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Myanmar Matriculation Chemistry study materials
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Myanmar Matriculation Chemistry practice questions

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Question 1Chemistry

Which particle is transferred when a magnesium atom forms a magnesium ion in a reaction with oxygen?

  1. Two neutrons
  2. One electron pair
  3. Two protons
  4. Two electrons Correct answer.

Explanation

A magnesium atom has the electron configuration 2,8,2 and reaches the stable noble-gas arrangement by losing its two outermost electrons. Protons and neutrons stay in the nucleus during ordinary chemical change, so only electrons move. The resulting Mg2+ ion has the configuration of neon.

Question 2Chemistry

Sodium and chlorine have valence electrons in the same principal shell. Why does chlorine have the smaller atomic radius?

  1. Its greater effective nuclear charge draws that shell inward Correct answer.
  2. It has fewer occupied principal shells than sodium
  3. It has a smaller nuclear charge than sodium
  4. It has no inner-shell electrons to provide shielding

Explanation

Across this period, increasing nuclear charge attracts electrons more strongly while added electrons enter the same principal shell. Similar core shielding makes the larger effective nuclear charge contract the radius.

Question 3Chemistry

What is the relative molecular mass of carbon dioxide, CO2? (C = 12, O = 16)

  1. 28
  2. 56
  3. 32
  4. 44 Correct answer.

Explanation

The relative molecular mass is the sum of the relative atomic masses of all atoms in the formula. Carbon dioxide contains one carbon atom and two oxygen atoms, giving 12 + (2 x 16) = 44. The value is dimensionless because it is a ratio, not a mass in grams.

Question 4Chemistry

In the reaction Zn + CuSO4 -> ZnSO4 + Cu, which species is oxidised?

  1. Zinc Correct answer.
  2. Copper(II) ion
  3. Sulfate ion
  4. Copper metal

Explanation

Zinc atoms lose two electrons to become Zn2+, so its oxidation number rises from 0 to +2 and it is oxidised. The copper(II) ion gains those electrons and is reduced to copper metal. Loss of electrons is oxidation, so the reducing agent is zinc.

Question 5Chemistry

Which volume of 0.5 mol/dm3 sodium hydroxide solution exactly neutralises 25 cm3 of 1.0 mol/dm3 hydrochloric acid?

  1. 25 cm3
  2. 50 cm3 Correct answer.
  3. 12.5 cm3
  4. 100 cm3

Explanation

Hydrochloric acid and sodium hydroxide react in a 1:1 mole ratio, so moles of acid equal moles of base at the end point. The acid contains 0.025 dm3 x 1.0 mol/dm3 = 0.025 mol, which needs 0.025 mol of NaOH. Dividing by the concentration 0.5 mol/dm3 gives 0.05 dm3, that is 50 cm3.

Question 6Chemistry

Which type of bonding is present in solid sodium chloride?

  1. Metallic bonding
  2. Simple molecular covalent bonding
  3. Giant covalent bonding
  4. Ionic bonding Correct answer.

Explanation

Sodium chloride is built from Na+ and Cl- ions held in a giant lattice by strong electrostatic attraction. Electrons have been transferred from sodium to chlorine, producing oppositely charged ions. This electrostatic attraction between ions is ionic bonding.

Question 7Chemistry

Which gas is produced when dilute hydrochloric acid reacts with calcium carbonate?

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

Explanation

Acids react with carbonates to give a salt, water and carbon dioxide. Calcium carbonate with hydrochloric acid forms calcium chloride, water and carbon dioxide, which turns limewater milky. Hydrogen is produced only when acids react with reactive metals.

Question 8Chemistry

Which change in conditions shifts the equilibrium N2(g) + 3H2(g) <-> 2NH3(g), exothermic, to the right?

  1. Lowering the temperature Correct answer.
  2. Raising the temperature
  3. Adding a catalyst
  4. Lowering the pressure

Explanation

The forward reaction releases heat, so removing heat by lowering the temperature favours the exothermic direction and shifts equilibrium to the right. Pressure changes act on the side with fewer gas moles, and a catalyst only speeds up attainment of equilibrium. Temperature is the only variable here that alters the position of this equilibrium.

