17.

(a) Magnesium Ribbon is heated in the presence of air to form a white powder called magnesium For example: When potassium iodide solu on is added to a solu on of lead nitrate, yellow
oxide.(b) When dilute sulphuric acid is poured over zinc granules 1. there will be a rise in precipitate of lead iodide is formed.
temperature 2. evolu on of hydrogen gas. (c)1. A yellow precipitate is formed. 2. There
24. (a) 2Al(OH)3+ 3H2SO4 —–> Al2(SO4)3+ 6H2O (b) MnO2+ 4HCl —–> MnCl2+ Cl2 + 2H2O
will be a change in color (from colourless to yellow).
25. (a) MgCO3 (s) + 2HCl (aq) MgCl2 (aq) + CO2 (g) + H2O (l) (b) 2NaOH (aq) + H2SO4 (aq) Na2SO4 (aq) +
18. (a) The method of represen ng a chemical reac on with the help of symbols and formulae of
2H2O (l)
substances involved in it is called a chemical equa on.
Example: Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. This
equa on is wri en as: Zn + H2SO4 —–> ZnSO4 + H2
(b) A balanced chemical equa on has an equal number of atoms of different elements in the
reactants and products. It has equal masses of various elements in the reactants and products. 26.
Example: Zn + H2SO4 —–> ZnSO4 + H2 The condi ons for this reac on to take place are: a pressure of 300 atmospheres (wri en as 300
An unbalanced chemical equa on has an unequal number of atoms of one or more elements in the atm), a temperature of 300oC, and a catalyst which is a mixture of zinc oxide and chromium oxide
reactants and products. It has unequal masses of various elements in the reactants and products. (ZnO + CrO3).
Example: H2 + O2 —–> 2H2O (c)2NH3 —–> N2 + 3H2 C + CO2 —–> 2CO 27. (a) 2KClO3(s) 2KCl (s) + 3O2 (g) (b) 2Mg + CO2 —–> 2MgO + C
19. H2 + CuO —–> Cu + H2O 1. Elements : H2 and Cu 2. Compounds : CuO and H2O 28. (a) CaCO3+ 2HCl —–> CaCl2 + H2O + CO2 (b) NaOH (aq) + HCl(aq) —–> NaCl (aq) + H2O (l)
3. Reactants: H2 and CuO 4. Products: Cu 29. 4NH3 (g)+ 3O2 (g) —–> 2N2 (g) + 6H2O (l)
5.Metal: Cu 6. Non-metal: H2
30. 6CO2 (g) + 6H2O C6 —–> H12O6 (aq) + 6O2(g)
20. (a) The various ways in which a chemical equa on can be made more informa ve are : Carbon dioxide, Water , Glucose, Oxygen
(i) By indica ng the physical states of the reactants and products. Example: Gaseous state is
indicated by the symbol (g). 31. 3BaCl2(aq) + Al2(SO4)3 (aq) —–> 3BaSO4 (s) + 2AlCl3(aq)
Zn (s) + H2SO4 (aq) —–> ZnSO4 (aq) + H2 (g)
32. 2KNO3 (s) —–> 2KNO2 (s) + O2(g)
(ii) By indica ng the heat changes taking place in the reac on. For xxample: An exothermic reac on
is indicated by wri ng “+Heat” or “+Heat energy” or “+Energy” on the products side of an equa on. 33. (a)Chemical reac ons are the processes in which new substances with new proper es are
C (s) + O2 (g) —–> CO2 (g) + Heat formed. For example: When magnesium ribbon is heated, it burns in air to form a white powder
(iii) By indica ng the “condi ons” under which the reac on takes place. called magnesium oxide. (b)1.The chemical reac on between zinc and dilute sulphuric acid.
Example: Delta stands for heat which is wri en over the arrow of the equa on.
2.The chemical reac on between citric acid and purple coloured potassium permanganate solu on
is characterised by change in colour (from purple to
colourless).3.The chemical reac on between potassium iodide and lead nitrate is characterised by
the forma on of a yellow precipitate of lead iodide.4.The reac on between quick lime and water to
(b) Ca(OH)2 (aq) + CO2 (g) —–> CaCO3 (s) + H2O (l) form slaked lime is characterised by a change in temperature. 5.When wax is burned, then water
and carbon dioxide are formed. Wax is a solid,water is a liquid whereas carbon dioxide is a gas.
21. (a) A balanced chemical equa on has an equal number of atoms of different elements in the
reactants and products. It has equal masses of various elements in the reactants and products. A (a) The various characteris cs of chemical reac ons are: Evolu on of a gas, Forma on of a
chemical equa on should be balanced to sa sfy the law of conserva on of chemical reac ons. precipitate, Change in colour, Change in temperature,Change in state. (b)Evolu on of carbon
(b) 2Al + 3Cl2 —–> 2AlCl3 (c) 2K + 2H2O —–> 2KOH + H2 dioxide gas, Change in colour from purple to colourless, Forma on of white precipitate of barium
sulphate, Change in temperature, Change in state from solid to liquid and gas.
22. (a) The physical
states of the reactants and products are shown by pu ng the “state symbols” 35. (a) Those reac ons in which heat is evolved are known as exothermic reac ons.
in an equa on. The reac ons in which heat is absorbed are known as endothermic reac ons.
For example: Zn (s)+ H2SO4 (aq) —–> ZnSO4 (aq) + H2(g) b) Zn (s) + 2HCl(aq) —–> ZnCl2 (aq) + H2 (g) (b) Example of exothermic reac on:
(c) FeSO4(aq)+ 2NaOH (aq) —–> Fe(OH)2 (s) + Na2SO4(aq) C (s) + O2 (g) —–> CO2 + Heat
Example of endothermic reac on:
23. Evolu on of gas. N2 (g) + O2 (g) + Heat —–> 2NO (g)
For example: When sodium carbonate reacts with dilute hydrochloric acid, carbon dioxide gas is (c) Endothermic reac ons: Photosynthesis, Electrolysis of water, Decomposi on of calcium
evolved. (b) Forma on of a precipitate. carbonate.
Exothermic reac ons: Burning of natural gas, Respira on
25. Oxida on Reac on: The addi on of oxygen (or removal of hydrogen) to a substance is called are called double displacement reac ons.
oxida on.(i) C (ii) ZnO Equa on: AgNO3 (aq) + NaCl (aq)—–>AgCl (s)+ NaNO3 (aq)
(b) Any reac on in which an insoluble solid (called precipitate) is formed that separates from the
26. (a) The oxida on and reduc on reac ons occurring together are called a redox reac on.
solu on is called a precipita on reac on.
Example:
Example: The reac on between barium chloride and sodium sulphate solu on to form a white
In this reac on, copper oxide is being reduced to copper whereas hydrogen is being oxidised to
barium sulphate precipitate (alongwith sodium chloride solu on) is an example of a precipita on
water.
reac on.
(b) Magnesium is oxidised as addi on of oxygen to magnesium takes place leading to forma on of

