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Karnataka SSLC Science (KSEAB) study materials
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Karnataka SSLC Science (KSEAB) practice questions

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Question 1Chemical Reactions and Equations

In the reaction Zn + 2HCl → ZnCl₂ + H₂, which substance is oxidised and which is reduced?

  1. Zinc is oxidised and hydrogen in HCl is reduced Correct answer.
  2. Neither zinc nor hydrogen changes oxidation state
  3. Zinc is reduced and hydrogen in HCl is oxidised
  4. Zinc is oxidised and chlorine in HCl is reduced

Explanation

Oxidation is loss of electrons, shown by an increase in oxidation number; reduction is gain of electrons. Zinc changes from 0 to +2, so it is oxidised, while hydrogen changes from +1 to 0, so it is reduced. Chlorine stays at −1 throughout.

Question 2Chemical Reactions and Equations

A white solid on heating gives a gas that turns lime water milky, and the solid is used in the manufacture of glass. Which reaction is this?

  1. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
  2. CaCO₃ → CaO + CO₂ Correct answer.
  3. CaO + H₂O → Ca(OH)₂
  4. 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Explanation

Limestone (calcium carbonate) is used in glass making and decomposes on strong heating into quicklime and carbon dioxide. The carbon dioxide released turns lime water milky. This is a thermal decomposition reaction.

Question 3Chemical Reactions and Equations

Which change is correctly described as a displacement reaction?

  1. Fe + CuSO₄ → FeSO₄ + Cu Correct answer.
  2. CaCO₃ → CaO + CO₂
  3. NaOH + HCl → NaCl + H₂O
  4. 2H₂ + O₂ → 2H₂O

Explanation

In a displacement reaction a more reactive element pushes a less reactive element out of its compound. Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The other equations show decomposition, neutralisation and combination.

Question 4Chemical Reactions and Equations

A student adds dilute sulphuric acid to sodium carbonate. Which observation confirms that carbon dioxide is produced?

  1. Brisk effervescence with a gas that turns lime water milky Correct answer.
  2. No visible change but the solution becomes warm
  3. A gas with a burning smell that relights a glowing splint
  4. A gas that burns with a pop sound

Explanation

Carbonates react with dilute acids to release carbon dioxide, seen as brisk effervescence. Carbon dioxide is identified by turning freshly prepared lime water milky. Heat alone is not a test for this gas.

Question 5Chemical Reactions and Equations

Which statement about a balanced chemical equation is correct?

  1. The number of molecules is equal on both sides
  2. The total mass of reactants is greater than that of products
  3. The physical states of all reactants must be the same
  4. The number of atoms of each element is equal on both sides Correct answer.

Explanation

Balancing an equation makes the number of atoms of every element the same on the reactant and product sides, which reflects conservation of mass. The number of molecules need not be equal, and physical states may differ.

Question 6Chemical Reactions and Equations

Consider the equation 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ on strong heating. Which type of reaction is this, and what colour is the nitrogen dioxide produced?

  1. Combination; colourless gas
  2. Double displacement; white fumes
  3. Thermal decomposition; reddish-brown gas Correct answer.
  4. Displacement; pale green gas

Explanation

A single compound breaking into simpler substances on heating is thermal decomposition. Lead nitrate gives lead oxide, brown nitrogen dioxide and oxygen. The brown fumes of nitrogen dioxide are characteristic of this reaction.

Question 7Chemical Reactions and Equations

In the reaction CuO + H₂ → Cu + H₂O, which statement is correct?

  1. CuO is reduced and acts as the oxidising agent Correct answer.
  2. Neither reactant changes its oxidation number
  3. CuO is oxidised and acts as the reducing agent
  4. H₂ is reduced and acts as the oxidising agent

Explanation

Copper in CuO goes from +2 to 0, so CuO is reduced and is itself the oxidising agent. Hydrogen goes from 0 to +1, so it is oxidised and acts as the reducing agent. The two processes always occur together.

Question 8Chemical Reactions and Equations

A shiny iron nail is kept in copper sulphate solution for some time. Which set of observations is expected?