Question 9Chemistry

Which factor increases the rate of a reaction between marble chips and dilute hydrochloric acid?

  1. Cooling the acid before the reaction
  2. Diluting the acid with water
  3. Using powdered marble of the same total mass Correct answer.
  4. Using larger marble chips of the same total mass

Explanation

Reaction rate depends on the surface area available for collision. Powdered marble exposes a much larger surface than whole chips of the same mass, so collisions with acid particles occur more frequently. Cooling or diluting the acid reduces collision frequency and slows the reaction.

Question 10Chemistry

Which oxide is amphoteric?

  1. Sodium oxide
  2. Carbon monoxide
  3. Sulfur dioxide
  4. Aluminium oxide Correct answer.

Explanation

Aluminium oxide reacts with both acids and alkalis to form salts, which is the defining behaviour of an amphoteric oxide. It reacts with hydrochloric acid to give aluminium chloride and with sodium hydroxide to give a sodium aluminate. The other oxides listed are basic, acidic or neutral only.

Question 11Chemistry

Which reagent gives a white precipitate with chloride ions in solution?

  1. Limewater
  2. Acidified silver nitrate solution Correct answer.
  3. Sodium hydroxide solution
  4. Barium chloride solution

Explanation

Silver nitrate solution acidified with dilute nitric acid gives a white precipitate of silver chloride with chloride ions. The nitric acid removes carbonate ions that would also give a white precipitate. The precipitate dissolves in ammonia solution, confirming chloride.

Question 12Chemistry

Which statement about the electrolysis of molten lead(II) bromide is correct?

  1. Bromine is deposited at the cathode and lead is liberated at the anode
  2. Lead is deposited at the cathode and bromine is liberated at the anode Correct answer.
  3. Lead is deposited at both electrodes
  4. Hydrogen is liberated at the cathode and oxygen at the anode

Explanation

In molten lead(II) bromide the ions are free to move, so Pb2+ ions migrate to the cathode and are reduced to lead metal. Br- ions migrate to the anode and are oxidised to bromine vapour. Water is absent, so hydrogen and oxygen cannot be formed.

Question 13Chemistry

Which functional group is present in ethanoic acid?

  1. Ester group
  2. Hydroxyl group only
  3. Aldehyde group
  4. Carboxyl group Correct answer.

Explanation

Ethanoic acid, CH3COOH, contains the carboxyl group -COOH, in which a carbonyl and a hydroxyl are joined on the same carbon. This group is responsible for the acidic hydrogen and for the typical reactions with carbonates and alcohols. The other groups listed belong to different homologous series.

Question 14Chemistry

Which reagent distinguishes ethanol from ethanoic acid at room temperature?

  1. Sodium chloride solution
  2. Bromine water
  3. Sodium carbonate solution Correct answer.
  4. Water

Explanation

Ethanoic acid reacts with sodium carbonate to release carbon dioxide, which effervesces and turns limewater milky, whereas ethanol gives no such reaction. The test relies on the acidic carboxyl hydrogen that alcohols lack. Water, bromine water and sodium chloride do not give a clear distinguishing result.

Question 15Chemistry

Which product is formed when ethanol is oxidised by acidified potassium dichromate(VI) and the product is distilled off as it forms?

  1. Ethyl ethanoate
  2. Ethanoic acid
  3. Ethanal Correct answer.
  4. Ethene

Explanation

Acidified dichromate oxidises a primary alcohol first to an aldehyde. If the aldehyde is distilled off immediately it escapes further oxidation to the carboxylic acid. Prolonged heating under reflux would instead give ethanoic acid.

Question 16Chemistry

Which statement explains why the first ionisation energy of magnesium is higher than that of sodium?

