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Question 1

(a) The circulatory system in horses is similar to in humans.
Fig. 1.1 is a simplified diagram of the circulatory system in horses.
The arrows represent the direction of blood flow.
(i) Identify from Fig. 1.1: blood vessel \(X\); organ \(Y\); blood vessel \(Z\).
(ii) Describe evidence from Fig. 1.1 that horses have a double circulatory system.
(b) The circulatory system transports blood around the body.
(i) State two blood groups in humans.
(ii) White blood cells are one component of blood.
Tick (✓) all the boxes that show the correct functions of white blood cells.
(c) Table 1.1 shows the average thickness of wall for an artery and a capillary.
(i) Calculate how many times thicker an artery wall is than a capillary wall.
(ii) Explain the reasons for this difference in thickness shown in Table 1.1.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic B9.1/B9.3 — Circulatory systems & blood vessels (Part (a))
• Topic B17.1 — Variation (ABO blood groups) (Part (b)(i))
• Topic B9.4 — Blood (Part (b)(ii))
• Topic B9.3 — Blood vessels (Part (c))

▶️ Answer/Explanation

(a)(i) X = pulmonary artery; Y = kidney; Z = vena cava

\(X\) carries blood from the heart to the lungs, so it is the pulmonary artery.
\(Y\) receives blood from the renal artery, so it must be the kidney.
\(Z\) returns blood from the head to the heart, so it is the vena cava.

(a)(ii) Two separate loops through the heart

The diagram shows blood passing through the heart twice in one full circuit.
One loop goes heart → lungs → heart (oxygenation).
The second loop goes heart → rest of body/head/kidney → heart (delivery to tissues), confirming a double circulation.

(b)(i) Any two of: A, B, AB, O

Human blood is classified into the ABO blood group system.
This is an example of discontinuous variation with no intermediate phenotypes.
Any two of the four groups (A, B, AB, O) are acceptable.

(b)(ii) 

White blood cells defend the body against pathogens.
Lymphocytes produce antibodies, while phagocytes engulf and digest pathogens.
Blood clotting, hormone transport, and oxygen transport are not functions of white blood cells (these belong to platelets, plasma, and red blood cells respectively).

(c)(i) 2000 times

\( \dfrac{1.0000}{0.0005} = 2000 \)
The artery wall is 2000 times thicker than the capillary wall.

(c)(ii) Different functions require different wall thickness

The artery wall is thick (and muscular/elastic) to withstand and maintain the high pressure of blood leaving the heart.
The capillary wall is only one cell thick to provide the shortest possible diffusion distance for the exchange of substances such as oxygen, carbon dioxide, and nutrients between blood and tissues.

Question 2

Carbon dioxide gas is made in many chemical reactions.
(a) Tick (✓) the two chemical reactions that make carbon dioxide.
(b) Scientists are concerned about increased concentrations of carbon dioxide in the atmosphere. Explain why.
(c) Oxides, such as carbon dioxide, can be classified as acidic, basic, or amphoteric.
Classify carbon dioxide by putting a ring around the correct type of oxide.
acidic
amphoteric
basic
Explain your answer.
(d) Carbon dioxide, \(CO_2\), is a simple molecule.
(i) State the type of chemical bonding in a molecule of carbon dioxide.
(ii) Draw a dot-and-cross diagram to show the bonding in carbon dioxide.
You only need to draw the outer-shell electrons.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic B12 — Respiration / Topic C9.4 — Extraction of metals (thermal decomposition) (Part (a))
• Topic C10.2 — Air quality and climate (Part (b))
• Topic C7.2 — Oxides (Part (c))
• Topic C2.5 — Simple molecules and covalent bonds (Part (d))

▶️ Answer/Explanation

(a) 

Respiration releases \(CO_2\) as a waste product: \(glucose + oxygen \rightarrow carbon\ dioxide + water\).
Thermal decomposition of calcium carbonate also releases \(CO_2\): \(CaCO_3 \rightarrow CaO + CO_2\).
A reaction between an acid and a metal produces hydrogen gas, not \(CO_2\), and an alkali metal with water also produces hydrogen gas.

