CBSE Class 10 Science Advanced Chemistry Chemical Reactions and Equations MCQs Set 01

Answer: (a) red in colour

Explanation:
1. Green crystals of ferrous sulphate first lose their water of crystallisation and turn white.
2. On strong heating they decompose: \( \ce{2FeSO4 -> Fe2O3 + SO2 + SO3} \).
3. The residue ferric oxide, \( \ce{Fe2O3} \), is red (reddish brown).

Teacher's Note:
1. Remember that the same reaction gives off the smell of burning sulphur (SO2).
2. Do not confuse the residue (red) with the original crystals (green).

Answer: (d) white

Explanation:
1. Zinc reacts with dilute HCl: \( \ce{Zn + 2HCl -> ZnCl2 + H2 ^} \).
2. Hydrogen bubbles come out and the surface of the grey granules loses its metallic look and turns whitish as it reacts.
3. Among the given colours the board key accepts white.

Teacher's Note:
1. The important observation of this reaction is the evolution of hydrogen gas, not a colour change.
2. Where a question offers only colours, choose the one that matches the board key.

Answer: (a) (i) and (ii)

Explanation:
1. Carbon takes oxygen from PbO, so carbon is oxidised (statement iii is true).
2. PbO loses oxygen, so lead oxide is reduced (statement iv is true).
3. \( \ce{CO2} \) is the product, it is not being oxidised, so (ii) is incorrect.
4. Statement (i) names the element lead, but it is lead oxide that is reduced to lead, so the board treats (i) as incorrect too.
5. The incorrect statements are (i) and (ii).

Teacher's Note:
1. Always ask which substance loses oxygen (reduced) and which gains it (oxidised).
2. A product of a reaction is not the one undergoing the change.

Answer: (c) A hissing sound

Explanation:
1. Quicklime reacts vigorously with water: \( \ce{CaO + H2O -> Ca(OH)2 + "heat"} \).
2. The reaction is very exothermic, so some water turns to steam and the mixture hisses.
3. The answer is a hissing sound.

Teacher's Note:
1. A pop sound belongs to burning hydrogen, not to this reaction.
2. Touching the dish also shows that it becomes hot.

Answer: (d) double displacement reaction

Explanation:
1. \( \ce{BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)} \).
2. The two compounds exchange their ions, and insoluble barium sulphate forms.
3. This is a double displacement (precipitation) reaction.

Teacher's Note:
1. Two compounds in, two new compounds out, with an exchange of ions, means double displacement.
2. A single displacement has one element and one compound as reactants.

Answer: (a) White

Explanation:
1. Barium sulphate is a white, insoluble solid.
2. It is the white precipitate formed when barium chloride and sodium sulphate solutions are mixed.
3. The answer is white.

Teacher's Note:
1. Link the colour with a well known test: white precipitate with barium chloride shows sulphate ions.
2. Lead iodide, in contrast, is yellow.

Answer: (c) White

Explanation:
1. The precipitate is barium sulphate, \( \ce{BaSO4} \).
2. Its colour is white.

Teacher's Note:
1. This is the same reaction as the one in the earlier board question, so the colour has to be the same.
2. Repeated board questions are common, so learn the colours of common precipitates.

Answer: (d) All of these

Explanation:
1. Mass is conserved in every chemical change (A is true).
2. A chemical change forms new substances which cannot easily be changed back (B is true).
3. A physical change gives no new substance and can usually be reversed (C is true).
4. The answer is all of these.

Teacher's Note:
1. Compare the two kinds of change under three heads: new substance, permanence and reversibility.
2. The mass remains the same in both kinds of change.

Answer: (d) All are correct

Explanation:
1. An equation names the reactants and products (A).
2. It uses their symbols and formulae (B).
3. Its coefficients show how many atoms or molecules take part (C).
4. So the answer is all are correct.

Teacher's Note:
1. A chemical equation is a short, quantitative way of describing a reaction.
2. What it does not tell us are the speed and the conditions unless they are written in.

Answer: (c) evolution of gas

Explanation:
1. Zinc displaces hydrogen from the acid.
2. Hydrogen gas bubbles out of the solution.
3. No solid is formed, so it is not a precipitation, and the visible change is evolution of gas.

Teacher's Note:
1. Look for the symbol (g) on the product side to spot gas evolution.
2. A precipitate is marked (s) on the product side.