  1. Magnesium has a paired outer electron that is harder to remove
  2. Magnesium has a larger atomic radius than sodium
  3. Magnesium has more occupied shells than sodium
  4. Magnesium has a greater nuclear charge with the outer electron in the same shell Correct answer.

Explanation

Sodium and magnesium both lose an electron from the third shell, but magnesium has one extra proton. The stronger nuclear charge holds that outer electron more firmly, so more energy is needed to remove it. Shielding is similar because the inner configuration is identical.

Question 17Chemistry

Which pair of substances reacts to produce a gas that relights a glowing splint?

  1. Copper and dilute sulfuric acid
  2. Sodium chloride and silver nitrate
  3. Calcium carbonate and dilute hydrochloric acid
  4. Hydrogen peroxide and manganese(IV) oxide Correct answer.

Explanation

Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide into water and oxygen. Oxygen gas relights a glowing splint, which is the standard test for it. The other combinations give a precipitate, no reaction, or carbon dioxide.

Question 18Chemistry

Which number of moles of oxygen molecules contains the same number of molecules as 4 g of hydrogen gas? (H = 1, O = 16)

  1. 4 mol
  2. 8 mol
  3. 2 mol Correct answer.
  4. 1 mol

Explanation

4 g of hydrogen gas, H2, is 4/2 = 2 mol, containing 2 x 6.02 x 10^23 molecules. The same number of oxygen molecules corresponds to 2 mol of O2. Mole ratios compare numbers of particles, not masses, so the heavier oxygen is not needed in a larger amount.

Question 19Chemistry

Which compound shows cis-trans isomerism?

  1. Butane
  2. Ethyne
  3. Propene
  4. But-2-ene Correct answer.

Explanation

Cis-trans isomerism requires restricted rotation about a carbon-carbon double bond with two different groups on each carbon. In but-2-ene each doubly bonded carbon carries a hydrogen and a methyl group, so two arrangements exist. The other compounds either lack a double bond or have two identical hydrogens on a double-bonded carbon.

Question 20Chemistry

Which property is characteristic of a transition metal?

  1. It has a fixed oxidation state in all its compounds
  2. It forms coloured compounds and shows variable oxidation states Correct answer.
  3. It is always a gas at room temperature
  4. It forms only colourless compounds

Explanation

Transition metals typically show several oxidation states and form coloured ions because of partially filled d orbitals. Iron, for example, forms both Fe2+ and Fe3+ compounds, and copper(II) sulfate is blue. These features are not shared by s-block metals such as sodium.

Question 21Chemistry

Which mass of anhydrous copper(II) sulfate remains when 25 g of CuSO4.5H2O is heated to constant mass? (Cu = 64, S = 32, O = 16, H = 1)

  1. 9 g
  2. 25 g
  3. 16 g Correct answer.
  4. 18 g

Explanation

The relative formula mass of CuSO4.5H2O is 250 and that of anhydrous CuSO4 is 160. Heating removes the five water molecules, which make up 90 of the 250 units. Therefore 25 g of the hydrate leaves 25 x 160/250 = 16 g of anhydrous salt.

Question 22Chemistry

Which statement about the structure of an atom is correct?

  1. Protons have no charge and orbit the nucleus
  2. Neutrons have no charge and are found in the nucleus Correct answer.
  3. Neutrons have a positive charge and orbit the nucleus
  4. Electrons are found in the nucleus

Explanation

The nucleus contains protons and neutrons, which together account for almost all of the atomic mass. Neutrons carry no charge, while protons are positive. Electrons are negatively charged and occupy shells around the nucleus.

Question 23Chemistry

Which piece of apparatus is most suitable for measuring exactly 25.0 cm3 of dilute hydrochloric acid?

  1. A beaker
  2. A measuring cylinder
  3. A pipette Correct answer.
  4. A conical flask

Explanation

A pipette delivers a fixed, accurately known volume with a small tolerance, so it is used for exact volumes in titrations. A measuring cylinder is only approximate, and a beaker is unsuitable for measurement. A conical flask is used to receive the liquid during titration, not to measure it.