  1. A gas is released and the solution turns colourless instantly
  2. Blue colour fades and a brownish deposit forms on the nail Correct answer.
  3. No change occurs because iron is less reactive than copper
  4. Blue colour deepens and the nail becomes silvery

Explanation

Iron is more reactive than copper, so it displaces copper from copper sulphate. The blue colour of the solution fades as copper sulphate is used up, and reddish-brown copper deposits on the nail. This is a displacement reaction.

Question 9Chemical Reactions and Equations

Which of the following is a decomposition reaction driven by electricity rather than by heat?

  1. 2KClO₃ → 2KCl + 3O₂ on heating
  2. 2H₂O → 2H₂ + O₂ when current is passed Correct answer.
  3. 2H₂O₂ → 2H₂O + O₂ using manganese dioxide
  4. CaCO₃ → CaO + CO₂ on heating

Explanation

Passing electric current through acidified water splits it into hydrogen and oxygen; this is electrolytic decomposition. The other reactions are decomposition brought about by heat or by a catalyst. The source of energy distinguishes them.

Question 10Chemical Reactions and Equations

Silver bromide is used in black-and-white photography. What happens to it when exposed to sunlight?

  1. It decomposes into silver and bromine, darkening the material Correct answer.
  2. It melts and then recrystallises unchanged
  3. It dissolves in water to give an acidic solution
  4. It combines with oxygen to form silver oxide

Explanation

Silver bromide undergoes decomposition in light, giving grey silver metal and bromine vapour. The deposited silver makes the exposed area appear dark, which is the basis of photographic imaging. Light supplies the energy for this decomposition.

Question 11Chemical Reactions and Equations

In the reaction Fe + S → FeS on heating, what type of reaction occurs and what is the nature of the product?

  1. Displacement reaction; a brown gas is released
  2. Combination reaction; a black compound is formed Correct answer.
  3. Decomposition reaction; a yellow powder is formed
  4. Double displacement; a white precipitate is formed

Explanation

Two elements combine to give a single compound, so this is a combination reaction. Iron filings and sulphur powder give black iron sulphide, whose properties differ from both elements. No gas is evolved.

Question 12Chemical Reactions and Equations

A solution of barium chloride is mixed with dilute sulphuric acid. A white precipitate forms. Which equation correctly represents this double displacement reaction?

  1. BaCl₂ + H₂SO₄ → BaS + 2HCl + O₂
  2. BaCl₂ + H₂SO₄ → BaO + H₂S + Cl₂
  3. BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl Correct answer.
  4. BaCl₂ + H₂SO₄ → BaH₂ + Cl₂SO₄

Explanation

In a double displacement reaction the ions of two compounds exchange partners. Barium ions pair with sulphate ions to give insoluble white barium sulphate, and hydrogen pairs with chloride to give hydrochloric acid. The equation must balance atoms on both sides.

Question 13Chemical Reactions and Equations

Zinc oxide reacts with sodium hydroxide solution to give sodium zincate and water. Which description of zinc oxide is correct?

  1. It is amphoteric, reacting with both acids and bases Correct answer.
  2. It is acidic only, since it reacts with a base
  3. It is basic only, since it contains a metal
  4. It is neutral, since it does not react with acids

Explanation

Zinc oxide reacts with acids to form salts and water, and also with bases such as sodium hydroxide to form sodium zincate. Substances showing both acidic and basic behaviour are called amphoteric. Aluminium oxide behaves similarly.

Question 14Chemical Reactions and Equations

Rusting of iron requires both oxygen and moisture. A student wants to slow rusting of an iron gate. Which method works by preventing contact between iron and the surroundings?

  1. Cleaning the gate with acid regularly
  2. Painting the surface to form a barrier Correct answer.
  3. Storing the gate in moist air
  4. Keeping the gate in salt water

Explanation

Rusting needs both oxygen and water, so blocking contact with either one slows it down. Painting forms a physical barrier that keeps air and moisture away from the iron. Galvanising and greasing work on the same principle.

Question 15Chemical Reactions and Equations

In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which species is oxidised and which is reduced?