Answer: (a) exothermic reaction

Explanation:
1. An acid reacting with a base gives salt and water, and heat is released.
2. The mixture becomes warm, so it is exothermic.
3. No oxidation number changes, so it is not a redox reaction.

Teacher's Note:
1. Heat released means exothermic and heat absorbed means endothermic.
2. Neutralisation is also a double displacement reaction.

Answer: (b) \( \ce{2C4H10 + 12O2 -> 8CO2 + 10H2O} \)

Explanation:
1. Count atoms in (B): C is 8 and 8, H is 20 and 20, but O is 24 on the left and 16 + 10 = 26 on the right.
2. 13 molecules of oxygen are needed, not 12.
3. (A), (C) and (D) have equal atoms on both sides.
4. The unbalanced equation is (B).

Teacher's Note:
1. Count every element on both sides, saving oxygen for the last.
2. Burning of hydrocarbons often needs an odd number of O atoms, so the whole equation may have to be doubled.

Answer: (c) 4 and 2

Explanation:
1. Hydrogen: 2H2O has 4 H, so x = 4.
2. Nitrogen: 4 = 2 (in the nitrate) + y, so y = 2.
3. Check oxygen: left 12, right 6 + 4 + 2 = 12.
4. So x = 4 and y = 2, option (C).

Teacher's Note:
1. Use hydrogen first because it occurs in only one product.
2. Finish by checking the oxygen count.

Answer: (a) hydrogen gas and ferrous chloride are produced.

Explanation:
1. Iron is above hydrogen in the reactivity series, so it displaces hydrogen from the acid.
2. \( \ce{Fe + 2HCl -> FeCl2 + H2 ^} \).
3. Iron forms ferrous chloride (iron(II)), not ferric chloride; hydrogen gas is the other product. The answer is (A).

Teacher's Note:
1. Dilute acids give iron(II) salts, because H+ is a weak oxidant and cannot take iron to +3.
2. Chlorine gas is never produced when a metal reacts with dilute HCl.

Answer: (b) 2, 2, 4

Explanation:
1. Balanced equation: \( \ce{2Pb(NO3)2 -> 2PbO + 4NO2 + O2} \).
2. Check: Pb 2 and 2, N 4 and 4, O 12 and 2 + 8 + 2 = 12.
3. So x = 2, y = 2, z = 4, option (B).

Teacher's Note:
1. Fix O2 = 1 as given and then balance nitrogen and lead.
2. Brown fumes of NO2 are the sign of this reaction.

Answer: (c) \( \ce{NaOH} \)

Explanation:
1. The products contain Na and OH, which must have come from the second reactant.
2. \( \ce{FeSO4 + 2NaOH -> Na2SO4 + Fe(OH)2 v} \).
3. So x is NaOH.

Teacher's Note:
1. Work backwards from the products to find the missing reactant.
2. The dirty green precipitate of ferrous hydroxide is the sign of this reaction.

Answer: (a) an oxidising agent

Explanation:
1. Chlorine takes electrons from magnesium and becomes \( \ce{Cl-} \), so chlorine is reduced.
2. The substance that is reduced is the oxidising agent.
3. So chlorine is an oxidising agent.

Teacher's Note:
1. Whatever gets reduced is the oxidising agent.
2. Magnesium, which loses electrons, is the reducing agent here.

Answer: (b) \( \ce{4Na + O2 -> 2Na2O} \)

Explanation:
1. In (A) two compounds combine; in (C) a compound combines with an element.
2. (D) is a displacement reaction.
3. Only (B) has two elements, Na and \( \ce{O2} \), forming one compound. The answer is (B).

Teacher's Note:
1. Sort the reactants as element or compound before choosing the type.
2. Only one product is formed in a combination reaction.

Answer: (d) All the above statements are correct

Explanation:
1. Oxidation means gain of oxygen or another electronegative element, loss of hydrogen, or loss of an electropositive element, and loss of electrons.
2. All of (A), (B) and (C) are among these, so the answer is all the above statements are correct.

Teacher's Note:
1. Reduction is the exact opposite of each of these points.
2. The modern definition, loss of electrons, covers all of them.