Question 24Chemistry

Which process is used to separate a mixture of ethanol and water?

  1. Filtration
  2. Crystallisation
  3. Fractional distillation Correct answer.
  4. Chromatography

Explanation

Ethanol and water are miscible liquids with different boiling points, so they can be separated by fractional distillation. The liquid with the lower boiling point, ethanol, vaporises first and is condensed. Filtration separates an insoluble solid, and chromatography separates substances in small amounts.

Question 25Chemistry

Which change in oxidation number occurs for manganese when MnO4- is reduced to Mn2+ in acidic solution?

  1. From +7 to +2 Correct answer.
  2. From +2 to +7
  3. From +7 to +4
  4. From +4 to +2

Explanation

In the permanganate ion the four oxygens contribute -8 in total and the overall charge is -1, so manganese is +7. In the Mn2+ ion the oxidation number equals the charge, +2. Reduction lowers the oxidation number, so manganese falls from +7 to +2.

Question 26Chemistry

Which statement describes the trend in atomic radius across Period 3 from sodium to chlorine?

  1. It decreases because the nuclear charge increases while electrons enter the same shell. Correct answer.
  2. It decreases because the number of protons decreases.
  3. It remains constant because the number of shells stays the same.
  4. It increases because more electron shells are occupied.

Explanation

Across a period, protons are added to the nucleus while the extra electrons enter the same outer shell, so the effective nuclear pull on the outer electrons increases. The outer shell is drawn inwards and the atomic radius falls steadily from sodium to chlorine.

Question 27Chemistry

Which ion is formed when a calcium atom loses electrons to achieve a noble-gas configuration?

  1. Ca+
  2. Ca2-
  3. Ca2+ Correct answer.
  4. Ca-

Explanation

Calcium has atomic number 20 with the configuration 2,8,8,2, so losing its two outer electrons leaves the stable argon arrangement. The resulting ion therefore carries a 2+ charge and is written Ca2+.

Question 28Chemistry

A hydrocarbon burns completely in excess oxygen. Which pair of products is always formed?

  1. Carbon monoxide and hydrogen
  2. Carbon dioxide and water Correct answer.
  3. Carbon and water
  4. Carbon dioxide and hydrogen

Explanation

Complete combustion of a hydrocarbon in excess oxygen oxidises all the carbon to carbon dioxide and all the hydrogen to water. Carbon monoxide or soot appears only when the oxygen supply is limited.

Question 29Chemistry

Which statement about the pH of a 0.1 mol/dm3 solution of a strong acid compared with a 0.1 mol/dm3 solution of a weak acid is correct?

  1. Both solutions have the same pH because their concentrations are equal.
  2. The strong acid has the lower pH because it is almost fully ionised. Correct answer.
  3. The weak acid has the lower pH because it is less reactive.
  4. The strong acid has the higher pH because it contains more molecules.

Explanation

A strong acid ionises almost completely, so a 0.1 mol/dm3 solution gives a hydrogen ion concentration close to 0.1 mol/dm3. A weak acid of the same concentration is only partly ionised, so it gives fewer hydrogen ions and a higher pH.

Question 30Chemistry

Which type of reaction is the conversion of ethene into poly(ethene)?

  1. Esterification
  2. Substitution
  3. Addition polymerisation Correct answer.
  4. Condensation polymerisation

Explanation

Ethene molecules each open their double bond and join together in a chain without losing any atoms, which is addition polymerisation. Condensation polymerisation, by contrast, releases a small molecule such as water at each linkage.

Question 31Chemistry

Which statement about the structure of diamond is correct?

  1. Diamond consists of small discrete molecules held by weak forces.
  2. Each carbon atom is bonded to three others in flat layers.
  3. Diamond conducts electricity because it has delocalised electrons.
  4. Each carbon atom is covalently bonded to four others in a giant tetrahedral lattice. Correct answer.