  1. Both manganese and chlorine are reduced in this reaction
  2. Manganese in MnO₂ is oxidised; chloride ion is reduced
  3. Chloride ion is oxidised to chlorine; manganese in MnO₂ is reduced Correct answer.
  4. Oxygen in MnO₂ is oxidised; hydrogen in HCl is reduced

Explanation

Chloride ions lose electrons, going from −1 in HCl to 0 in Cl₂, so they are oxidised. Manganese gains electrons, going from +4 in MnO₂ to +2 in MnCl₂, so it is reduced. Hydrogen and oxygen keep their usual oxidation numbers.

Question 16Chemical Reactions and Equations

A student writes the equation for the reaction of aluminium with dilute hydrochloric acid as Al + HCl → AlCl₃ + H₂. Which correctly balanced equation represents the reaction?

  1. Al + 2HCl → AlCl₃ + H₂
  2. 2Al + 6HCl → 2AlCl₃ + 3H₂ Correct answer.
  3. Al + 3HCl → AlCl₃ + H₂
  4. 2Al + 3HCl → 2AlCl₃ + 3H₂

Explanation

Aluminium forms Al³⁺ and hydrogen is released as H₂, so the equation must show equal atoms of each element on both sides. Using 2Al, 6HCl, 2AlCl₃ and 3H₂ balances aluminium, chlorine and hydrogen. This also matches the oxidation numbers, since each aluminium atom loses three electrons.

Question 17Acids, Bases and Salts

A solution turns blue litmus paper red. Which type of substance is dissolved in it?

  1. A neutral salt
  2. A base
  3. An acid Correct answer.
  4. An indicator

Explanation

Blue litmus turns red only in acidic solutions, because acids release hydrogen ions (H+) in water. Bases turn red litmus blue, and neutral salt solutions leave litmus unchanged. Therefore the dissolved substance must be an acid.

Question 18Acids, Bases and Salts

Which gas is produced when dilute hydrochloric acid reacts with zinc granules?

  1. Carbon dioxide
  2. Chlorine
  3. Oxygen
  4. Hydrogen Correct answer.

Explanation

A reactive metal such as zinc displaces hydrogen from a dilute acid, giving zinc chloride and hydrogen gas. The gas burns with a pop sound, which is the standard test for hydrogen. Carbon dioxide is produced by carbonates, not by metals.

Question 19Acids, Bases and Salts

What is the pH of a solution that is neutral at 25 degrees Celsius?

  1. 0
  2. 7 Correct answer.
  3. 14
  4. 10

Explanation

On the pH scale, 7 marks neutrality at room temperature, where hydrogen ion and hydroxide ion concentrations are equal. Values below 7 are acidic and values above 7 are basic. Pure water and neutral salt solutions such as sodium chloride solution show pH 7.

Question 20Acids, Bases and Salts

Which of the following is the correct chemical name of baking soda?

  1. Calcium carbonate
  2. Sodium hydrogen carbonate Correct answer.
  3. Sodium carbonate
  4. Sodium chloride

Explanation

Baking soda is sodium hydrogen carbonate, formula NaHCO3. It is a mild base used in cooking and in antacid preparations. Sodium carbonate (washing soda) is a different, stronger base, and calcium carbonate is limestone or chalk.

Question 21Acids, Bases and Salts

Which one of these substances is used as an indicator in acid-base titrations and is obtained from lichens?

  1. Litmus Correct answer.
  2. Methyl orange
  3. Turmeric
  4. Phenolphthalein

Explanation

Litmus is a natural indicator obtained from lichens; it is red in acid and blue in base. Phenolphthalein and methyl orange are synthetic indicators, and turmeric is a natural indicator obtained from a plant root. Thus only litmus fits the source described.

Question 22Acids, Bases and Salts

A student adds dilute sulphuric acid to sodium hydroxide solution until the solution becomes neutral. Which salt is formed in this reaction?

  1. Sodium sulphide
  2. Sodium sulphate Correct answer.
  3. Sodium hydrogen sulphate
  4. Sodium sulphite

Explanation

An acid reacting with a base gives a salt and water. Sulphuric acid supplies the sulphate ion and sodium hydroxide supplies the sodium ion, so the salt is sodium sulphate. The other options contain sulphur in a different oxidation state or an incomplete neutralisation product.

Question 23Acids, Bases and Salts

A solution of a metal oxide in water turns red litmus blue. What does this tell us about the metal oxide?