Answer: (b) (i) and (ii)

Explanation:
1. An acid reacts with a base to give salt and water, so it is a neutralisation reaction.
2. The ions of the two compounds exchange partners, so it is also a double displacement reaction.
3. Two products are formed from two reactants, so it is neither a decomposition nor an addition reaction.
4. The answer is (i) and (ii).

Teacher's Note:
1. Every neutralisation reaction is also a double displacement reaction.
2. Decomposition has one reactant and addition gives only one product.

Answer: (c) oxidation

Explanation:
1. \( \ce{2Mg + O2 -> 2MgO} \).
2. Magnesium gains oxygen (and loses electrons), so it is oxidised.

Teacher's Note:
1. Burning in air is mostly oxidation by oxygen.
2. Sublimation is a physical change, not a chemical one.

Answer: (b) a reducing agent

Explanation:
1. A substance that is oxidised itself gives electrons or hydrogen (or takes oxygen) to another substance, which is thereby reduced.
2. Such a substance is a reducing agent.

Teacher's Note:
1. A reducing agent is oxidised; an oxidising agent is reduced.
2. A short rule: the agent does the opposite to itself of what it does to the other substance.

Answer: (d) one substance is oxidised, while the other is reduced.

Explanation:
1. Oxidation and reduction always occur together.
2. One substance loses electrons (is oxidised) and the other gains them (is reduced).
3. The answer is (D).

Teacher's Note:
1. The word redox is made from reduction and oxidation.
2. One cannot take place without the other.

Answer: (a) butylated hydroxyl anisole.

Explanation:
1. Antioxidants such as BHA (butylated hydroxy anisole) and BHT (butylated hydroxy toluene) are added to foods containing fats and oils.
2. They stop the fats from being oxidised, so the answer is butylated hydroxyl anisole.

Teacher's Note:
1. Remember BHA and BHT as the two food antioxidants.
2. Sodium hydroxide and sodium carbonate are bases and would spoil the food.

Answer: (d) \( \ce{Ag2S} \)

Explanation:
1. Silver reacts with hydrogen sulphide present in the air.
2. A black coating of silver sulphide forms, so the answer is \( \ce{Ag2S} \).

Teacher's Note:
1. Silver tarnishes black, copper turns green and iron turns red-brown.
2. Rubbing with toothpaste or a polish removes the black layer.

Answer: (a) \( \ce{Sn^2+ - 2e- -> Sn^4+} \)

Explanation:
1. Oxidation is loss of electrons.
2. In (A) tin loses 2 electrons and goes from +2 to +4.
3. In (B) and (C) electrons are gained, so those are reductions.
4. The answer is (A).

Teacher's Note:
1. Electrons on the left side of an arrow mean reduction; electrons on the right side (or subtracted) mean oxidation.
2. Oxidation number increases in an oxidation.

Answer: (b) aqueous.

Explanation:
1. Slaked lime dissolves in water to a small extent, giving lime water, and the equation is written with it as dissolved.
2. The state symbol used is (aq), so the answer is aqueous.

Teacher's Note:
1. Ca(OH)2 is a solid when dry, but in this reaction it is in the excess water.
2. It cannot be a gas or a pure liquid.

Answer: (c) (a)-(iii), (b)-(i), (c)-(iv), (d)-(ii)

Explanation:
1. (iii) two compounds give one compound: combination.
2. (i) one compound breaks into two compounds: decomposition.
3. (iv) iron displaces copper: displacement.
4. (ii) water breaks into its elements: analysis.
5. So (a)-(iii), (b)-(i), (c)-(iv), (d)-(ii), option (C).

Teacher's Note:
1. An analysis reaction is a decomposition in which the compound breaks into its elements.
2. Check each pair in turn and strike out options that fail.

Answer: (d) (i) to (iv) all

Explanation:
1. In each of the four reactions two compounds exchange their parts to give two new compounds.
2. So all of them are double displacement reactions, option (D).

Teacher's Note:
1. Spot the pattern AB + CD giving AD + CB.
2. Reactions (iii) and (iv) are also not redox, since no atom changes its oxidation number.

Answer: (c) \( \ce{AgCl} \)(s).

Explanation:
1. Silver chloride decomposes in sunlight: \( \ce{2AgCl -> 2Ag + Cl2} \).
2. The silver formed turns the white solid grey.
3. The answer is \( \ce{AgCl} \).