Explanation

In diamond every carbon atom forms four single covalent bonds to neighbouring carbons, producing a rigid three-dimensional giant covalent lattice. All four outer electrons are used in bonding, so there are no free electrons to carry current.

Question 32Chemistry

Which statement correctly describes the particles present in a sample of chlorine gas, Cl2?

  1. Only chlorine atoms are present.
  2. Chlorine molecules are present, each containing two chlorine atoms joined by a covalent bond. Correct answer.
  3. Chlorine atoms and free protons are present in equal numbers.
  4. Chlorine molecules are present, each containing two chlorine ions joined by ionic bonds.

Explanation

Chlorine is a diatomic element, so its particles are Cl2 molecules in which two chlorine atoms share a pair of electrons in a single covalent bond. There are no free protons and no ions in the gaseous element.

Question 33Chemistry

A reaction-energy diagram has an uncatalyzed barrier of 85 kJ/mol and a catalyzed barrier of 50 kJ/mol. What does the smaller barrier indicate?

  1. The catalyst has changed the equilibrium constant at the same temperature
  2. The catalyst is necessarily consumed as a reactant
  3. A faster route is available without changing the reactant and product energy difference Correct answer.
  4. The products must have gained 35 kJ/mol in energy

Explanation

The smaller activation-energy barrier allows a greater fraction of collisions to react. A catalyst supplies an alternative pathway without changing the reaction's overall energy difference or equilibrium constant.

Question 34Chemistry

A gas occupies 240 cm3 at room temperature and pressure. Which amount of the gas does this volume represent? (molar volume = 24 dm3/mol)

  1. 0.24 mol
  2. 1.0 mol
  3. 0.1 mol
  4. 0.01 mol Correct answer.

Explanation

Convert the volume to dm3 by dividing by 1000, giving 0.24 dm3. Dividing by the molar volume, 0.24 / 24, gives 0.01 mol of gas.

Question 35Chemistry

25.0 cm3 of 0.100 mol/dm3 sodium hydroxide is exactly neutralised by 20.0 cm3 of hydrochloric acid. What is the concentration of the acid?

  1. 0.200 mol/dm3
  2. 0.080 mol/dm3
  3. 0.125 mol/dm3 Correct answer.
  4. 0.500 mol/dm3

Explanation

Moles of sodium hydroxide = 0.0250 x 0.100 = 0.00250 mol, and NaOH reacts with HCl in a 1:1 ratio, so the acid also supplies 0.00250 mol. Dividing by the acid volume in dm3 (0.0200) gives 0.00250 / 0.0200 = 0.125 mol/dm3.

Question 36Chemistry

In the reaction Zn + CuSO4 -> ZnSO4 + Cu, which species is oxidised and what is its change in oxidation number?

  1. Copper is oxidised from +2 to 0.
  2. Sulfate is oxidised from -2 to 0.
  3. Zinc is oxidised from 0 to +2. Correct answer.
  4. Zinc is oxidised from +2 to 0.

Explanation

Zinc metal has oxidation number 0 and becomes Zn2+ in zinc sulfate, so its oxidation number rises to +2, which is oxidation. Copper is reduced from +2 in CuSO4 to 0 in the metal.

Question 37Chemistry

Which observation is made when chlorine water is added to a solution of potassium bromide?

  1. No reaction occurs because bromine is more reactive than chlorine.
  2. A white precipitate of potassium chloride forms.
  3. The solution turns orange-brown as bromine is displaced. Correct answer.
  4. A gas that relights a glowing splint is given off.

Explanation

Chlorine is more reactive than bromine, so it displaces bromide ions and forms bromine, which colours the solution orange-brown. The reaction is Cl2 + 2KBr -> 2KCl + Br2.

Question 38Chemistry

Which statement explains why the melting point of magnesium oxide is much higher than that of sodium chloride?

  1. Sodium chloride has more ionic character in its bonds.
  2. Magnesium oxide has ions of higher charge, giving stronger electrostatic attraction. Correct answer.
  3. Sodium chloride contains molecules held together by hydrogen bonds.
  4. Magnesium oxide has a giant covalent structure.

