17.

(a) Magnesium Ribbon is heated in the presence of air to form a white powder called magnesium
oxide.(b) When dilute sulphuric acid is poured over zinc granules 1. there will be a rise in
temperature 2. evolution of hydrogen gas. (c)1. A yellow precipitate is formed. 2. There
will be a change in color (from colourless to yellow).

18. (a) The method of representing a chemical reaction with the help of symbols and formulae of
substances involved in it is called a chemical equation.
Example: Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. This
equation is written as: Zn + H2SO4 —–> ZnSO4 + H2
(b) A balanced chemical equation 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.
Example: Zn + H2SO4 —–> ZnSO4 + H2
An unbalanced chemical equation has an unequal number of atoms of one or more elements in the
reactants and products. It has unequal masses of various elements in the reactants and products.
Example: H2 + O2 —–> 2H2O (c)2NH3 —–> N2 + 3H2 C + CO2 —–> 2CO

19. H2 + CuO —–> Cu + H2O 1. Elements : H2 and Cu 2. Compounds : CuO and H2O

3. Reactants: H2 and CuO 4. Products: Cu

5.Metal: Cu 6. Non-metal: H2

20. (a) The various ways in which a chemical equation can be made more informative are :
(i) By indicating the physical states of the reactants and products. Example: Gaseous state is
indicated by the symbol (g).
Zn (s) + H2SO4 (aq) —–> ZnSO4 (aq) + H2 (g)
(ii) By indicating the heat changes taking place in the reaction. For xxample: An exothermic reaction
is indicated by writing “+Heat” or “+Heat energy” or “+Energy” on the products side of an equation.
C (s) + O2 (g) —–> CO2 (g) + Heat
(iii) By indicating the “conditions” under which the reaction takes place.
Example: Delta stands for heat which is written over the arrow of the equation.

(b) Ca(OH)2 (aq) + CO2 (g) —–> CaCO3 (s) + H2O (l)

21. (a) A balanced chemical equation 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
chemical equation should be balanced to satisfy the law of conservation of chemical reactions.
(b) 2Al + 3Cl2 —–> 2AlCl3 (c) 2K + 2H2O —–> 2KOH + H2

22. (a) The physical

states of the reactants and products are shown by putting the “state symbols”
in an equation.
For example: Zn (s)+ H2SO4 (aq) —–> ZnSO4 (aq) + H2(g) b) Zn (s) + 2HCl(aq) —–> ZnCl2 (aq) + H2 (g)
(c) FeSO4(aq)+ 2NaOH (aq) —–> Fe(OH)2 (s) + Na2SO4(aq)

23. Evolution of gas.

For example: When sodium carbonate reacts with dilute hydrochloric acid, carbon dioxide gas is
evolved. (b) Formation of a precipitate.
For example: When potassium iodide solution is added to a solution of lead nitrate, yellow
precipitate of lead iodide is formed.

24. (a) 2Al(OH)3+ 3H2SO4 —–> Al2(SO4)3+ 6H2O (b) MnO2+ 4HCl —–> MnCl2+ Cl2 + 2H2O

25. (a) MgCO3 (s) + 2HCl (aq) MgCl2 (aq) + CO2 (g) + H2O (l) (b) 2NaOH (aq) + H2SO4 (aq) Na2SO4 (aq) +
2H2O (l)

26.
The conditions for this reaction to take place are: a pressure of 300 atmospheres (written as 300
atm), a temperature of 300oC, and a catalyst which is a mixture of zinc oxide and chromium oxide
(ZnO + CrO3).

27. (a) 2KClO3(s) 2KCl (s) + 3O2 (g) (b) 2Mg + CO2 —–> 2MgO + C

28. (a) CaCO3+ 2HCl —–> CaCl2 + H2O + CO2 (b) NaOH (aq) + HCl(aq) —–> NaCl (aq) + H2O (l)

29. 4NH3 (g)+ 3O2 (g) —–> 2N2 (g) + 6H2O (l)

30. 6CO2 (g) + 6H2O C6 —–> H12O6 (aq) + 6O2(g)

Carbon dioxide, Water , Glucose, Oxygen

31. 3BaCl2(aq) + Al2(SO4)3 (aq) —–> 3BaSO4 (s) + 2AlCl3(aq)

32. 2KNO3 (s) —–> 2KNO2 (s) + O2(g)

33. (a)Chemical reactions are the processes in which new substances with new properties are
formed. For example: When magnesium ribbon is heated, it burns in air to form a white powder
called magnesium oxide. (b)1.The chemical reaction between zinc and dilute sulphuric acid.

2.The chemical reaction between citric acid and purple coloured potassium permanganate solution
is characterised by change in colour (from purple to
colourless).3.The chemical reaction between potassium iodide and lead nitrate is characterised by
the formation of a yellow precipitate of lead iodide.4.The reaction between quick lime and water to
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.

(a) The various characteristics of chemical reactions are: Evolution of a gas, Formation of a
precipitate, Change in colour, Change in temperature,Change in state. (b)Evolution of carbon
dioxide gas, Change in colour from purple to colourless, Formation of white precipitate of barium
sulphate, Change in temperature, Change in state from solid to liquid and gas.

