSO4/AgNO3 → No
reaction, i.e., D is less reactive than iron, copper, zinc, and silver
(i) B is the most reactive metal.
(ii) If B is added to a solution of copper (II) sulphate, then
it would displace copper.
B + CuSO4 → Displacement
(iii) The arrangement of the metals in the order of decreasing
reactivity is:
B > A > C > D
B > A > C > D
Question 6:
Which gas is produced when dilute hydrochloric acid is added to
a reactive metal? Write the chemical reaction when iron reacts with dilute H2SO4.
Answer: Hydrogen gas is evolved when dilute hydrochloric acid is
added to a reactive metal.
When iron reacts with dilute H2SO4, iron
(II) sulphate with the evolution of hydrogen gas is formed.
Question 7:
What would you observe when zinc is added to a solution of iron
(II) sulphate? Write the chemical reaction that takes place.
Answer
Zinc is more reactive than iron. Therefore, if zinc is added to
a solution of iron (II) sulphate, then it would displace iron from the
solution.
(i) Write the electron-dot structures for sodium, oxygen and
magnesium.
(ii) Show the formation of Na2O and MgO by the
transfer of electrons.
(iii) What are the ions present in these compounds?
Answer
Question 9:
Why do ionic compounds have high melting points?
Answer
Ionic compounds have strong electrostatic forces of attraction
between the ions. Therefore, it requires a lot of energy to overcome these
forces. That is why ionic compounds have high melting points.
Question 10:
Define the following terms.
(i) Mineral (ii) Ore (iii) Gangue
Answer
(i) Mineral: Most of the
elements occur in nature as in combined state as minerals. The chemical
composition of minerals is fixed.
(ii) Ore: Minerals from which metals can be extracted profitably
are known as ores.
(iii) Gangue: The impurities (sand, silt, soil, gravel, etc.)
present in the ore are called gangue.
Question 11:
Name two metals which are found in nature in the free state.
Answer
The metals at the bottom of the reactivity series are
mostly found in free state. For example: gold, silver, and platinum.
Question 12:
What chemical process is used for obtaining a metal from its
oxide?
Answer
The chemical process used for obtaining a metal from its oxide
is reduction. In this process, metal oxides are reduced by using suitable
reducing agents such as carbon or by highly reactive metals to displace the
metals from their oxides.
Manganese dioxide is reduced to manganese by treating it
with aluminium powder. In this case, aluminium displaces manganese from its
oxide.
Oxides of more reactive metals are reduced by electrolysis.
Question 13:
Metallic oxides of zinc, magnesium and copper were heated with
the following metals.
Metal
|
Zinc
|
Magnesium
|
Copper
|
Zinc oxide
|
-
|
-
|
-
|
Magnesium oxide
|
-
|
-
|
-
|
Copper oxide
|
-
|
-
|
-
|
In which cases will you find displacement reactions taking
place?
Answer
Metal
|
Zinc
|
Magnesium
|
Copper
|
Zinc oxide
|
No reaction
|
Displacement
|
No reaction
|
Magnesium oxide
|
No reaction
|
No reaction
|
No reaction
|
Copper oxide
|
Displacement
|
Displacement
|
No reaction
|
Question 14:
Which metals do not corrode easily?
Answer
More reactive a metal is, more likely it is to be corroded.
Therefore, less reactive metals are less likely to get corroded. This is
why gold plating provides high resistance to corrosion.
Question 15:
Which metals do not corrode easily?
Answer
More reactive a metal is, more likely it is to be corroded.
Therefore, less reactive metals are less likely to get corroded. This is
why gold plating provides high resistance to corrosion.
Question 16:
What are alloys?
Answer
Alloys are homogeneous mixtures of two or more elements. The
elements could be two metals, or a metal and a non-metal. An alloy is formed by
first melting the metal and then dissolving the other elements in it. For
example, steel is an alloy of iron and carbon.
Question 17:
Which of the following pairs will give displacement reactions?
(a) NaCl solution and copper metal
(b) MgCl2 solution and aluminium metal
(c) FeSO4 solution and silver metal
(d) AgNO3 solution and copper metal.
Answer
(d) AgNO3 solution
and copper metal
Question 18:
Which of the following methods is suitable for preventing an
iron frying pan from rusting?
(a) Applying grease
(b) Applying paint
(c) Applying a coating of zinc
(d) all of the above.
(c) Applying a coating of
zinc
(We can also apply grease and paint to prevent iron from
rusting. However, in case of iron frying pan, grease and paint cannot be
applied because when the pan will be heated and washed again and again, the
coating of grease and paint would get destroyed.)
Question 19:
An element reacts with oxygen to give a compound with a high
melting point. This compound is also soluble in water. The element is likely to
be
(a) calcium
(b) carbon
(c) silicon
(d) iron
Answer
Question 20:
Food cans are coated with tin and not with zinc because
(a) zinc is costlier than tin.
(b) zinc has a higher melting point than tin.
(c) zinc is more reactive than tin.
(d) zinc is less reactive than tin.
Answer
(c) Food cans are coated
with tin and not with zinc because zinc is more reactive than tin.
Question 21:
You are given a hammer, a battery, a bulb, wires and a switch.
(a) How could you use them to distinguish between samples of
metals and non-metals?
(b) Assess the usefulness of these tests in distinguishing
between metals and non-metals.
Answer
(a) With the hammer, we
can beat the sample and if it can be beaten into thin sheets (that is, it is
malleable), then it is a metal otherwise a non-metal. Similarly, we can use the
battery, bulb, wires, and a switch to set up a circuit with the sample. If the
sample conducts electricity, then it is a metal otherwise a non-metal.