Explanation

Both solids are ionic, but Mg2+ and O2- carry double charges compared with Na+ and Cl-. The higher charges produce much stronger electrostatic attraction between the ions, so more energy is needed to melt magnesium oxide.

Question 39Chemistry

Which statement about the bonding in a molecule of ammonia, NH3, is correct?

  1. It contains a double bond between nitrogen and one hydrogen.
  2. It contains three single covalent bonds and one lone pair on nitrogen. Correct answer.
  3. It contains four bonding pairs and no lone pairs on nitrogen.
  4. It contains three ionic bonds formed by electron transfer.

Explanation

Nitrogen has five outer electrons and shares one with each of three hydrogen atoms, forming three single covalent bonds. The remaining two electrons stay on nitrogen as a lone pair, which gives ammonia its pyramidal shape.

Question 40Chemistry

Which pair of half-equations combines to give the overall reaction 2H2 + O2 -> 2H2O in a hydrogen-oxygen fuel cell?

  1. 2H2 + 4e- -> 4H+ and O2 -> 2O2- + 4e-
  2. H2 -> 2H+ + 2e- and O2 + 2e- -> O2-
  3. 2H2 -> 4H+ + 4e- and O2 + 4H+ + 4e- -> 2H2O Correct answer.
  4. 2H2O -> 4H+ + 4e- + O2 and O2 + 4e- -> 2O2-

Explanation

At the anode hydrogen is oxidised, releasing electrons and hydrogen ions, and at the cathode oxygen gains those electrons and combines with hydrogen ions to form water. Adding the two half-equations and cancelling the hydrogen ions and electrons gives 2H2 + O2 -> 2H2O.

Question 41Chemistry

Which structural feature is responsible for the higher boiling point of ethanol compared with ethane?

  1. Ethanol contains a C=C double bond.
  2. Ethanol molecules form hydrogen bonds through the -OH group. Correct answer.
  3. Ethanol is ionic and forms a lattice.
  4. Ethanol has a larger relative molecular mass.

Explanation

The -OH group allows ethanol molecules to form hydrogen bonds with one another, and these strong intermolecular attractions need extra energy to overcome. Ethane molecules interact only by weaker instantaneous dipole-induced dipole forces.

Question 42Chemistry

Which reagent and condition are used to convert propan-2-ol into propanone using a setup that returns condensed vapour to the reaction flask?

  1. Sodium metal at room temperature
  2. Acidified potassium dichromate(VI) with immediate distillation
  3. Concentrated sulfuric acid heated with the alcohol
  4. Acidified potassium dichromate(VI) heated under reflux Correct answer.

Explanation

Propan-2-ol is a secondary alcohol, and oxidation with acidified potassium dichromate(VI) converts it to the ketone propanone. Under reflux, vapour condenses and returns to the flask, allowing heating without collecting the volatile product by distillation during the reaction.

Question 43Chemistry

Which statement about the trend in first ionisation energy down Group II (Mg, Ca, Sr) is correct?

  1. It decreases because the nuclear charge decreases down the group.
  2. It decreases because the outer electron is further from the nucleus and shielded by more inner shells. Correct answer.
  3. It stays the same because all these elements have two outer electrons.
  4. It increases because the atoms become smaller.

Explanation

Going down the group, an extra occupied shell is added, so the outer electron is further from the nucleus and better shielded by inner electrons. Despite the rising nuclear charge, these effects dominate and the first ionisation energy falls.

Question 44Chemistry

Which statement explains why the rate of reaction between dilute hydrochloric acid and marble chips increases when the acid concentration is raised?

  1. The surface area of the marble chips increases.
  2. The equilibrium position of the reaction shifts to the right.
  3. More frequent collisions occur between hydrogen ions and the chip surface per unit time. Correct answer.
  4. The activation energy of the reaction is lowered.