(b) Carbon dioxide is a greenhouse gas

Carbon dioxide absorbs and re-emits thermal energy radiated from the Earth, contributing to the (enhanced) greenhouse effect.
Increasing concentrations of this and other greenhouse gases lead to global warming and climate change.

(c) Acidic

Carbon is a non-metal, and oxides of non-metals are classified as acidic oxides.
Carbon dioxide dissolves in water to form a weakly acidic solution (carbonic acid).

(d)(i) Covalent bonding

Carbon dioxide is a simple molecule formed when atoms share pairs of electrons.
Each oxygen atom shares two pairs of electrons with the carbon atom, forming two C=O double covalent bonds.

(d)(ii) 

The diagram should show \(O::C::O\) with two double bonds.
Each double bond consists of two shared pairs of electrons (one shown as dots, one as crosses) between carbon and each oxygen.
Each oxygen atom also has two non-bonding (lone) pairs of electrons shown only as dots or crosses belonging to that oxygen, giving carbon 8 outer electrons and each oxygen 8 outer electrons.

Question 3

Fig. 3.1 shows a wind turbine used to generate electricity.
(a) State one advantage of generating electricity using wind turbines.
(b) The wind turbine contains an alternating current (a.c.) generator.
On Fig. 3.2, sketch a graph of output voltage against time for the a.c. generator when the wind turbine is turning at a constant speed.
(c) A step-up transformer is used to increase the voltage from the generator.
Describe the construction of a basic step-up transformer.
You may include a labelled diagram to aid your description.
(d) The wind exerts a pressure of \(7200\,\text{Pa}\) on each blade of the wind turbine.
Each blade has a surface area of \(90\,\text{m}^2\).
Calculate the force exerted by the wind on each turbine blade.
(e) The wind turbines produce a low-pitch sound when they turn.
(i) State the minimum frequency of sound which can be heard by a healthy human ear.
(ii) Sound waves are longitudinal waves.
Describe, in terms of oscillations and energy transfer, what is meant by a longitudinal wave.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic P1.6.3 — Energy resources (Part (a))
• Topic P4.5.2 — The a.c. generator (Part (b))
• Topic P4.5.6 — The transformer (Part (c))
• Topic P1.7 — Pressure (Part (d))
• Topic P3.4 — Sound / Topic P3.1 — General properties of waves (Part (e))

▶️ Answer/Explanation

(a) Does not release \(CO_2\) / renewable / no fuel costs

Wind turbines generate electricity without burning fossil fuels.
This means no greenhouse gases are released and the energy source (wind) is renewable.

(b) Sinusoidal waveform

The trace should be a smooth sine wave, alternating above and below the time axis.
Both the amplitude and the time period of the wave must stay constant, since the turbine turns at constant speed.

(c) Soft-iron core with two coils

A basic transformer has a soft iron core linking two separate coils of wire.
The primary and secondary coils are both wound around the same core.
For a step-up transformer, the number of turns on the secondary coil is greater than the number of turns on the primary coil.

(d) \(648\,000\,\text{N}\) (≈ \(6.5\times10^{5}\,\text{N}\))

Force is calculated using \(F = P \times A\).
\(F = 7200 \times 90 = 648\,000\,\text{N}\), which rounds to \(650\,000\,\text{N}\) (2 s.f.).

(e)(i) \(20\,\text{Hz}\)

The typical range of human hearing is approximately \(20\,\text{Hz}\) to \(20\,000\,\text{Hz}\).
\(20\,\text{Hz}\) is therefore the accepted minimum audible frequency for a healthy human ear.

(e)(ii) Oscillations parallel to energy transfer

In a longitudinal wave, the particles oscillate back and forth in the same direction that the energy is being transferred.
This produces alternating regions of compression and rarefaction along the direction of travel.