  1. It is a basic oxide such as magnesium oxide. Correct answer.
  2. It is a neutral oxide such as water.
  3. It is an amphoteric oxide such as zinc oxide.
  4. It is an acidic oxide such as carbon dioxide.

Explanation

Metal oxides that dissolve in water to give hydroxide ions are basic oxides, and their solutions turn red litmus blue. Magnesium oxide behaves in this way, forming magnesium hydroxide. Non-metal oxides such as carbon dioxide instead give acidic solutions.

Question 24Acids, Bases and Salts

Why does dry hydrogen chloride gas not turn blue litmus paper red, while its aqueous solution does?

  1. The gas is a base until it dissolves.
  2. The gas contains no hydrogen atoms.
  3. Acidity requires the presence of water to produce hydrogen ions. Correct answer.
  4. Litmus paper cannot be used with gases.

Explanation

Acids show acidic behaviour only in aqueous solution, because hydrogen ions are released when the compound ionises in water. Dry hydrogen chloride has no free hydrogen ions, so it cannot change litmus. Once dissolved, it ionises and the solution becomes acidic.

Question 25Acids, Bases and Salts

A farmer finds that his soil is too acidic for a crop. Which substance should be added to reduce the acidity?

  1. Vinegar
  2. Quicklime (calcium oxide) Correct answer.
  3. Common salt
  4. Dilute hydrochloric acid

Explanation

Excess soil acidity is corrected by adding a base that neutralises the acid. Quicklime, calcium oxide, is a basic substance and is commonly used for this purpose. Adding an acid or a neutral salt would not reduce acidity.

Question 26Acids, Bases and Salts

What happens when carbon dioxide gas is passed through lime water for a long time?

  1. The solution remains clear throughout.
  2. The solution turns blue immediately.
  3. The solution turns red and then milky.
  4. The solution turns milky and then becomes clear. Correct answer.

Explanation

Carbon dioxide first forms insoluble calcium carbonate, which makes lime water milky. On passing excess carbon dioxide, the calcium carbonate is converted into soluble calcium hydrogen carbonate, so the milkiness disappears and the solution becomes clear again.

Question 27Acids, Bases and Salts

Which pair of solutions, when mixed in equal volumes, produces a salt and water with no gas evolved?

  1. Hydrochloric acid and sodium hydroxide Correct answer.
  2. Hydrochloric acid and sodium sulphite
  3. Hydrochloric acid and sodium carbonate
  4. Hydrochloric acid and zinc granules

Explanation

The reaction of an acid with a base is neutralisation, which produces only a salt and water. Hydrochloric acid with sodium hydroxide gives sodium chloride and water. The other pairs involve a carbonate, a metal or a sulphite, all of which release a gas.

Question 28Acids, Bases and Salts

A solution has a pH of 3. If it is diluted ten times with pure water, what is the most likely new pH?

  1. 7
  2. 4 Correct answer.
  3. 10
  4. 1

Explanation

Diluting an acidic solution reduces the hydrogen ion concentration and moves the pH towards 7. A tenfold dilution lowers the hydrogen ion concentration ten times, raising the pH by about one unit, so pH 3 becomes close to pH 4. It cannot become basic on simple dilution with water.

Question 29Acids, Bases and Salts

Washing soda crystals are heated gently. Which observation and product are correct?

  1. They lose water of crystallisation and form anhydrous sodium carbonate. Correct answer.
  2. They turn into sodium chloride and water.
  3. They melt into sodium hydrogen carbonate.
  4. They release carbon dioxide and leave sodium oxide.

Explanation

Washing soda is hydrated sodium carbonate, Na2CO3.10H2O. On gentle heating it loses its ten molecules of water of crystallisation and becomes anhydrous sodium carbonate, a white powder. No carbon dioxide or chlorine-containing product is formed in this change.

Question 30Acids, Bases and Salts

A white powder is added to dilute hydrochloric acid and a gas is produced that turns lime water milky. The same powder, when dissolved in water, turns red litmus blue. Which substance is the powder most likely to be?

