CBSE Class 10 Science Chemical Reactions and Equations Assignment

Question. When ferrous sulphate is heated strongly it undergoes decomposition to form ferric oxide as a main product accompanied by a change in color from: 
(a) Blue to green.
(b) Green to blue.
(c) Green to brown.
(d) Green to yellow.
Answer : C

Question. In the equation, NaOH + HNO₃ -> NaNO₃ + H₂O nitric acid is acting as 
(a) An oxidizing agent
(b) An acid
(c) A nitrating agent
(d) A dehydrating agent
Answer : B

Question. Select the oxidising agent for the following reaction: 
H₂S + I₂ > 2HI + S
(a) I₂
(b) H₂S
(C) HI
(d) S
Answer : A

Question. The Following Reaction Is An Example Of : 4NH₃(G) + 5O₂(G) → 4NO(G) + 6H2O(G) 
(a) Displacement reaction and Neutralisation reaction
(b) Combination reaction and Redox reaction
(c) Displacement reaction and Redox reaction
(d) Neutralisation reaction and Redox reaction
Answer : C

Question. A substance which oxidized itself and reduces other is known as 
(a) Oxidizing agent
(b) Reducing agent
(c) Both of these
(d) None of these
Answer : B

Question. Consider reaction: Al(s) + O₂ (g) → Al₂O₃. Moles of Al(s) needed to balance equation are 
(a) 1
(b) 2
(c) 3
(d) 4A
nswer : D

Question. It is necessary to balance a chemical equation in order to satisfy the law of: 
(a) Conservation of motion
(b) Conservation of momentum
(c) Conservation of energy
(d) Conservation of mass
Answer : D

Question. Which of the following does show an oxidation reaction? 
(a) Gain of oxygen
(b) Loss of oxygen
(c) Gain of hydrogen
(d) None of the above
Answer : A

Question. MnO₂ + 4HCl → 2 + 2H₂O + Cl₂ Identify the substance oxidized in the above equation. 
(a) MnCl2
(b) HCl
(c) H₂O
(d) MnO₂
Answer : D

Question. When crystals of lead nitrate are heated strongly in a dry test tube 
(a) crystals immediately melt
(b) a brown residue is left
(c) white fumes appear in the tube
(d) a yellow residue is left
Answer : B

Question. Which of the following is Not True with respect to the neutralisation reaction? 
(a) Salt is formed.
(b) Reaction occurs between an acid and a base.
(c) Reactive element displaces less reactive element.
(d) Reactants are in gaseous state.
Answer : D

Question. An element X on exposure to moist air turns reddish-brown and a new compound Y is formed. The substances X and Y are 
(a) X = Fe, Y = Fe₂O₃
(b) X = Ag, Y = Ag₂S
(c) X = Cu, Y = CuO
(d) X = Al, Y = Al₂O₃
Answer : A

Question. If magnesium is gently heated, it forms 
(a) Magnesium oxide
(b) Magnesium sulfide
(c) Magnesium nitrite
(d) Magnesium carbonate
Answer : A

Question. What chemicals are used in fireworks? 
(a) Copper chloride
(b) Calcium chloride
(c) Barium chloride
(d) All of above
Answer : D

Question. In a chemical reaction between sulphuric acid and barium chloride solution the white precipitates formed are of: 
(a) Hydrochloric acid
(b) Barium sulphate
(c) Chlorine
(d) Sulphur
Answer : B

Question. Color of Solid magnesium is 
(a) Dark grey
(b) Silver grey
(c) Black
(d) Whitish silver
Answer : B

Question. Consider equation: Cu⁺² (aq) + 2OH⁻(aq) → Cu(OH)₂ (s). precipitate of Copper Hydroxide (Cu(OH)₂) will be of 
(a) Green color
(b) Blue color
(c) Brown color
(d) White color
Answer : B

Question. Consider The Following Statements, Choose The Incorrect One(S): 
(a) The chemical equation is balance to satisfy the law of conservation of mass in chemical reaction.
(b) We should never change the formula of an element or compound to balance and equation.
(c) Those reaction in which heat is evolved are known as exothermic reactions.
(d) When Methane (CH4) burns in oxygen of air it forms carbon monoxide and water vapour.
Answer : D

Question. Which of the following reactions can also be termed as thermal decomposition reaction? 
(a) Combination Reaction
(b) Decomposition Reaction
(c) Displacement reaction
(d) Double displacement reaction
Answer : B

Question. What is the name of the process where fatty foods become rancid? 
(a) Corrosion
(b) Oxidation
(c) Reduction
(d) Hydrogenation
Answer : B