magnesium oxide.

(c)1.HCl 2. MnO2 3. MnO2 4.HCl 35. (a) (i) H2S + Cl2—–>S + 2HCl
In this reac on, H2S is changing into S. That is, Hydrogen is being removed from hydrogen sulphide.
27. (a) Those reac ons in which two or more substances combine to form a single substance is called Now, by defini on, the removal of hydrogen from a compound is called oxida on, so, we can say
a combina on reac on. that hydrogen sulphide is being oxidised to sulphur. On the other hand, Cl2 is changing into HCl. That
is, hydrogen is being added to chlorine. By defini on, the addi on of hydrogen to a substance is
called reduc on, so we can say that chlorine is being reduced to hydrogen chloride.
(b) 2Mg + O2—–>2MgO (i) Mg (ii) O2
28.
In this reac on, copper oxide is being reduced to copper whereas hydrogen is being oxidised to 36. (a) Those reac ons in which one element takes the place of another element in a compound, are
water. known as displacement reac ons.
Example: CuSO4 (aq) + Zn (s) —–>ZnSO4 + Cu
29. (a) The colour of ferrous sulphate is green. It changes to brown a er hea ng.
When a strip of zinc metal is placed in copper sulphate solu on, then zinc sulphate solu on and
(b) The product formed is ferric oxide. This is a decomposi on reac on.
copper are obtained. In this reac on, zinc displaces copper from copper sulphate compound so that
30. copper is set free. The blue colour of copper sulphate solu on fades due to the forma on of zinc
sulphate. (b) Those reac ons in which two compounds react by an exchange of ions to form two
new compounds are called double displacement reac ons.
Example: AgNO3 (aq) + NaCl (aq)
When silver nitrate solu on is added to sodium chloride solu on, then a white precipitate of silver
chloride is formed along with sodium nitrate solu on. In this reac on, two compounds, silver nitrate
and sodium chloride, react to form two new compounds, silver chloride and sodium nitrate.