Question 4

(a) State the balanced symbol equation for photosynthesis.
(b) Photosynthesis is an enzyme-controlled reaction.
Fig. 4.1 is a graph showing the effect of temperature on the rate of photosynthesis.
Complete the sentences to describe and explain the graph in Fig. 4.1.
As the temperature increases, the rate of photosynthesis increases until it reaches the optimum temperature of ____ °C.
The rate increases because the particles gain more ____ energy causing more ____ collisions.
At 45°C the enzymes are ____. Photosynthesis stops because the ____ of the enzyme has changed shape so the substrate can no longer fit.
(c) Complete the sentence to describe the transfer of energy during photosynthesis.
During photosynthesis chlorophyll transfers ____ energy into ____ energy.
(d) Carbohydrates are produced during photosynthesis.
State the name of the carbohydrate in plants that is used for storage and for transport.
(e) Plant cells are adapted for their function.
State the name of the cells
  • in the leaf that are adapted for efficient photosynthesis
  • that are adapted for absorption of water from the soil.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic B6.1 — Photosynthesis (Parts (a) and (c))
• Topic B5.1 — Enzymes (Part (b))
• Topic B4.1 — Biological molecules / Topic B8 — Transport in plants (Part (d))
• Topic B2.1 — Cell structure (Part (e))

▶️ Answer/Explanation

(a) \(6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2\)

Carbon dioxide and water are the reactants, combined using light energy absorbed by chlorophyll.
Glucose and oxygen are the products, and the equation must be balanced on both sides.

(b) 35; kinetic; successful/frequent; denatured; active site

The rate of photosynthesis is highest at the optimum temperature of \(35\,^{\circ}C\), as read from the peak of the graph.
Increasing temperature gives particles more kinetic energy, causing more frequent/successful collisions between enzyme and substrate.
At \(45\,^{\circ}C\) the enzymes are denatured, because the shape of the active site has changed permanently so the substrate can no longer bind.

(c) Light energy into chemical energy

Chlorophyll absorbs light energy from sunlight.
This light energy is converted into chemical energy, stored in the bonds of glucose molecules.

(d) Storage = starch; Transport = sucrose

Glucose made in photosynthesis is converted to starch for storage, since starch is insoluble and does not affect water potential.
Glucose is converted to sucrose for transport around the plant in the phloem, as sucrose is more soluble and stable for translocation.

(e) Palisade mesophyll cells; root hair cells

Palisade mesophyll cells are packed with chloroplasts and are positioned near the top of the leaf for efficient photosynthesis.
Root hair cells have a large surface area, adapting them for efficient absorption of water (and mineral ions) from the soil.

Question 5

Fig. 5.1 shows the structures of four carbon compounds.
(a)(i) State which compound is ethene.
Choose from A, B, C or D.
(ii) State which compound is an unsaturated hydrocarbon.
Choose from A, B, C or D.
(iii) Compound B is made from compound D.
State what type of reaction takes place.
(iv) Describe how compound C is made from compound D.
(v) Compound A is a member of a homologous series.
State two similarities between the members of a homologous series.
(b) Fig. 5.2 shows the structure of a carbon atom.
(i) Carbon is in Group IV of the Periodic Table.
Describe how to deduce from Fig. 5.2 that carbon is in Group IV.
(ii) Carbon exists as a mixture of isotopes.
Complete Fig. 5.3 to show a different isotope of carbon from that shown in Fig. 5.2.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic C11.5 — Alkenes (Parts (a)(i)–(iii))
• Topic C11.6 — Alcohols (Part (a)(iv))
• Topic C11.1 — Formulas and terminology (Part (a)(v))
• Topic C2.2 — Atomic structure and the Periodic Table (Part (b)(i))
• Topic C2.3 — Isotopes (Part (b)(ii))

▶️ Answer/Explanation

(a)(i) D

Compound D has the structure \(H_2C=CH_2\), which is ethene.

(a)(ii) D

Compound D contains a C=C double bond, making it an unsaturated hydrocarbon.
Compounds A and B contain only single C–C bonds (saturated), and compound C contains an oxygen atom so is not a hydrocarbon.

(a)(iii) Addition polymerisation

Many ethene (D) monomer molecules join together by opening their double bonds.
This forms the long-chain polymer poly(ethene), shown as compound B.

(a)(iv) Addition of steam using a catalyst

Ethene (D) reacts with steam (\(H_2O\)) in the presence of a catalyst (e.g. phosphoric acid).
This addition reaction converts the C=C double bond into a single bond and adds an –OH group, forming ethanol (C).