Question. The reaction between aqueous solutions of sodium chloride and silver nitrate is -
a. displacement reaction
b. synthesis reaction
c. double displacement reaction
d. analysis reaction
Answer. C

Question. In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate?
a) Lead sulphate (insoluble)
(b) Lead acetate
(c) Ammonium nitrate
(d) Potassium sulphate
Answer : B

Question. The brown gas evolved on heating of copper nitrate is
(a) O₂
(b) NO₂
(c) N₂
(d) NO
Answer : B

Question. Pb + CuCl₂ → PbCl₂ + Cu
The above reaction is an example of:
(a) combination
(b) double displacement
(c) decomposition
(d) displacement
Answer : D

Question. 2HNO₃ + Ca (OH)₂ → Ca (NO₃)₂ + 2H₂O; is an example of
(i) displacement reaction (ii) double displacement reaction
(iii) neutralisation reaction (iv) combination reaction.
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Answer : B

Question. Oxidation involves
(i) gain of electron
(ii) loss of electron
(iii) addition of oxygen or electronegative element
(iv) removal of hydrogen or electropositive element
(a) (i), (ii), (iii)
(b) (ii), (iii), (iv)
(c) (i), (iii), (iv)
(d) (i), (ii), (iv)
Answer : B

Question. CH₄ + Cl₂ → CH₃ Cl + HCl reaction is an example of -
a. Synthetic
b. Analytical
c. Displacement
d. Neutralisation
Answer. C

Question. A Brown and bright element “x” when heated in presence of air turns into black substance “y”. If Hydrogen gas is passed over this heating material again “x” is obtained. “x” and “y” are –
a. Cu & CuO
b. S & SO₂
c. C & CO₂
d. Na & NaH
Answer. A

Question. H₂S(g) + Cl₂ (g) →  2 HCl(g) + S(s), The reaction is interpreted as : 
a. H₂S is getting oxidised and Cl₂ is getting reduced
b. H₂S is getting reduced and Cl₂ is getting oxidised
c. Only H₂S is oxidised
d. Both H₂S and Cl₂ are reduced
Answer. A

Question. What is the oxidation number of sulphur in peroxy mono sulphuric acid (H₂SO₅) ?
a. 8
b. 6
c. 5
d. 4
Answer. D

Question. Oxidation is defined as : 
a. loss of electron
b. gain of electron
c. loss of proton
d. gain of proton
Answer. A

Question. From the following metals whose nitrate produces NO₂ gas on heating
a. Na
b. K
c. Pb
d. None of these
Answer. C

Question. The equation given below indicates; NaCl + AgNO₃ → AgCl + NaNO₃ :
a. Chemical decomposition
b. Chemical combination
c. Chemical displacement
d. Chemical double displacement
Answer. D

Question. From the given reactions which is a redox reaction ? 
a. CuO(s) + H₂O(g) → Cu(s) + H₂O(l)
b. 2Cu(s) + O₂(g) → 2CuO(s)
c. C(s) + 2H₂(g) → CH₄(g)
d. 4Al(s) + 3O₂(g) → 2Al₂O₃(s)
Answer. C

Question. Displacement reaction is : 
a. CaO (s) + H₂O(l) → Ca(OH)₂ (aq)
b. Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)
c. MnO₂(s) + 4HCl(l) → MnCl₂\(s) + 2H₂O(l) + Cl₂(g)
d. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Answer. B

Question. Identify the type of reaction for each of the following as 
Combination –(p)
Decomposition –(q)
Displacement –(r)
Double displacement –(s) or combustion – (t)
(i) A compound break apart into its elements
(ii) A metal and a non metal react to form an ionic compound.
(iii) A compound of hydrogen and carbon reacts with oxygen to produce carbon-dioxide and water.
(iv) Silver ion from Ag(NO3) (aq.) forms a precipitate with bromide ion from KBr aq.
a. (i)-(q), (ii)-(p), (iii)-(r), (iv)-(s)
b. (i)-(q), (ii)-(p), (iii)-(t), (iv)-(s)
c. (i)-(s), (ii)-(r), (iii)-(q), (iv)-(p)
d. (i)-(p), (ii)-(t), (iii)-(q), (iv)-(r)
Answer. B

Question. Which of the following is endothermic reaction ?
a. C(s) + O₂(g) → CO₂(g)
b. N₂(g) + O₂(g) → 2NO(g)
c. 2H₂(g) + O₂(g) → 2H₂O(l)
d. 2CH₃OH (l) + 3O₂(g) → 2CO₂(g)+4H₂O(l)
Answer. B