  1. Ammonium chloride
  2. Sodium chloride
  3. Sodium hydrogen carbonate Correct answer.
  4. Copper sulphate

Explanation

A carbonate or hydrogen carbonate gives carbon dioxide with acid, and carbon dioxide turns lime water milky. Sodium hydrogen carbonate is also basic in solution, so it turns red litmus blue. Only this option satisfies both observations together.

Question 31Acids, Bases and Salts

Phenolphthalein is added to a dilute acid, and then sodium hydroxide solution is added drop by drop. What colour change is seen at the end point?

  1. Blue to red
  2. Colourless to pink Correct answer.
  3. Pink to colourless
  4. Yellow to orange

Explanation

Phenolphthalein is colourless in acidic and neutral solutions and pink in basic solutions. As sodium hydroxide is added, the solution becomes neutral and then basic, so the indicator turns pink at the end point. The change is therefore from colourless to pink.

Question 32Acids, Bases and Salts

Which statement correctly explains why a solution of sodium hydroxide conducts electricity?

  1. It contains uncharged water molecules only.
  2. It contains mobile ions formed when the base dissolves in water. Correct answer.
  3. It contains hydrogen gas dissolved in it.
  4. It contains free sodium metal atoms.

Explanation

Bases such as sodium hydroxide ionise in water to give mobile sodium and hydroxide ions. These free ions carry charge through the solution, so it conducts electricity. The conduction is ionic, not due to metal atoms or dissolved gas.

Question 33Metals and Non-metals

In the reactivity series of metals, which metal is placed immediately below magnesium?

  1. Iron
  2. Sodium
  3. Zinc
  4. Aluminium Correct answer.

Explanation

The reactivity series arranges metals in decreasing order of reactivity: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, copper, silver, gold. Aluminium sits directly after magnesium, so it is less reactive than magnesium but more reactive than zinc. Sodium and calcium lie above magnesium, while zinc and iron lie well below it.

Question 34Metals and Non-metals

Which property of metals is directly responsible for their use in making thin foils such as aluminium foil for wrapping food?

  1. Malleability Correct answer.
  2. Sonority
  3. High density
  4. High melting point

Explanation

Malleability is the ability of a metal to be beaten or rolled into thin sheets without breaking. Aluminium is highly malleable, so it can be rolled into very thin foil used for wrapping. Density, sonority and melting point do not describe the flattening of a metal into sheets.

Question 35Metals and Non-metals

Sodium is stored immersed in kerosene oil in the laboratory. Which statement best explains this precaution?

  1. Kerosene increases the reactivity of sodium for later use
  2. Kerosene dissolves the oxide layer that forms on sodium
  3. Kerosene keeps sodium cool so that it does not melt
  4. Sodium reacts vigorously with air and moisture, so kerosene isolates it Correct answer.

Explanation

Sodium is so reactive that it catches fire on contact with atmospheric oxygen and reacts violently with water vapour to release hydrogen. Kerosene is a hydrocarbon that does not react with sodium, so it forms a protective layer that keeps air and moisture away. This is why sodium and potassium are stored under kerosene.

Question 36Metals and Non-metals

When dilute hydrochloric acid is added to zinc granules, a gas is evolved. Which test confirms the identity of this gas?

  1. It turns moist blue litmus paper red
  2. It turns limewater milky
  3. It burns with a pop sound Correct answer.
  4. It relights a glowing splint

Explanation

Zinc displaces hydrogen from dilute hydrochloric acid, forming zinc chloride and hydrogen gas. Hydrogen is combustible and burns with a characteristic pop sound when a burning splint is brought near it. Limewater, a glowing splint and moist litmus test other gases or acidity, not hydrogen.

Question 37Metals and Non-metals

Which of the following is an alloy rather than a pure metal?

  1. Graphite
  2. Sulphur
  3. Silicon
  4. Bronze Correct answer.

Explanation

An alloy is a homogeneous mixture of a metal with one or more other elements, and bronze is a mixture of copper and tin. Graphite, silicon and sulphur are non-metals and are not alloys at all. Hence bronze alone fits the definition of an alloy.

Question 38Metals and Non-metals

A student adds copper turnings to dilute hydrochloric acid and warms the mixture. No gas is observed. Which conclusion is correct?