(a)(v) Same general formula; similar chemical properties

Members of a homologous series all fit the same general formula.
They show a gradual change in physical properties (e.g. boiling point) but similar chemical properties, differing by a \(CH_2\) unit between consecutive members.

(b)(i) 4 electrons in the outer shell

Fig. 5.2 shows carbon has 4 electrons in its outer shell.
The Group number of an element corresponds to the number of electrons in its outer shell, so carbon is in Group IV.

(b)(ii) Same protons, different neutrons

The diagram should keep 6 protons (same as Fig. 5.2, since isotopes are atoms of the same element).
The electron arrangement of 2,4 must also be shown to be correct.
The number of neutrons drawn must be different from 6 (e.g. 7 or 8), since isotopes differ only in their number of neutrons.

Question 6

Angler fish live in the sea at depths of up to \(2000\,\text{m}\).
Fig. 6.1 shows an angler fish.
(a) The angler fish has a light-emitting lure to attract smaller fish.
(i) The frequency of light emitted by the lure is \(5.0 \times 10^{14}\,\text{Hz}\) and the wavelength of the light is \(4.5 \times 10^{-7}\,\text{m}\).
Calculate the speed of light in water.
(ii) Use your answer to (a)(i) to calculate the refractive index of water.
(b) An angler fish of mass \(28\,\text{kg}\) moves at \(0.11\,\text{m/s}\).
(i) Calculate the kinetic energy of the angler fish.
(ii) Fig. 6.2 shows the forces acting on the angler fish when it moves at a constant speed of \(0.11\,\text{m/s}\).
Use Fig. 6.2 to complete the sentences to explain why the angler fish is moving at a constant speed.
Force A is called the ____.
Force A has the same magnitude as the ____, and force B has the same magnitude as the ____.
There is no resultant force acting on the angler fish. Therefore, there is no ____, so the angler fish moves at constant speed.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic P3.2.2 — Refraction of light (Part (a))
• Topic P1.6.1 — Energy (Part (b)(i))
• Topic P1.5.1 — Effects of forces (Part (b)(ii))

▶️ Answer/Explanation

(a)(i) \(2.3 \times 10^{8}\,\text{m/s}\)

Wave speed is calculated using \(v = f\lambda\).
\(v = (5.0\times10^{14}) \times (4.5\times10^{-7}) = 2.25\times10^{8} \approx 2.3\times10^{8}\,\text{m/s}\).

(a)(ii) \(1.3\)

Refractive index is calculated using \(n = \dfrac{c}{v}\), where \(c = 3.0\times10^{8}\,\text{m/s}\) is the speed of light in a vacuum/air.
\(n = \dfrac{3.0\times10^{8}}{2.3\times10^{8}} \approx 1.3\).

(b)(i) \(0.17\,\text{J}\)

Kinetic energy is calculated using \(KE = \tfrac{1}{2}mv^2\).
\(KE = 0.5 \times 28 \times 0.11^2 = 0.5 \times 28 \times 0.0121 \approx 0.17\,\text{J}\).

(b)(ii) Weight; upthrust and drag; acceleration

Force A acts downwards on the fish, so it is the weight of the fish.
Since the fish moves at constant velocity, the resultant force is zero: force A (weight) balances the upthrust, and force B balances the drag.
With no resultant force, there is no acceleration, so the fish continues to move at a constant speed.

Question 7

(a) Fig. 7.1 summarises mitosis and meiosis in an organism.
(i) The parental cells in Fig. 7.1 have been drawn showing just four chromosomes. The actual number of chromosomes in each parental cell is 24.
Identify the actual number of chromosomes in cell A and cell B.
(ii) Identify the name of the type of cells represented by cell B in Fig. 7.1.
(iii) Describe the process occurring at stage 1 in Fig. 7.1.
(iv) State the name of the type of nucleus in the parental cell in Fig. 7.1.
(v) Describe the arrangement of chromosomes in the parental cell in Fig. 7.1.
(b) State two uses of mitosis.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic B16.2 — Cell division (all parts)

▶️ Answer/Explanation

(a)(i) Cell A = 24; Cell B = 12

Mitosis produces daughter cells with the same chromosome number as the parent cell, so cell A has 24 chromosomes.
Meiosis halves the chromosome number, so cell B (a gamete) has 12 chromosomes.