Question. In which of the following reactions H₂O₂ acts as a reducing agent :
a. H₂O₂ + 2H+ + 2e– → 2H₂O
b.  H₂O₂ – 2e– → O₂ + 2H+
c.  H₂O₂ + 2e– → 2OH–
d.  H₂O₂ + 2OH– – 2e– → O₂ + 2H₂O
a. a.,c.
b. b.,d.
c. a.,b.
d. c., d.
Answer. C

Question. Precipitate formation during chemical reaction is indicated by this arrow :
a. ↑
b. →
c. ↓
d. ←
Answer. B

Question. The chemical reaction HNO₃ + KOH → KNO₃ + H₂O is an example of
a. neutralization
b. double displacement
c. neutralization and double displacement
d. combination
Answer. B

Question. In the reaction SO₂ (g) + 2H₂S(g) → 3S + 2H₂O(g) the reducing agent is
a. SO₂
b. H₂O
c. H₂S
d. S
Answer. C

Question. Which of the following is reduction reaction ?
a. Bleaching of coloured objects using moist chlorine
b. Rancidity of fats
c. Thermite process involving the reaction of chromium oxide (or) iron oxide with alumini
d. The poling process involving the removal of impurities from a molten metals
Answer. C

Question. Hydrolysis of water is which type of following reactions? 
a. Endothermic
b. Decomposition
c. Both 1 and 2
d. Combination
Answer. C

Question. When a burning splinter is brought near the gas jar containing hydrogen gas a popping sound is observed. It is due to
a. exothermic
b. endothermic
c. exothermic and endothermic
d. none of these
Answer. A

Question. 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. A

Question. Identify the correct oxidant and reductant in the following reaction : 
PbS + 4H₂O₂ → PbSO₄ + 4H₂O
a. PbS – Oxidant
H₂O₂ – Reductant
b. PbS – Reductant
PbSO₄ – Oxidant
c. PbS – Reductant
H₂O₂ – Oxidant
d. H₂O₂ – Oxidant
H₂O – Reductant
Answer. C

Question. Sodium carbonate is kept open in the air it looses crystalline water. The process is called:
a. Hygroscopy
b. Deliquescent
c. Efflorescence
d. None of the above
Answer. C

Question. Which out of the following solutions will give a white precipitate when common salt solution is added to it : 
a. KNO₃ solution
b. AgNO₃ solution
c. Ba(NO₃)₂ solution
d. Mg(NO₃)₂ solution
Answer. B

Question. Match the given column-I with column-II and choose the correct option : 
                I                                       II
a. 2Mg + O₂→ 2MgO                  i. Exothermic reaction
b. CaO+H₂O→Ca(OH)₂              ii. Dissociation
c. 2 AgCl → 2Ag +Cl₂                iii. Reduction
d. CuO +H₂ → Cu+H₂O             iv. Oxidation
a. a-i, b-ii, c-iii, d-iv
b. a-ii, b-i, c-iv, d-iii
c. a-iv, b-iii, c-i, d-ii
d. a-iv, b-i, c-ii, d-iii
Answer. D

Question. When KMnO₄ acts as an oxidising agent and ultimately forms MnO₄^2–, MnO₂, Mn₂O₃ and Mn²⁺, then the number of electrons transferred in each case is : 
a. 4, 3, 1, 5
b. 1, 5, 3, 7
c. 1, 3, 4, 5
d. 3, 5, 7, 1
Answer. C

Question. Which of the following is a redox reaction ? 
a. NaCl + KNO₃ → NaNO₃ + KCl
b. CaC₂O₄ + 2 HCl → CaCl₂ + H₂C₂O₄
c. Mg (OH)₂ + 2 NH₄Cl → MgCl₂ + 2 NH₄OH
d. Zn + 2 AgCN → 2 Ag + Zn (CN)₂
Answer. D

Question. The oxidation state of Cr in [Cr(NH₃)₄Cl₂]⁺ is : 
a. + 3
b. + 2
c. + 1
d. 0
Answer. A

Question. Which of the following chemical reactions depicts the oxidizing behaviour of H₂SO₄?
a. 2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O
b. Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O
c. NaCl + H₂SO₄ → NaHSO₄ + HCl
d. 2PCl₅ + H₂SO₄ → 2POCl₃ + 2HCl + SO₂Cl₂
Answer. A

Question. In which of the following reactions H₂O₂ acts as a reducing agent ? 
a. H₂O₂ + 2H+ + 2e¯ → 2H₂O
b. H₂O₂ – 2e¯ → O₂ + 2H+
c. H₂O₂ + 2e¯ → 2OH¯
d. H₂O₂ + 2OH¯ – 2e¯ → O₂ + 2H₂O
a. a., b.
b. c., d.
c. a., c.
d. b., d.
Answer. D

