Q._________ gas is used to store fat and oil-containing foods for a long time?
Ans: Nitrogen gas.
Q. Grapes hanging on the plant do not ferment, but after being plucked from the plant can
be fermented. Under what conditions do these grapes ferment? Is it a chemical or a
physical change?
Ans: When attached to the plants, Grapes are living, and therefore, their immune system
prevents fermentation. The microbes can grow in the plucked grapes, which can be
fermented under anaerobic conditions. This is a chemical change.
Q. Write the balanced chemical equations for the following reactions and identify the type of
reaction in each case.
(a ) In the thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and
aluminium oxide.
(b ) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium
nitride.
(c ) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium
chloride solution and solid iodine.
Answer:
(a ) Fe2O3 (s) + 2 Al (s) → Al2O3 (s) + 2 Fe (l) + Heat
It is a displacement or redox reaction.
(b ) 3 Mg (s) + N2 (g) → Mg3N2 (s)
It is a combination reaction.
(c ) 2 KI (aq) + Cl2 (g) → 2 KCl (aq) + I2 (s)
Q. A substance X, an oxide of a group 2 element, is used intensively in the cement
industry. This element is present in bones also. On treatment with water, it forms a
solution which turns red litmus blue. Identify X and also write the chemical reactions
involved.
Ans: Here, X is calcium oxide.
Calcium oxide is used in the cement industry.
The element present in it is calcium.
On treatment with water, calcium oxide forms a solution of calcium hydroxide [Ca(OH) 2], which is an
alkali. Hence, it turns red litmus blue.
CaO (s) + H2O (l) → Ca(OH)2 (aq) + HeaT
Q. Balance the following chemical equations and identify the type of chemical reaction.
(a ) Mg (s) + Cl2 (g) → MgCI2 (s) (b ) HgO (s) + Heat → Hg (l) + O2 (g)
(c ) Na (s) + S (s) → Na2S (s) (d ) TlCl4 (l) + Mg (s) → Tl (s) + MgCl2 (s)
(e ) CaO (s) + SiO2 (s) → CaSiO3 (s) (f ) H2O2 (l) + UV → H2O (l) + O2 (g)
Ans: (a ) Mg (s) + Cl2 (g) → MgCI2 (s) ,It is a combination reaction.
(b ) 2 HgO (s) + Heat → 2 Hg (l) + O2 (g) ,It is a thermal decomposition reaction.
(c ) 2 Na (s) + S (s) → Na2S (s) ,It is a combination reaction.
(d ) TlCl4 (l) + 2 Mg (s) → Tl (s) + 2 MgCl2 (s) , It is a displacement reaction.
(e ) CaO (s) + SiO2 (s) → CaSiO3 (s) ,It is a combination reaction.
(f ) 2 H2O2 (l) + UV → 2 H2O (l) + O2 (g) ,It is a decomposition reaction.
Q. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper
does not. Explain why?
Ans: Zinc is more reactive than copper as Zinc is placed above hydrogen, and copper is
placed below hydrogen in the activity series of metals. Thus, zinc liberates hydrogen gas
when reacted with dilute hydrochloric acid, whereas copper does not.
Q. A silver article generally turns black when kept in the open for a few days. The article,
when rubbed with toothpaste again, starts shining.
(a ) Why do silver articles turn black when kept in the open for a few days? Name the
phenomenon involved.
(b ) Name the black substance formed and give its chemical formula.
Ans: (a ) The silver article turns black when kept in the air because the silver article reacts
with sulphur compounds such as hydrogen sulphide (H2S) present in the air to form silver
sulphide Ag2S. This phenomenon is called corrosion. It is also known as tarnishing of silver.
(b ) The black substance is silver sulphide. Its chemical formula is Ag2S.
Q. On heating blue coloured powder of copper (I) nitrate in a boiling tube, copper oxide
(black), oxygen gas, and a brown gas X is formed
(a) Write a balanced chemical equation of the reaction
.(b) Identity the brown gas X evolved.
(c) Identify the type of reaction.
(d) What could be the pH range of the aqueous solution of the gas X?