(a)(ii) Gametes (sex cells / haploid cells)

Cell B is produced by meiosis and contains half the chromosome number, identifying it as a gamete.

(a)(iii) Duplication/replication of chromosomes

Before division, each chromosome is copied exactly, producing two identical chromatids joined together.

(a)(iv) Diploid

The parental cell contains chromosomes in pairs (the full chromosome number), making the nucleus diploid.

(a)(v) Paired (arranged in homologous pairs)

In a diploid nucleus, chromosomes are arranged in pairs, with one chromosome of each pair inherited from each parent.

(b) Growth and repair of damaged tissues (any two of: growth, repair, replacement of cells, asexual reproduction)

Mitosis produces genetically identical cells, allowing an organism to grow by increasing cell number.
It is also used to repair damaged tissues and replace worn-out cells.

Question 8

A student investigates the reaction between calcium carbonate and dilute hydrochloric acid.
The student does the experiment using three different sets of conditions, A, B and C.
All other variables are kept the same.
Fig. 8.1 shows the three sets of conditions.
(a) State which set of conditions, A, B or C, will give the fastest rate of reaction.
(b) The equation for the reaction in the student’s experiment is shown.
\(CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\)
Calculate the mass of \(1.2\,\text{dm}^3\) of carbon dioxide gas measured at room temperature and pressure (r.t.p.).
The volume of one mole of any gas is \(24\,\text{dm}^3\) at r.t.p. Show your working.
[\(A_r\): C, 12; O, 16]
(c) The reaction between calcium carbonate and dilute hydrochloric acid is exothermic.
State what is meant by an exothermic reaction.
(d) A food company is making new self-heating cans of soup.
They investigate three different reactions.
Fig. 8.2 shows the reaction profiles for the three reactions, X, Y and Z.
The reaction profiles show:
  • the energy change in the reaction
  • the activation energy of the reaction.
(i) State what is meant by activation energy.
(ii) Suggest which reaction, X, Y or Z, would be most suitable to use in the self-heating cans. Explain your answer.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic C6.2 — Rate of reaction (Part (a))
• Topic C3.1 — Formulas (Part (b))
• Topic C5.1 — Exothermic and endothermic reactions (Parts (c) and (d))

▶️ Answer/Explanation

(a) C

Condition C uses powdered calcium carbonate, which has the largest surface area of the three options.
A larger surface area allows more frequent collisions with acid particles, giving the fastest rate of reaction (both A and C are at the same temperature and acid concentration, isolating surface area as the variable).

(b) \(2.2\,\text{g}\)

The relative molecular mass of \(CO_2 = 12 + (16\times2) = 44\).
Moles of \(CO_2 = \dfrac{1.2}{24} = 0.05\,\text{mol}\).
Mass of \(CO_2 = 0.05 \times 44 = 2.2\,\text{g}\).

(c) A reaction that gives out heat/energy

In an exothermic reaction, energy is transferred from the reacting chemicals to the surroundings, usually shown by a temperature rise.

(d)(i) Minimum energy needed for a reaction to occur

Activation energy is the minimum amount of energy that colliding particles must have for a reaction to take place.
It corresponds to the energy difference between the reactants and the peak of the reaction profile curve.

(d)(ii) Reaction X

Reaction X is exothermic, releasing heat as the products have less energy than the reactants — essential for a self-heating can.
Reaction Z is endothermic (it absorbs heat rather than releasing it), so it would not heat the soup.
Reaction X has a lower activation energy than reaction Y, meaning it releases its heat more readily/quickly, which is more suitable for a “hot in 3 minutes” product.