Question. In balancing the reaction.
xH₂S + 2NaNO₃ + 2HCl → y S + zNO + kNaCl + 4H₂O
one would get x,y, z and k, respectively, as 
a. 3,3,2 and 2
b. 2,2,3 and 3
c. 3,3,4 and 4
d. 4,4,3 and 3
Answer. A

Question : The chemical formula of lead sulphate is 
a. Pb₂SO₄
b. PbSO₄
c. Pb₂(SO₄)₃
d. Pb(SO₄)₂
Answer : B

Question :  Heat is evolved during 
a. Combination Reaction
b. Displacement Reaction
c. Endothermic Reaction
d. Combustion Reaction
Answer :  D

Explanation: Burning takes place in combustion so it is an exothermic reaction

Question : When zinc metal is dipped in copper sulphate solution 
a. No reaction takes place
b. The solution remains blue and copper metal gets deposited
c. The solution becomes colourless and reddish brown copper metal gets deposited
d. The solution becomes green and copper metal gets deposited
Answer : C

Explanation:

Zinc is more reactive than copper. zinc displace copper from copper sulphate and Copper metal is deposited.

Question : Which of the following decolourizes a blue solution of copper sulphate? 
a. Al
b. Zn
c. Fe
1. (A), (B) and (C)
2. (B) only
3. (A) only
4. (C) only
Answer : A

Explanation: All Zn , Al and Fe are above Cu in the reactivity series so they can displace Cu from CuSO₄ Solution.

Question :  Choose a displacement reaction. 
a. burning of metals
b. addition of more active metal to a solution of a less active metal compound.
c. extraction of metals.
d. electrolysis
Answer : B

Explanation: Reactions in which atoms or ions move from one compound to other to form new compound are known as Displacement reaction.
A general displacement reaction can be represented using chemical equation as follows:
A + BC → AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taken place.
Example: When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.

ASSERTION- REASON TYPE QUESTIONS

DIRECTION: Each of these questions contains an assertion followed by reason. Read them carefully and answer the question on the basis of following options. You have to select the one that best describes the two statements.
(a) If both Assertion and Reason are correct and reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Question. Assertion (A): AgBr is used in photographic and X-ray film.
Reason (R): AgBr is photosensitive and changes to Ag and bromine in presence of sunlight and undergoes decomposition reaction.
Answer : A

Question. Assertion (A): Magnesium ribbon keeps on burning in atmosphere of nitrogen.
Reason (R): Magnesium reacts with nitrogen to form magnesium nitrides and this reaction is combination reaction.
Answer : A

Question. Assertion (A): Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas and it is displacement reaction.
Reason (R): Zinc reacts with oxygen to form zinc oxide.
Answer : B

Question. Assertion (A): MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O is redox reaction.
Reason (R): MnO₂ oxidises HCl to Cl₂ and gets reduced to MnCl₂.
Answer : A

Question. Assertion (A): White silver chloride turns grey in sunlight.
Reason (R): Decomposition of silver chloride in presence of sunlight takes place to form silver metal and chlorine gas 
Answer : A

Short Answer type Questions

Question : Why are food items packed in aluminium foils? 
Answer : A protective coating of aluminium oxide (Al₂O₃) is formed on the surface of the foil which prevents it from getting corroded in the presence of air and water. Hence, food items are saved from getting spoilt when packed in aluminium foils.

Question : Nidhi wanted the image of her pencil to be double the size of its original size. Name the mirror used for getting such image.
Answer : Concave mirror.

Question : Write the laws of reflection.
Answer : (a) The angle of incidence is equal to angle of reflection.
(b) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Question : Name two types of spherical mirror.
Answer : • Concave mirror
• Convex mirror

Question : What are the two types of reflection?
Answer : (i) Regular (ii) Irregular

Question : Give uses of plane mirror.
Answer : • Looking glass
• Used in submarines
• Solar cooker
• Kaleidoscope

Question : Amit visited a fair and saw a mirror in which he got a very funny image. The above part of his body was big in size, middle part was of normal size and the lower part of the body showed very small size. What kind of mirror is this?
Answer : Upper part – Concave mirror; Middle part : Plane mirror; Lower part : Convex mirror.

Question : Give uses of concave mirror.
Answer : • Used as reflectors in car headlights, searchlights, etc.
• Used as shaving mirror.
• Used in solar cooker to focus the sunlight on one point.

Question : Give uses of convex mirror.
Answer : • It is used as the rear view mirror in cars.
• It is used in street lights as it diverge the light over larger area.