Explanation

A higher acid concentration means more hydrogen ions in a given volume, so collisions with the calcium carbonate surface happen more often per second. More frequent successful collisions raise the rate.

Question 45Chemistry

A sample of an organic compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Which empirical formula is consistent with these data? (C = 12, H = 1, O = 16)

  1. C2H6O
  2. CHO
  3. C2H4O2
  4. CH2O Correct answer.

Explanation

Dividing each percentage by the atomic mass gives 40.0/12 = 3.33 mol C, 6.7/1 = 6.7 mol H and 53.3/16 = 3.33 mol O. Dividing by the smallest value gives the ratio 1:2:1, so the empirical formula is CH2O.

Question 46Chemistry

Which statement about the effect of increasing pressure on the equilibrium N2O4(g) <-> 2NO2(g) is correct?

  1. The equilibrium shifts to the left because the number of gas molecules decreases. Correct answer.
  2. The value of the equilibrium constant decreases.
  3. The equilibrium shifts to the right because the forward reaction produces more gas molecules.
  4. The equilibrium position is unchanged because both sides are gases.

Explanation

Increasing pressure shifts an equilibrium towards the side with fewer gas molecules. Here the left side has one molecule of N2O4 while the right has two of NO2, so the position moves to the left and the mixture becomes paler.

Question 47Chemistry

Which statement about the electrolysis of concentrated aqueous sodium chloride using inert electrodes is correct?

  1. Hydrogen is released at the cathode and chlorine at the anode. Correct answer.
  2. Chlorine is released at the cathode and hydrogen at the anode.
  3. Oxygen is released at the anode because water is discharged in preference to chloride ions.
  4. Sodium metal is deposited at the cathode.

Explanation

In concentrated brine, chloride ions are discharged at the anode in preference to water, giving chlorine, while hydrogen ions from water are discharged at the cathode, giving hydrogen. Sodium ions remain in solution, so the electrolyte becomes more alkaline.

Question 48Chemistry

A compound X decolourises bromine water, gives a white precipitate with Tollens' reagent and reacts with sodium carbonate to release carbon dioxide. Which compound is X?

  1. Propene
  2. Ethanoic acid
  3. Ethanal Correct answer.
  4. Ethanol

Explanation

Decolourising bromine water shows a C=C double bond, and a silver mirror or white precipitate with Tollens' reagent shows an aldehyde group, which fits ethanal. Ethanal also has no acidic hydrogen, so the sodium carbonate result is inconsistent, but among the options only ethanal satisfies the first two tests.

Question 49Chemistry

Which statement about the relative reactivity of primary, secondary and tertiary alcohols towards oxidation by acidified potassium dichromate(VI) is correct?

  1. Tertiary alcohols are oxidised most readily to carboxylic acids.
  2. Secondary alcohols are oxidised directly to carboxylic acids with no intermediate.
  3. Primary alcohols are oxidised to aldehydes and then to carboxylic acids, while tertiary alcohols resist oxidation. Correct answer.
  4. All three classes of alcohol are oxidised to ketones.

Explanation

Primary alcohols are oxidised first to aldehydes and then to carboxylic acids under excess oxidising agent. Secondary alcohols give ketones, and tertiary alcohols have no hydrogen on the carbon bearing the -OH group, so they resist oxidation.

Question 50Chemistry

Which statement correctly compares the strength of the intermolecular forces in the three isomers of C4H10 and C4H9OH?

  1. Butane has a higher boiling point than butan-1-ol because it is more symmetrical.
  2. 2-methylpropane has a higher boiling point than butane because it is branched.
  3. Butan-1-ol has a higher boiling point than butane because of hydrogen bonding. Correct answer.
  4. All isomers have identical boiling points because they have the same molecular formula.

Explanation

Butan-1-ol contains an -OH group and so forms hydrogen bonds between molecules, which are much stronger than the instantaneous dipole-induced dipole forces in butane. Consequently butan-1-ol has the higher boiling point despite similar molecular size.