This decomposi on takes place in the presence of heat and catalyst. In this decomposi on reac on, 37.Those reac ons in which a compound splits up into two or more simpler substances are known as
a single compound, potassium chlorate, is spli ng up into two simpler substances, potassium decomposi on reac ons.
chloride and oxygen. This decomposi on reac on is used for preparing oxygen gas in the laboratory. Example: When calcium carbonate is heated, it decomposes to give calcium oxide and carbon
dioxide. They are called opposite of combina on reac ons because in a combina on reac on, two
31. ZnO + C (i) Zinc oxide. (ii) Carbon.
or more substances combine to form a single substance.
Example: Magnesium and oxygen combine, when heated, to form magnesium oxide.
(b) Cu (s) + 2AgNO3 (aq)—–>Cu(NO3)2 (aq) + 2Ag (s)

32.

33. (a) Those reac ons in which one element takes the place of another element in a compound, are
known as displacement reac ons.
38. (a) When a piece of iron metal is placed in copper sulphate solu on, then magnesium sulphate
Equa on: CuSO4 (aq)+ Zn (s)—–>ZnSO4+ Cu
solu on and copper metal are formed. This is a type of displacement reac on.
Those reac ons in which two compounds react by an exchange of ions to form two new compounds
 (c) Solid sodium sulphate and Solid barium chloride
(d) Double displacement reac on.
39.(a) H2 (b) CuO (c) CuO (d) H2 (e) Double displacement reac on between silver nitrate solu on and sodium chloride solu on forms
a white precipitate of silver chloride and sodium nitrate solu on.
40.When silver nitrate solu on is added to sodium chloride solu on, then a white precipitate of
silver chloride is formed alongwith sodium nitrate solu on.
(a) AgNO3(aq) + NaCl (aq)
(b) Double displacement reac on.

41.When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

This reac on is used in black and white photography.

42.When a strip of zinc metal is placed in copper sulphate solu on, then zinc sulphate solu on and
copper are obtained.

43. (a) Corrosion is the process in which metals are eaten up gradually by the ac on of air, moisture
or a chemical (such as an acid) on their surface. Rus ng of iron metal is the most common form of
corrosion.
During the corrosion of iron, iron metal is oxidised by the oxygen of air in the presence of water
(moisture) to form hydrated iron oxide called rust.

(b) Rus ng. (c) Oxida on. (d) Railings, car bodies and bridges.

44. (a) The condi on produced by aerial oxida on of fats and oils in foods marked by unpleasant
smell and taste is called rancidity. It spoils the food materials prepared in fats and oils which have
been kept for a considerable me and makes them unfit for ea ng.
(b) Oxida on.
(c)1.Rancidity can be prevented by adding an -oxidants to foods containing fats and oils.2.It can be
prevented by packaging fat and oil containing foods in nitrogen gas.3.Rancidity can be retarded by
keeping food in a refrigerator.4.Rancidity can be prevented by storing food in air- ght
containers.5.Storing foods away from light can also prevent rancidity.

45.(a) When barium chloride solu on is added to sodium sulphate solu on, then a white precipitate
of barium sulphate is formed along with sodium chloride solu on.