Question : What are the two types of refractive index?
Answer : • Relative refractive index– It is the ratio of speed of light in one medium to the speed of light in another medium
• Absolute refractive index– It is the ratio of light in vacuum to the speed of light in another medium

Question : Why do we prefer a convex mirror as a rear view mirror in vehicles?
Answer : Convex mirrors are used as rear view mirror in cars because it produces erect and diminished image of the traffic behind the vehicle. It also gives a wider view.

Question : Name the type of mirror used in the following situations.
(a) Headlights of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Answer : (a) Concave mirror
(b) Convex mirror
(c) Concave mirror

Question : What are double displacement reaction? 
Answer : Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.
AB + CD → AC + BD
These are the reactions in which two different atoms or groups of atoms are displaced by another atoms/groups of atoms.
Example :
i. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄ (s)¯ + 2NaCl (aq)
ii. AgNO₃ (aq) + NaCl (aq) → AgCl(s)¯ + NaNO₃ (aq)
Such reactions usually occur in ionic compound and are very fast reactions. 

Question : Oil and fat containing food items are flushed with nitrogen. Why? 
Answer : In the presence of oxygen in the air, the fats present in the fatty food are oxidised to compounds which have a bad smell i.e. the food becomes rancid. Flushing with nitrogen cuts off the contact of food with oxygen and protects the food from rancidity.

Question : Write the formula and then balance the following equations. 
a. Butane (C₄H₁₀) + Oxygen Carbon dioxide + Water
b. Magnesium + Silver nitrate Magnesium nitrate + Silver
c. Lime water + Carbon dioxide Calcium carbonate + Water
d. Sodium + Water Sodium hydroxide + Hydrogen
e. Calcium carbonate + Water + Carbon dioxide Calcium bicarbonate 
Answer : The formula and balanced equations are:
a. Butane (C₄H₁₀) + Oxygen → Carbon dioxide + Water
2C₄H₁₀ + 13O₂ + → 8CO₂ + 10H₂O
b. Magnesium + Silver nitrate →Magnesium nitrate + Silver
Mg + 2AgNO₃ → Mg(NO₃)₂ + 2Ag
c. Lime water + Carbon dioxide →Calcium carbonate + Water
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
d. Sodium + Water →Sodium hydroxide + Hydrogen
2Na + 2H₂O → 2NaOH + H₂
e. Calcium carbonate + Water + Carbon dioxide →Calcium bicarbonate
CaCO₃ + H₂O + CO₂ →Ca(HCO₃)₂

Question :Write the formula and then balance the following equation.
Red lead oxide →Lead monoxide + Oxygen
Answer : Red lead oxide Lead monoxide + Oxygen

Question : Name the method used to balance a chemical equation. 
Answer : There are two methods of balancing chemical equation:
1. Hit and Trail Method:
(i)The symbols and formulae of the reactant and products are written as a skeletal equation.
(ii) Any elementary gas (O₂, H₂, N₂ etc.) appearing on either side of the skeletal equation, is written in the atomic state.
(iii)The formula containing the maximum number of atoms is selected to begin the process of balancing. If this method is not convenient, then balancing of the atoms begin with atoms, which appear minimum number of times.
(iv)Atoms of elementary gases are balanced at the last.
(v) When the balancing is complete, the equation is converted to the molecular form.
2. Partial equation method: When equations contain many reactants and products they cannot be balanced by the hit and trial method. They are then balanced by the partial equation method. In this method the overall reaction is assumed to take place through two or more simpler reactions, which can be represented by partial equations. The steps involved are:
(i)The given chemical equation is split into two or more partial equations. 
(ii)Each partial equation is separately balanced by the hit and trial method.
(iii)These balanced partial equations are multiplied with suitable coefficients in order to exactly cancel out those common substances which do not appear in the overall chemical equations.
(iv)The balanced partial equations so obtained, are added to arrive at the balanced chemical equation.

Question : Name the type of reaction :Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen. 
Answer : Reactions in which atoms or ions move from one compound to other to form new compound are known as Displacement reaction.
Zn + H₂SO₄ →ZnSO₄ + H₂
Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen this is displacement reaction.

Question : Mention five ways by which rusting can be prevented. 
Answer : 1. By painting metal surface with enamel paints.
2. By applying oils and greases on surfaces.
3. By plastic coating on the surfaces.
4. By galvanization.
5. By forming alloys.
6. By Plating
7. Keep away from moisture.