  1. The acid was too dilute and copper would react if concentrated hydrochloric acid were used
  2. Copper reacts only with concentrated acids and never with dilute acids of any kind
  3. Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid Correct answer.
  4. Copper forms a protective oxide layer that stops all reactions with acids

Explanation

A metal displaces hydrogen from a dilute acid only if it lies above hydrogen in the reactivity series. Copper lies below hydrogen, so it cannot reduce hydrogen ions from dilute hydrochloric acid and no hydrogen gas is evolved. This is why copper vessels are not corroded by dilute acids.

Question 39Metals and Non-metals

Aluminium is extracted from purified bauxite by electrolysis rather than by heating with carbon. Which statement gives the correct reason?

  1. Carbon reacts with aluminium to form an alloy instead of reducing the oxide
  2. Aluminium oxide is a poor conductor of electricity in the molten state
  3. Aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide Correct answer.
  4. Bauxite contains no oxygen, so carbon reduction is impossible

Explanation

Carbon can reduce the oxide of a metal only if the metal is less reactive than carbon. Aluminium is more reactive than carbon, so aluminium oxide cannot be reduced by carbon and electrolysis of molten alumina dissolved in cryolite is used instead. Metals such as iron and zinc, which are less reactive than carbon, are extracted by carbon reduction.

Question 40Metals and Non-metals

Iron articles are often coated with a layer of zinc, a process called galvanising. How does this coating protect the iron from rusting?

  1. Zinc lowers the temperature of the iron surface below the dew point
  2. Zinc forms a hard layer that physically blocks oxygen but dissolves in water
  3. Zinc is more reactive than iron, so it corrodes in preference to iron even when the coating is scratched Correct answer.
  4. Zinc reacts with iron to form an alloy that cannot rust

Explanation

Galvanising protects iron in two ways: the zinc layer excludes air and moisture, and because zinc is more reactive than iron it loses electrons preferentially when the coating is broken. This sacrificial protection continues to protect exposed iron, which is why galvanised sheets resist rust even at scratches.

Question 41Metals and Non-metals

Which pair of substances reacts to produce hydrogen gas?

  1. Gold and water
  2. Copper and dilute sulphuric acid
  3. Magnesium and dilute hydrochloric acid Correct answer.
  4. Silver and dilute hydrochloric acid

Explanation

Metals above hydrogen in the reactivity series displace hydrogen from dilute acids. Magnesium lies well above hydrogen, so it reacts with dilute hydrochloric acid to give magnesium chloride and hydrogen gas. Copper, silver and gold lie below hydrogen and do not liberate hydrogen from dilute acids.

Question 42Metals and Non-metals

In the reaction CuO + H2 → Cu + H2O, which substance is oxidised and which is reduced?

  1. Copper(II) oxide is reduced and water is oxidised
  2. Copper(II) oxide is oxidised and hydrogen is reduced
  3. Both copper(II) oxide and hydrogen are oxidised
  4. Hydrogen is oxidised and copper(II) oxide is reduced Correct answer.

Explanation

Oxidation is loss of hydrogen or gain of oxygen, and reduction is gain of hydrogen or loss of oxygen. Hydrogen gains oxygen to form water, so hydrogen is oxidised; copper(II) oxide loses oxygen to give copper, so it is reduced. The reaction is therefore a redox reaction.

Question 43Metals and Non-metals

Which statement correctly compares the oxides of sodium and sulphur?

  1. Sodium oxide is acidic and sulphur dioxide is basic
  2. Sodium oxide is basic and sulphur dioxide is acidic Correct answer.
  3. Both oxides dissolve in water to give acidic solutions
  4. Both oxides are neutral and insoluble in water

Explanation

Metal oxides are generally basic, and sodium oxide dissolves in water to form sodium hydroxide, an alkali. Non-metal oxides are generally acidic, and sulphur dioxide dissolves to give sulphurous acid. This difference in oxide character follows from whether the element is a metal or a non-metal.

Question 44Metals and Non-metals

Which property of metals explains why they conduct electricity in the solid state?