Answer: (a ) 2 CuNO3 (s) + Heat → 2 CuO (s) + 4 NO2 (g) + O2 (g)
(b ) The brown gas is of nitrogen dioxide.
(c ) It is a thermal decomposition reaction.
(d ) NO2 gas reacts with water to produce nitric acid. Thus, its pH range will be less than 7.
Q. Why is it necessary to balance a chemical equation?
ans: according to the law of conservation of mass, mass can neither be created nor e
destroyed in a chemical reaction.Thus balancing of chemical equation is necessary so that
the law of conservation of mass may be obeyed.
Q. In the equations given below, state giving reasons, whether substances have been
oxidised or reduced.
(i) PbO + CO –> Pb + CO2
(ii) H2S + Cl2–>2HCl + S.
Solution: (i) Carbon monoxide is oxidised as it gains oxygen.
(ii) Chlorine is reduced as it gains hydrogen.
Q : State one basic difference between a physical change and a chemical change.
Ans: In physical change, no new substance is formed, whereas in a chemical change, new
substance(s) is/are formed.
Question : Identify the type of reaction(s) in the following equations.
(i) CH4 + 2O2 → CO2 + 2H2O (ii) Pb(NO3)2 + 2KI → Pbl2 + 2KNOs
(iii) CaO + H2O → Ca(OH)2 (iv) CuSO4 + Zn → ZnSO4 + Cu
Answer: (i) Combustion reaction and oxidation reaction.
(ii) Double displacement reaction and precipitation reaction.
(iii) Combination reaction.
(iv) Displacement reaction.
Q What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an
exothermic or an endothermic reaction?
Ans: The reaction in which oxygen or electronegative element is added or hydrogen or
electropositive element is removed or loss of electrons takes place, is called an oxidation
reaction. For example:
Fe2+ → Fe3+ + e‒ (loss of electron)
C + O2 → CO2 + Heat
Q14. Name a metal for each case:
(i) it does not react with cold as well as hot water but reacts with any other physical state of water,
(ii) it does not react with any physical state of water.
Answer: (i) Aluminium (Al)
(ii) Copper (Cu)
Q. A sample of water weed was placed in water and exposed to sunlight. Bubbles of a gas
are seen on the surface of the leaves.
(i) Name the gas evolved.
(ii) Name the process taking place.
(iii) Write a balanced equation of reaction taking place.
Solution: (i) Oxygen
(ii) Photosynthesis
(iii) 6CO2 + 6H2O —> C6H12O6 + 6O2.
Q. What is corrosion?
ANS: Corrosion is defined as a natural process that causes the transformation of pure metals into
undesirable substances when they react with substances like water or air. This reaction causes
damage and disintegration of the metal starting from the portion of the metal exposed to the
environment and spreading to the entire bulk of the metal.
Q. Explain four different types of chemical reactions with suitable examples.
ANS: Combination Reaction
1. A reaction in which two or more reactants combine to form a single product is known as a
combination reaction.
2. It takes the form of X + Y → XY
3. A combination reaction is also known as a synthesis reaction.
4. Example of combination reaction: 2Na(s)+Cl2(g)⟶2NaCl(s)
Decomposition Reaction
1. A reaction in which a single compound breaks into two or more simpler compounds is known as a
decomposition reaction.
2. It takes the form of XY → X + Y
3. A decomposition reaction is just the opposite of a combination reaction.
4. Example of a decomposition reaction: CaCO3(s)⟶CaO(s)+CO2(g)
Displacement Reaction
1. A chemical reaction in which a more reactive element displaces a less reactive element from its
aqueous salt solution.
2. It takes the form X + YZ → XZ + Y
3. It is also called a substitution reaction
4. Example of displacement reaction: Zn(s)+CuSO4(aq)⟶ZnSO4(aq)+Cu(s)
Double Displacement Reaction
1. A chemical reaction in which ions get exchanged between two reactants which form a new
compound is called a double displacement reaction.
2. It takes the form of XY + ZA → XZ + YA
3. It is also called a metathesis reaction
4. Example of double displacement reaction: BaCl2(aq)+Na2SO4(aq)⟶BaSO4(s)+2NaCl(aq)