Question : Translate the following statement into a chemical equation and then balance it :
Barium chloride reacts with aluminium sulphate to give aluminium. 
Answer : Barium chloride reacts with aluminium sulphate to give aluminium and separates Barium sulfate(BaSO₄).
3BaCl₂(aq) + Al₂(SO₄)₃ (aq) → 2AlCl₃(aq) + 3BaSO₄(s)↓

Question : i. Identify the type of reactions taking place in each of the following cases and write the balanced chemical equations for the reactions
a. Barium chloride solution is mixed, with copper sulphate solution and a white precipitate is obtained.
b. On heating copper powder in air, the surface of the copper powder turns black.
ii. What happens when hydrogen gas is passed over the heated copper oxide? Write the chemical equation involved in this reaction. 
Answer : i. a. BaCl₂ (aq) + CuSO₄ (aq) → BaSO₄ (s) + CuCl₂ (aq)
Double decomposition reaction

Redox reaction, here Cu is changing into CuO. This is oxidised and oxygen is getting reduced by adding metal to it.
ii. If hydrogen gas is passed over heated material (CuO) the black coating on the surface turns brown as the reaction takes place and copper is obtained. 

Question : State one basic difference between a physical change and a chemical change.
Answer : In a physical change, no new substance is forme(d) In a chemical change, a new substance is forme(d)

Question :  Why eatables are preferably packed in aluminum foils?
Answer : Aluminum foils do not corrode in atmosphere even if kept for a long time. Actually, a protective coating of aluminum oxide (Al₂O₃) is formed on the surface of the metal. It stops any further reaction of the metal with air (oxygen) and water. The eatables do not get spoile(d)

Question : What happens when ZnCO₃ is heated in the absence of air? Give the relevant equation.
Answer : ZnO(s) and CO₂(g)CO₂(g) are forme(d) Chemical Equation:
ZnCO₃ → ZnO + CO₂

Question : What happens when an iron nail is put inside the copper sulfate solution? Write a reaction with observation.
Answer : Iron nail turns brown, blue color of CuSO4 changes to colorless. (or light green)
 Fe(s)+CuSO4(aq)→FeSO4(aq)-+Cu(s)

Question :  You are given the following materials:
(i) Iron nails
(ii) Copper sulphate solution
(iii) Barium chloride solution
(iv) Copper powder
(v) Ferrous sulphate crystals
(vi) Quick lime.
Identify the type of chemical reaction taking place when :
(a) Barium chloride solution is mixed with copper sulphate solution and a white precipitate is observe(d)
(b) On heating, copper powder in air in a china dish, the surface of copper powder becomes black.
(c) On heating green ferrous sulphate crystals, reddish brown solid is left and a gas having smell of burning sulphur is notice(d)
(d) Iron nails when left dipped in blue copper sulphate solution become brownish in colour and blue colour of copper sulphate solution fades away.
(e) Quick lime reacts vigorously with water releasing a large amount of heat.

Question : Define a combination reaction. Give one example of a combination reaction which is also exothermi(c)
Answer : A combination reaction is said to have occurred when two or more than two substances combine to form a single substance.

Question : Which of the following is a displacement reaction?
Answer: B

Question : Why do we store silver chloride in dark colored bottles? Explain in brief.
Answer : Silver chloride on exposure to sunlight may decompose as per the following reaction
                     2AgCL → 2Ag + Cl₂
Therefore it is stored in dark colored bottles.

Question : (i) Account for the following :
(a) White silver chloride turns grey in sunlight.
(b) Brown coloured copper powder on heating in air turns into black coloured substance.
(ii) What do you mean by
(a) Displacement reaction
(b) Reduction reaction
(c) Combination reaction?
Write balanced chemical equation in support for all
Answer : (i) (a) White coloured silver chloride undergoes decomposition in the presence of sunlight and forms silver (grey in colour) and chlorine.
(b) Brown coloured copper powder on heating in air gets oxidised to copper oxide which is black in colour.
(ii) For the different types of reactions,
(a) In a displacement reaction, one element takes the place of another in a compound dissolved in a solution. For example,
Fe(s) + CuSO₄ (aq) ———> FeSO₄ (aq) + Cu(s)
(b) Combination reaction may be defined as the reaction in which two or more substances combine under suitable conditions to form a new substance. For example,
(c) A decomposition reaction may be defined as the reaction in which a single substance decomposes or splits into two or more substances under suitable conditions.
For example,
It may be concluded that a certain substance is formed or synthesised in combination reaction and it breaks or splits in decomposition reaction. Therefore, the two reactions oppose each other. 

 

Question : Find the incorrect statement : 

(I) Oxygen is highly combustible and hydrogen is supporter of combustion,

(II) Oxygen and hydrogen both are highly combustible,

(III) Oxygen and hydrogen both are supporters of combustion,

(IV) Hydrogen is highly combustible and oxygen is supporter of combustion

a. I, II and III

b. I, III and IV

c. IV, I and II

d. I, II and IV

Answer : A

Explanation: Only Hydrogen is combustible and Oxygen is a supporter of combustion

 

Question : Which of the following is correct conclusion? 

