CBSE Std 8 Chemistry Ch-2 Question-answer

Chemistry

Ch-2

(Question-answer)

1) Fill in the blanks.
a) Non-metals are generally soft but diamond is an exception.
b) Among non-metals, nitrogen does not react with bases at all.
c) Hydrogen is the only non-metal included in the reactivity series of metals.
d) Sulpher is also used in hardening of rubber.
e) The impurities that are added to semiconductors to alter their electrical conductivity are called dopants.

2) Mark true or false against the following statements.
a) Metals are good conductors of heat and electricity. ✔
b) No metal can form an amphoteric oxide upon reaction with oxygen. ✖
c) Non-metals do not react with dilute acids. ✔
d) Copper can displace iron from ferrous sulphate. ✖
e) Some metals are used in chemotherapy. ✔

3) Complete the following chemical equations.

a) MgO + H₂O ⇾ Mg(OH)₂
b) Zn + H₂SO₄ ⇾ ZnSO₄ + H₂
c) Mg + 2HNO₃ ⇾ Mg(NO₃)₂ + H₂
d) Fe + CuSO₄ ⇾ FeSO₄ + Cu

4) Name a metal that reacts violently with water and metal that reacts very slowly with water.
Ans – Sodium reacts violently with water and magnesium reacts very slowly with water.

5) How do non-metals react with acids?
Ans – Non-metals do not react with dilute acids. They react with concentrated acids. e.g. Concentrated nitric acid reacts with sulphur,carbon,phosphorous to form their respective oxides.

 

 

6) What is the essential condition for the occurrence of a displacement reaction?
Ans – A more reactive metal can displace a less reactive metal from its salt. For displacement reaction ‘AB+C ⇾ CB+A’ to occur,C must be more reactive than A.

7) Magnesium oxide formed by burning magnesium in oxygen is dissolved in water.Is the resulting solution acidic or basic? How will you prove this?
Ans – When magnesium oxide formed by burning magnesium in oxygen is dissolved in water,it forms magnesium hydroxide( Mg(OH)₂). It is base because in this OH⁻ is present.

8) ‘A metal reacts with a base to produce a gas. The gas produced is nitrogen.’ Is this statement correct ? Why?
Ans – Incorrect because when a metal reacts with a base it produces hydrogen gas not nitrogen gas.
2 Al + 6 NaOH ⇾ Na₃AlO₃ + 3 H₂

9) How can metal salts be prepared from metals? Explain with an example.
Ans – When metals react with dilute acids it gives metal salts and hydrogen gas.
e.g. Mg + 2HCl ⇾ MgCl₂ + H₂

10) Write a short note on metalloids and their applications.
Ans – Other than metals and non-metals,there are some elements that posses some characteristics of metals and some of non-metals.They are known as metalloids.

  • applications
  • silicon is used in solar cells,transistors,computers etc.
  • Arsenic is used in pesticides.
  • Boron is used in production of ceramics and glass.
  • Polonium is used as lightweight heat source in space satellites.

 

11) Give two properties of non-metals that can be observed with the help of our senses and two properties of non-metals that can be examined with the help of some experiments.
  • Two properties of non-metals that can be observed with the help of our senses
  1. Hardness
  2. Physical state
  • Two properties of non-metals that can be examined with the help of some experiments.
  1. Melting points and boiling points
  2. Thermal and electrical conductivity
12) Give reasons for the following :

a) Copper is unable to displace zinc from its salt solution.
Ans – Because zinc is more reactive than copper.

b) Metals are used for making automobiles.
Ans – Because Some metals are lighter and heavier. It can be also converted into thin sheets and wires.

c) Graphite is used for making pencil leads.
Ans – Because graphite is solid but soft and has ability to leave mark on paper.

13) On the basis of the reactivity series of metals, discuss the displacement reactions of metals.

from text book

14) Why do screwdrivers have plastic handles?
Ans – Because plastic can’t conduct electricity. So we have no fear of electric shock while working with screwdrivers.

15) Which material,wood or iron, is better suited for making the handle of a metallic cooking pan? Why?
Ans – Wood is better suited for making the handle of a metallic cooking pan because iron is metal and good conductor of heat and wood is not conductor of heat.so by heating cooking pan handle will not get heat and we can easily hold it.

16) Why are metallic substances such as copper and aluminium used for making immersion rods for heating water?
Ans – Metallic substances such as copper and aluminium used for making immersion rods for heating water because they are good conductor of heat and electricity. so they can heat water in less time.

17) Can you store lemon pickle in an aluminium utensil?
Ans – we can’t store lemon pickle in an aluminium utensil. Because lemon pickle is acidic and aluminium reacts with acid. so pickle will be spoiled.

18) Comment on the validity of the statement ‘Reactivity of metals varies from one metal to another and this influences their uses.’
Ans – This statement is valid because all metals have different reactivity and on this base their uses are also different. e.g. We use iron in construction because it is harder than other metals.

 

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