Question 9

A student investigates the use of cotton wool to insulate a beaker of hot water at \(90\,^{\circ}C\).
(a) The student uses a digital thermometer to measure the temperature of the water in the beaker as it cools.
The student repeats the experiment using different thicknesses of cotton wool.
Fig. 9.1 shows a graph of the results.
(i) Predict the temperature after 5.0 minutes of a beaker of water which is insulated with 2.0 cm of insulation.
Use the results shown in Fig. 9.1 to explain your answer.
(ii) Complete the sentences about the digital thermometer.
The digital thermometer contains two wires made of different metals.
The wires are joined together at each end to form two junctions.
This arrangement is known as a ____.
(b) The student uses an electric kettle to heat the water for the investigation.
The electric kettle has a power rating of \(1800\,\text{W}\) when a potential difference of \(240\,\text{V}\) is applied.
Calculate the resistance of the kettle.
(c) Plastic is an electrical insulator.
Fig. 9.2 shows two lightweight, plastic sheets suspended by insulating threads.
Each plastic sheet is positively charged.
(i) Explain what is observed when the two plastic sheets are moved close to each other.
(ii) Describe how a plastic sheet can become positively charged.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic P2.3.1 — Conduction (Part (a)(i))
• General physics knowledge — temperature sensors (Part (a)(ii))
• Topic P4.2.4 — Resistance (Part (b))
• Topic P4.2.1 — Electrical charge (Part (c))

▶️ Answer/Explanation

(a)(i) Any value from 36°C to 50°C

A 2.0 cm thickness lies between the 1.0 cm and 3.0 cm curves shown on the graph.
Since 2.0 cm reduces the rate of heat loss (conduction to the surroundings) more than 1.0 cm of insulation but less than 3.0 cm, its final temperature should lie between the two curves at 5.0 minutes.

(a)(ii) Thermocouple

Two different metal wires joined at two junctions form a thermocouple, which generates a small voltage dependent on the temperature difference between the junctions, allowing temperature to be measured electronically.

(b) \(32\,\Omega\)

Current is found using \(I = \dfrac{P}{V} = \dfrac{1800}{240} = 7.5\,\text{A}\).
Resistance is then found using \(R = \dfrac{V}{I} = \dfrac{240}{7.5} = 32\,\Omega\).

(c)(i) They move apart

Both plastic sheets carry the same (positive) charge.
Like charges repel each other, so the sheets will move apart/swing away from each other.

(c)(ii) Charging by friction

Rubbing the plastic sheet against another surface causes friction between the two materials.
This transfers electrons from the plastic sheet to the other surface, leaving the plastic sheet with an overall positive charge (a deficit of electrons).

Question 10

(a) Fig. 10.1 shows a diagram of human skin.
Identify the parts labelled A, B and C in Fig. 10.1.
(b) Describe the role of the blood vessels in maintaining a constant internal body temperature when external temperature increases.
(c) State the name of the type of feedback used to maintain a constant internal body temperature.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic B13.3 — Homeostasis (all parts)

▶️ Answer/Explanation

(a) A = (hair) erector muscle; B = sweat gland; C = fatty (adipose) tissue

A points to the small muscle attached to the hair follicle, which is the erector muscle.
B points to the coiled structure that produces sweat, the sweat gland.
C points to the layer beneath the dermis, made of fatty/adipose tissue, which provides insulation.

(b) Vasodilation of blood vessels

When external temperature increases, arterioles near the skin surface widen (vasodilation).
This allows more blood to flow through the capillaries near the surface of the skin.
More heat is therefore lost from the blood (by radiation/conduction) at the skin surface, helping to cool the body back towards normal temperature.

(c) Negative feedback

The body detects a change in temperature and triggers a response that counteracts (reverses) that change, which is the definition of negative feedback.