(a) Al is more reactive than Cu and Fe but less reactive than Zn

(b) Al is more reactive than Cu but less reactive than Zn and Fe

(c) Al is more reactive than Zn and Cu but less reactive than Fe

(d) Al is more reactive than Zn, Cu, Fe

Answer : D

Explanation: 2Al + 3ZnSO₄ → Al₂(SO₄)₃ + 3 Zn

2Al + 3CuSO₄ → Al₂(SO₄)₃ + 3 Cu

2Al + 3FeSO₄ → Al₂(SO₄)₃ + 3Fe

Al is more reactive than Zn, Cu and Fe because it displaces them from their salt solution. 

 

Question : Four students A, B, C and D noted the initial colour of the solutions in beakers I, II, III and IV. After inserting zinc rods in each solution and leaving it undisturbed for two hours, noted the colour of each solution again.

They recorded their observations in the form of table given below:

Which student noted the colour change in all the four beakers correctly? 

a. C

b. A

c. B

d. D

Answer : A

Explanation: In I and II no reaction takes place, therefore. In III, green solution of FeSO₄ will change to colourless due to formation of zinc sulphate.

 

 

Question : What happens when copper powder is heated ? 

Answer : Copper is oxidised to copper oxide.It means addition of oxygen.

Oxygen is reduced to copper oxide. It means addition of metal i.e reduction.

So, balanced chemical equation is: 2Cu + O₂→ 2CuO

A black colour copper (II) oxide is forme(d) 

 

Question :  What does one mean by exothermic and endothermic reactions ? Give examples. 

Answer : Exothermic reaction: A chemical reaction in which heat is given out is known as exothermic reaction. For example

C(s) + O₂ (g) → CO₂(g) + Heat energy

NaOH (aq) + HCl(aq) → NaCl(aq) + H₂O(l) + Heat energy

Endothermic reaction: A chemical reaction in which heat energy is absorbed is known as endothermic reaction. For example

  

 

Question : There are different types of chemical reactions occurring around us or being carried out for the benefit of mankind, e.g. combination reactions, decomposition reactions, displacement reactions, precipitation reactions, reduction-oxidation (redox) reactions, photochemical reactions etc

Now answer the following questions:  

i. Combustion of coke is a combination reaction. CO₂ is not a pollutant. Then why is combustion of coke harmful?

ii. Which reaction followed by two combination reactions are involved in white wash of walls?

iii. Give one use of tin plating in daily life.

iv. How photochemical reactions have played an important role in photography? 

Answer : i. CO₂ is not considered a pollutant up to a certain level. Rather, it helps in maintaining the temperature of the Earth. Combustion of coke is harmful as it increases the concentration of CO₂ in the atmosphere which causes global warming due to the greenhouse effect.

ii. The reaction of calcium with oxygen gives quicklime (CaO) which when mixed with water forms slaked lime Ca(OH)₂ [combination reaction] and when slaked lime is applied on walls then it reacts with CO₂ of the air to form CaCO₃.

iii. Tin-plating is preferred in the tiffin boxes because tin is non-poisonous and hence, does not contaminate the food kept in them. 

 

Question :. Why does the colour of heated copper powder becomes black when air is passed over it? 

Answer : When copper powder is heated in the presence of air,a black coating of copper oxide is formed. 

 

Question : Two beakers A and B contain Iron (II) sulphate solution. In the beaker A is placed a small piece of copper and in the beaker B is placed a small piece of zinc. It is found that a grey deposit forms on the zinc but not on the copper. What can be concluded from these observations? 

Answer : From these observation we can conclude that zinc is most active metal followed by iron and copper.

Copper does not react, but zinc reacts with ferrous sulphate solution. Thus, zinc is most reactive, followed by iron and copper.

Zn + FeSO₄ +→ ZnSO₄ + Fe

Cu + FeSO₄ →No reaction 

 

Question : What are the types of combination reactions ? Give example of each type. 

Answer : Combination reactions: A combination reaction is one in which two or more reactants combine to form a single product. Combination reactions are again of three types.

Types of Combination reactions:

i. Combination reactions between elements.

ii. Combination reactions between compounds.

iii. Combination reactions between elements and compounds.

iv. Combination reactions between elements: In these reactions elements were combined to form a product.

Examples : Formation of calcium oxide by the combination elements calcium and oxygen.