(b) The above tests are useful in distinguishing between metals
and non-metals as these are based on the physical properties. No chemical
reactions are involved in these tests.
Question 22:
What are amphoteric oxides? Give two examples of amphoteric
oxides.
Answer
Those oxides that behave as both acidic and basic oxides are
called amphoteric oxides.
Examples: aluminium oxide (Al2O3), zinc
oxide (ZnO)
Question 23:
Name two metals which will displace hydrogen from dilute acids,
and two metals which will not.
Answer
Metals that are more reactive than hydrogen displace it from
dilute acids. For example: sodium and potassium. Metals that are less reactive
than hydrogen do not displace it. For example: copper and silver.
Question 24:
In the electrolytic refining of a metal M, what would you take
as the anode, the cathode and the electrolyte?
Answer
In the electrolytic refining of a metal M:
Anode → Impure metal M
Cathode → Thin strip of pure metal M
Electrolyte → Solution of salt of the metal M
Question 25:
(a) What will be the action of gas on
(i) dry litmus paper?
(ii) moist litmus paper?
(b) Write a balanced chemical equation for the reaction taking
place.
(a) (i) There will be no
action on dry litmus paper.
(ii) Since the gas is sulphur dioxide (SO2), it
turns moist blue litmus paper to red because sulphur dioxide reacts with
moisture to form sulphurous acid.
Question 26:
State two ways to prevent the rusting of iron.
Answer
Two ways to prevent the rusting of iron are:
(i) Oiling, greasing, or painting: By applying oil,
grease, or paint, the surface becomes water proof and the moisture and oxygen
present in the air cannot come into direct contact with iron. Hence, rusting is
prevented.
(ii) Galvanisation: An iron article is coated with a
layer of zinc metal, which prevents the iron to come in contact with oxygen and
moisture. Hence, rusting is prevented.
Question 27:
What type of oxides are formed when non-metals combine with
oxygen?
Answer
Non-metals combine with oxygen to form acidic oxides.
Question 28:
Give reasons
(a) Platinum, gold and silver are used to make jewellery.
(b) Sodium, potassium and lithium are stored under oil.
(c) Aluminium is a highly reactive metal, yet it is used to make
utensils for cooking.
(d) Carbonate and sulphide ores are usually converted into
oxides during the process of extraction.
Answer
(a) Platinum, gold, and
silver are used to make jewellery because they are very lustrous. Also,
they are very less reactive and do not corrode easily.
(b) Sodium, potassium, and lithium are very reactive metals and
react very vigorously with air as well as water. Therefore, they are kept
immersed in kerosene oil in order to prevent their contact with air and
moisture.
(d) Carbonate and sulphide ores are usually converted into
oxides during the process of extraction because metals can be easily extracted
from their oxides rather than from their carbonates and sulphides.
Question 29:
You must have seen tarnished copper vessels being cleaned with
lemon or tamarind juice. Explain why these sour substances are effective in
cleaning the vessels.
Answer
Copper reacts with moist carbon dioxide in air to form copper
carbonate and as a result, copper vessel loses its shiny brown surface forming
a green layer of copper carbonate. The citric acid present in the lemon or
tamarind neutralises the basis copper carbonate and dissolves the layer. That
is why, tarnished copper vessels are cleaned with lemon or tamarind juice to
give the surface of the copper vessel its characteristic lustre.
Question 30:
Differentiate between metal and non-metal on the basis of their
chemical properties.
Answer
Metal
|
Non-metal
|
Metals are
electropositive.
|
Non-metals are
electronegative.
|
|
They react with
oxygen to form acidic or neutral oxides.
These have covalent bonds.
|
They react with
water to form oxides and hydroxides. Some metals react with cold water, some
with hot water, and some with steam.
|
They do not react
with water.
|
They react with
dilute acids to form a salt and evolve hydrogen gas. However, Cu, Ag, Au, Pt,
Hg do not react.
|
They do not react
with dilute acids. These are not capable of replacing hydrogen.
|
They react with the
salt solution of metals. Depending on their reactivity, displacement reaction
can occur.
|
These react with the
salt solution of non-metals.
|
|
|
Question 31:
A man went door to door posing as a goldsmith. He promised to
bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave
a set of gold bangles to him which he dipped in a particular solution. The
bangles sparkled like new but their weight was reduced drastically. The lady
was upset but after a futile argument the man beat a hasty retreat. Can you
play the detective to find out the nature of the solution he had used?
Answer
He must have dipped the gold metal in the solution of aqua regia
− a 3:1 mixture of conc. HCl and conc. HNO3. Aqua regia is a
fuming, highly corrosive liquid. It dissolves gold in it. After dipping the
gold ornaments in aqua regia, the outer layer of gold gets dissolved and the
inner shiny layer appears. That is why the weight of gold ornament reduced.
Give reasons why copper is used to make hot water tanks and not
steel (an alloy of iron).
Answer
Copper does not react with cold water, hot water, or steam. However, iron reacts with steam. If the hot water tanks are made of steel (an alloy of iron), then iron would react vigorously with the steam formed from hot water.
That is why copper is used to make hot water tanks, and not steel.
Answer
Copper does not react with cold water, hot water, or steam. However, iron reacts with steam. If the hot water tanks are made of steel (an alloy of iron), then iron would react vigorously with the steam formed from hot water.
That is why copper is used to make hot water tanks, and not steel.
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