Question 11

(a) Complete the sentence about electrolysis.
Electrolysis is the breakdown of an ____ compound when ____ or in aqueous solution by the passage of electricity.
(b) The products of the electrolysis of any binary salt can be predicted.
The binary salt will always break down into its elements.
Complete the sentence about the electrolysis of a binary salt.
The ____ is formed at the cathode and the ____ is formed at the anode.
(c) Chlorine gas is made at the anode during the electrolysis of concentrated aqueous sodium chloride.
\(2Cl^{-} \rightarrow Cl_2 + 2e^{-}\)
This is an example of oxidation.
(i) Explain why.
(ii) Construct the ionic half-equation for the formation of the product at the cathode.
(d) Sodium chloride has a melting point of \(801\,^{\circ}C\).
Chlorine has a melting point of \(-102\,^{\circ}C\).
Explain the difference in the melting points.
Use ideas about
  • the bonding in sodium chloride and in chlorine
  • attractive forces.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic C4.1 — Electrolysis (Parts (a), (b), (c))
• Topic C2.4 — Ions and ionic bonds / Topic C2.5 — Simple molecules and covalent bonds (Part (d))

▶️ Answer/Explanation

(a) Ionic compound; molten

Electrolysis only works on ionic compounds, because the ions must be free to move and carry charge.
This requires the compound to be either molten or dissolved in water (aqueous).

(b) Metal at the cathode; non-metal at the anode

Positively charged metal ions are attracted to and discharged at the negative cathode.
Negatively charged non-metal ions are attracted to and discharged at the positive anode.

(c)(i) Loss of electrons

Oxidation is defined as the loss of electrons.
In the equation, chloride ions (\(Cl^-\)) lose electrons to form chlorine gas, so this is oxidation.

(c)(ii) \(2H^{+} + 2e^{-} \rightarrow H_2\)

Hydrogen gas is produced at the cathode during electrolysis of concentrated sodium chloride solution.
Positive hydrogen ions gain electrons (reduction) at the cathode, and the equation must be balanced for charge and atoms.

(d) Ionic vs covalent bonding

Sodium chloride is an ionic compound, held together by strong electrostatic forces of attraction between oppositely charged ions, which require a large amount of energy to overcome.
Chlorine is a simple covalent (molecular) substance, where only weak intermolecular forces exist between separate \(Cl_2\) molecules.
Since the weak intermolecular forces in chlorine need much less energy to break than the strong ionic bonds in sodium chloride, chlorine has a much lower melting point.

Question 12

Polonium is a highly radioactive metal with no stable isotopes.
(a) Polonium-210 (\(^{210}_{84}Po\)) decays to form lead-206 (\(^{206}_{82}Pb\)).
The decay of polonium-210 is a one-step process.
(i) State the type of ionising radiation emitted when polonium-210 decays to lead-206.
(ii) The half-life of polonium-210 is 140 days.
The activity of a sample of polonium-210 is measured as 680 counts per minute.
Calculate the time taken, in days, for the activity to decrease to 85 counts per minute.
(b) Polonium is a solid at room temperature. The melting point of polonium is \(254\,^{\circ}C\).
(i) Explain, in terms of the motion and arrangement of atoms, why a fixed mass of solid polonium will occupy a smaller volume than the same mass of liquid polonium.
(ii) The density of solid polonium is \(9.4\,\text{g/cm}^3\).
Calculate the volume occupied by \(235\,\text{g}\) of solid polonium.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic P5.2.1 — Detection of radioactivity (Part (a))
• Topic P2.1.2 — Particle model (Part (b)(i))
• Topic P1.4 — Density (Part (b)(ii))

▶️ Answer/Explanation

(a)(i) Alpha particle

The mass number decreases by 4 (\(210 \rightarrow 206\)) and the atomic/proton number decreases by 2 (\(84 \rightarrow 82\)).
This change is characteristic of the emission of an alpha particle (\(^4_2He\)).

(a)(ii) 420 days

The activity falls from 680 to 85 counts per minute: \(\dfrac{680}{85} = 8 = 2^3\), so 3 half-lives have passed.
Time taken \(= 3 \times 140 = 420\) days.

(b)(i) Atoms vibrate in a fixed, regular arrangement

Both samples contain the same number of atoms, since they have the same mass.
In the solid, atoms vibrate about fixed positions in a regular, closely-packed arrangement.
In the liquid, atoms are free to move and are arranged randomly/irregularly, with more space between them, so the liquid takes up a larger volume for the same mass.

(b)(ii) \(25\,\text{cm}^3\)

Volume is calculated using \(V = \dfrac{m}{\rho}\).
\(V = \dfrac{235}{9.4} = 25\,\text{cm}^3\).

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