2Ca + O₂ →  2CaO

Formation of ammonia by the combination of elements nitrogen and hydrogen.

N₂ + 3H₂ → 2NH₃

v. Combination reactions between compounds: In these reactions compounds were combined to form product.

Example: Reaction of calcium oxide in water to form calcium hydroxide

CaO + H₂O → Ca(OH)₂

vi. Combination reactions between elements and compounds: In these reactions elements and compounds combined to form product.

Example:Formation of sulphur trioxide by the combination of sulphur dioxide and oxygen.

2SO₂ + O₂ → 2SO₃

 

Question : Write the balanced chemical equation for the following reaction and identify the type of reaction.

Zinc carbonate (s)→ Zinc oxide (s) + Carbon dioxide (g) 

Answer : Zinc carbonate (s)→ Zinc oxide (s) + Carbon dioxide (g)

ZnCO₃(s) → ZnO(s) + CO₂(g)

This is decomposition reaction.

 

Question : Write the formula and then balance the following equations. 

a. Chlorine + Sodium hydroxide →Sodium chloride + Sodium chlorate + water

b. Iron sulphate + Hydrochloric acid → Iron chloride + Hydrogen sulphide

c. Calcium + Water  →Calcium hydroxide + Hydrogen

d. Zinc carbonate → Zinc oxide + Carbon dioxide

Answer : The formula and balance equations are as follows:

a. Chlorine + Sodium hydroxide →Sodium chloride + Sodium chlorate + water

3Cl₂ + 6NaOH →5NaCl + NaClO₃ + 3H₂O

b. Iron sulphate + Hydrochloric →acid Iron chloride + Hydrogen sulphide

FeS + 2HCl →FeCl₂ + H₂S

c. Calcium + Water →Calcium hydroxide + Hydrogen

Ca + 2H₂O →Ca(OH)₂ + H₂ 

d. Zinc carbonate → Zinc oxide + Carbon dioxide

ZnCO₃ →ZnO + CO₂

e. Sodim nitrate + Sulphuric acid (conc.) →Sodium Sulphate + Nitric acid

2NaNO₃ + H₂SO₄ →Na₂SO₄ + 2HNO₃ 

 

Question : Define corrosion. 

Answer : When a material reacts with the external environment, over a time, its structure will be deteriorated, and breaks down into small pieces. Ultimately, it can disintegrate into the atomic level. This is known as corrosion. Most commonly this happens to metals.

It is the slow eating up of the metals by the action of air and moisture of their surfaces. 

 

Question : Write the balanced chemical equations for the following reactions and identify the type of reaction in each case. 

i. In thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

ii. Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.  

Answer : The thermite reaction is a displacement reaction. The formation of magnesium nitride as above is a combination reaction.

 

 

Long Answer type Questions

 

Question : A convex lens has a focal length of 15 cm. At what distance from the lens should the object be placed so that is forms on its other side a real and inverted image 30 cm away from the lens? What would be the size of image formed if the object is 5 cm high? With the help of a ray diagram show the formation of the image by the lens in this case.
Answer : f = 15 cm
ν = + 30 cm as image is real and inverted

 

Question : With the help of a ray diagram show the position, size and the nature of the image formed by a convex lens for various positions of the object.

Answer :

 

 

Question : With the help of a ray diagram show the type of images formed when object is placed at the following positions in front of concave mirror.
(a) at infinity (b) beyond C
(c) at F (d) at C
(e) between F and C (f) between F and O
∴ C = centre of curvature O = optical centre of the mirror
F = focus
Answer : 

Question : A convex lens has a focal length of 12 cm. At what distance from the lens should an object of height 6 cm be placed so that on the other side of the lens its real and inverted image is formed 24 cm away from the lens? What would be the size of the image formed?
Draw a ray diagram to show the image formed in this case.
Answer : f = +12 cm
Real and inverted image so v = +24 cm

 

Question : Name the type of mirror used in the following situations:
(i) Rear view mirror in vehicles
(ii) Solar furnace
(iii) Torch
(iv) Solar cooker
(v) To get the full length image of tall building.
Answer : (i) Rear view mirror in vehicles – convex mirror as it gives virtual image, diminished and cover the wider view.
(ii) Solar furnace – concave mirror to concentrate all parallel beam of light.
(iii) Torch – concave mirror is used.
(iv) Solar cooker – concave mirror is used to concentrate the heat rays at a point.
(v) Convex mirror is used to view a full length tall building.

 

Question : Draw and explain the ray diagram formed by a convex mirror when
(a) object is at infinity. (b) object is at finite distance from the mirror.
Answer : (a) When the object is at infinity, the image is formed at focus. ‘F’.