35. (a) Those reactions in which heat is evolved are known as exothermic reactions.
The reactions in which heat is absorbed are known as endothermic reactions.
(b) Example of exothermic reaction:
C (s) + O2 (g) —–> CO2 + Heat
Example of endothermic reaction:
N2 (g) + O2 (g) + Heat —–> 2NO (g)
(c) Endothermic reactions: Photosynthesis, Electrolysis of water, Decomposition of calcium
carbonate.
Exothermic reactions: Burning of natural gas, Respiration
25. Oxidation Reaction: The addition of oxygen (or removal of hydrogen) to a substance is called
oxidation.(i) C (ii) ZnO

26. (a) The oxidation and reduction reactions occurring together are called a redox reaction.
Example:
In this reaction, copper oxide is being reduced to copper whereas hydrogen is being oxidised to
water.
(b) Magnesium is oxidised as addition of oxygen to magnesium takes place leading to formation of

magnesium oxide.

(c)1.HCl 2. MnO2 3. MnO2 4.HCl

27. (a) Those reactions in which two or more substances combine to form a single substance is called
a combination reaction.

28.
In this reaction, copper oxide is being reduced to copper whereas hydrogen is being oxidised to
water.

29. (a) The colour of ferrous sulphate is green. It changes to brown after heating.
(b) The product formed is ferric oxide. This is a decomposition reaction.

30.

This decomposition takes place in the presence of heat and catalyst. In this decomposition reaction,
a single compound, potassium chlorate, is splitting up into two simpler substances, potassium
chloride and oxygen. This decomposition reaction is used for preparing oxygen gas in the laboratory.

31. ZnO + C (i) Zinc oxide. (ii) Carbon.

32.

33. (a) Those reactions in which one element takes the place of another element in a compound, are
known as displacement reactions.
Equation: CuSO4 (aq)+ Zn (s)—–>ZnSO4+ Cu
Those reactions in which two compounds react by an exchange of ions to form two new compounds
are called double displacement reactions.
Equation: AgNO3 (aq) + NaCl (aq)—–>AgCl (s)+ NaNO3 (aq)
(b) Any reaction in which an insoluble solid (called precipitate) is formed that separates from the
solution is called a precipitation reaction.
Example: The reaction between barium chloride and sodium sulphate solution to form a white
barium sulphate precipitate (alongwith sodium chloride solution) is an example of a precipitation
reaction.

35. (a) (i) H2S + Cl2—–>S + 2HCl

In this reaction, H2S is changing into S. That is, Hydrogen is being removed from hydrogen sulphide.
Now, by definition, the removal of hydrogen from a compound is called oxidation, so, we can say
that hydrogen sulphide is being oxidised to sulphur. On the other hand, Cl 2 is changing into HCl. That
is, hydrogen is being added to chlorine. By definition, the addition of hydrogen to a substance is
called reduction, so we can say that chlorine is being reduced to hydrogen chloride.
(b) 2Mg + O2—–>2MgO (i) Mg (ii) O2

36. (a) Those reactions in which one element takes the place of another element in a compound, are
known as displacement reactions.
Example: CuSO4 (aq) + Zn (s) —–>ZnSO4 + Cu
When a strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and
copper are obtained. In this reaction, zinc displaces copper from copper sulphate compound so that
copper is set free. The blue colour of copper sulphate solution fades due to the formation of zinc
sulphate. (b) Those reactions in which two compounds react by an exchange of ions to form two
new compounds are called double displacement reactions.
Example: AgNO3 (aq) + NaCl (aq)
When silver nitrate solution is added to sodium chloride solution, then a white precipitate of silver
chloride is formed along with sodium nitrate solution. In this reaction, two compounds, silver nitrate
and sodium chloride, react to form two new compounds, silver chloride and sodium nitrate.

37.Those reactions in which a compound splits up into two or more simpler substances are known as
decomposition reactions.
Example: When calcium carbonate is heated, it decomposes to give calcium oxide and carbon
dioxide. They are called opposite of combination reactions because in a combination reaction, two
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)

38. (a) When a piece of iron metal is placed in copper sulphate solution, then magnesium sulphate
solution and copper metal are formed. This is a type of displacement reaction.
39.(a) H2 (b) CuO (c) CuO (d) H2

40.When silver nitrate solution is added to sodium chloride solution, then a white precipitate of
silver chloride is formed alongwith sodium nitrate solution.
(a) AgNO3(aq) + NaCl (aq)
(b) Double displacement reaction.

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

This reaction is used in black and white photography.

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

43. (a) Corrosion is the process in which metals are eaten up gradually by the action of air, moisture
or a chemical (such as an acid) on their surface. Rusting 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) Rusting. (c) Oxidation. (d) Railings, car bodies and bridges.

44. (a) The condition produced by aerial oxidation 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 time and makes them unfit for eating.
(b) Oxidation.
(c)1.Rancidity can be prevented by adding anti-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-tight
containers.5.Storing foods away from light can also prevent rancidity.

45.(a) When barium chloride solution is added to sodium sulphate solution, then a white precipitate
of barium sulphate is formed along with sodium chloride solution.
(c) Solid sodium sulphate and Solid barium chloride
(d) Double displacement reaction.
(e) Double displacement reaction between silver nitrate solution and sodium chloride solution forms
a white precipitate of silver chloride and sodium nitrate solution.