Teacher's Note:
1. Silver salts, especially AgCl and AgBr, are light sensitive, which is why they are used in black and white photography.
2. This is a photodecomposition reaction.

Answer: (d) All of these are correct

Explanation:
1. (A) is true: for example hydrogen peroxide can act in both ways.
2. (B) is true: one mole of H+ neutralises one mole of OH-.
3. (C) is true: a catalyst is not used up.
4. The answer is all of these are correct.

Teacher's Note:
1. Neutralisation of equal moles needs equal volumes only when both solutions have the same molarity.
2. A catalyst only changes the rate of reaction.

Answer: (b) Intramolecular redox change

Explanation:
1. \( \ce{2KClO3 -> 2KCl + 3O2} \): chlorine is reduced and oxygen is oxidised.
2. Both changes occur within the same compound, so it is an intramolecular redox change.

Teacher's Note:
1. Intermolecular means two different substances take part; intramolecular means within one substance.
2. In disproportionation the same element is oxidised and reduced.

Answer: (d) (A) and (B) both

Explanation:
1. An insoluble solid, AgCl, is formed, so it is a precipitation reaction.
2. The ions of the two compounds exchange partners, so it is a double displacement reaction.
3. The answer is (A) and (B) both.

Teacher's Note:
1. Most precipitation reactions are double displacement reactions.
2. Two reactants and two products rule out a combination reaction.

Answer: (a) reduction of S

Explanation:
1. In \( \ce{PbSO4} \), sulphur has oxidation number +6.
2. In \( \ce{PbS} \), sulphur has oxidation number -2.
3. The oxidation number decreases, so sulphur is reduced.

Teacher's Note:
1. Use the sulphate ion total charge of -2: x + 4(-2) = -2 gives x = +6.
2. Loss of oxygen from a substance is also reduction.

Answer: (d) \( \ce{SnCl2 + HgCl2 -> SnCl4 + Hg} \)

Explanation:
1. (A) and (B) are double displacement reactions and (C) is also an exchange of ions with no change of oxidation numbers.
2. In (D) tin goes from +2 to +4 (oxidised) and mercury from +2 to 0 (reduced).
3. So (D) is the redox reaction.

Teacher's Note:
1. To spot a redox reaction look for an element changing its oxidation number.
2. Precipitation and neutralisation reactions are not redox reactions.

Answer: (c) double displacement reaction

Explanation:
1. \( \ce{NaCl + AgNO3 -> AgCl v + NaNO3} \).
2. The two compounds exchange ions to give two new compounds, so it is a double displacement reaction.

Teacher's Note:
1. A synthesis reaction forms one product from simple substances.
2. Analysis is the breaking of a compound into elements.

Answer: (c) Displacement

Explanation:
1. One hydrogen atom of methane is replaced (displaced) by a chlorine atom.
2. This is a substitution, which is a displacement reaction.

Teacher's Note:
1. The reaction needs light.
2. It is not a combination reaction because two products are formed.

Answer: (a) Cu & CuO

Explanation:
1. Brown shiny copper on heating in air gives black copper oxide: \( \ce{2Cu + O2 -> 2CuO} \).
2. Hydrogen reduces it back: \( \ce{CuO + H2 -> Cu + H2O} \).
3. So x = Cu and y = CuO, option (A).

Teacher's Note:
1. Only copper is brown and metallic among the choices.
2. The reverse change needs a reducing agent such as hydrogen.

Answer: (a) H₂S is getting oxidised and Cl₂ is getting reduced

Explanation:
1. Hydrogen is removed from \( \ce{H2S} \) (sulphur changes from -2 to 0), so \( \ce{H2S} \) is oxidised.
2. Chlorine gains hydrogen (changes from 0 to -1), so \( \ce{Cl2} \) is reduced.
3. The answer is (A).

Teacher's Note:
1. Option (C) is incomplete because oxidation never happens without reduction.
2. Removal of hydrogen is oxidation, and addition of hydrogen is reduction.

Answer: (a) loss of electron

Explanation:
1. In the electronic concept, oxidation is the loss of electrons and reduction is the gain of electrons.
2. The answer is loss of electron.

Teacher's Note:
1. Memory aid: OIL RIG, oxidation is loss and reduction is gain.
2. Protons are not exchanged in redox reactions.