  1. They contain loosely held protons that carry current
  2. They contain free electrons that can move through the lattice Correct answer.
  3. They have low melting points and melt easily
  4. Their ions are free to move in the solid lattice

Explanation

In a metal lattice, the positive ions are fixed in position but the valence electrons are delocalised and free to move throughout the structure. When a potential difference is applied, these free electrons drift and carry current. Ionic solids do not conduct because their ions are not free to move until molten or dissolved.

Question 45Metals and Non-metals

A student places an iron nail in copper sulphate solution and observes a reddish-brown deposit. Which equation and inference are both correct?

  1. Cu + FeSO4 → CuSO4 + Fe; copper is more reactive than iron
  2. Fe + CuSO4 → FeSO4 + Cu; iron is more reactive than copper Correct answer.
  3. 2Fe + 3CuSO4 → Fe2(SO4)3 + 3Cu; iron forms only iron(III) salts
  4. Fe + CuSO4 → FeSO4 + Cu; copper is more reactive than iron

Explanation

Iron lies above copper in the reactivity series, so it displaces copper from copper sulphate solution, giving iron(II) sulphate and a reddish-brown deposit of copper. The observation itself is evidence that iron is the more reactive metal. The reverse reaction does not occur.

Question 46Metals and Non-metals

Molten sodium chloride conducts electricity but solid sodium chloride does not. Which explanation is correct?

  1. In the solid, electrons are free but ions are not; melting frees the electrons
  2. Solid sodium chloride contains molecules, which split into ions only on heating
  3. Melting converts sodium chloride into a metal, which then conducts
  4. In the solid, ions are held in fixed positions; melting frees the ions to move Correct answer.

Explanation

Sodium chloride is an ionic solid in which sodium and potassium-like ions are locked in a rigid lattice, so there are no mobile charge carriers and it does not conduct. On melting, the lattice breaks down and the ions become free to move, so the melt conducts electricity and is electrolysed. Conduction in ionic compounds is therefore by ions, not electrons.

Question 47Metals and Non-metals

An ore contains zinc sulphide. In the extraction of zinc, the sulphide ore is first roasted in air and the oxide formed is then reduced with carbon. Which statement about this two-step process is correct?

  1. Roasting converts the sulphide to sulphate, which is then reduced by carbon
  2. Carbon reduces zinc sulphide directly, so roasting is unnecessary
  3. Roasting converts the sulphide to oxide, and carbon reduces the oxide because zinc is less reactive than carbon Correct answer.
  4. Roasting converts the oxide to sulphide, which is then reduced by carbon

Explanation

Roasting a sulphide ore in excess air converts the metal sulphide to the metal oxide with the release of sulphur dioxide. Zinc is less reactive than carbon, so the oxide can then be reduced by carbon to give zinc metal and carbon monoxide. Metals more reactive than carbon, such as aluminium, cannot be obtained this way.

Question 48Metals and Non-metals

A metal M reacts with dilute acid to give a salt and hydrogen, but it does not react with cold water. Its oxide is reduced by carbon on strong heating. Which metal is most consistent with all three observations?

  1. Copper
  2. Calcium
  3. Zinc Correct answer.
  4. Potassium

Explanation

Zinc lies above hydrogen, so it displaces hydrogen from dilute acids, but it is not reactive enough to react with cold water. Being less reactive than carbon, its oxide is reduced by carbon on heating, which is how zinc is extracted. Potassium and calcium react with cold water, and copper lies below hydrogen and is not attacked by dilute acids.

Question 49Carbon and its Compounds

Which homologous series is characterised by the general formula CnH2n+2?

  1. Alkene
  2. Alkane Correct answer.
  3. Alkyne
  4. Alcohol

Explanation

Saturated hydrocarbons in which every carbon is joined to four other atoms by single bonds follow the formula CnH2n+2, and this series is called the alkane series. Members such as methane, ethane and propane differ by one CH2 unit each step.

Question 50Carbon and its Compounds

A carbon atom forms four covalent bonds with other atoms. Which property of carbon best explains this?

  1. It exists as graphite
  2. It has a high melting point
  3. It has a large atomic radius
  4. It has four valence electrons Correct answer.

Explanation

Carbon's electronic configuration is 2,4, so it has four electrons in its outermost shell and needs four more to attain a stable noble-gas arrangement. It therefore shares four electrons, forming four covalent bonds, which is the basis of its tetravalency.