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Question 1

(a) Fig. 1.1 is a diagram of the male reproductive system in humans.
(i) State the letter from Fig. 1.1 that identifies the part that:
carries urine and semen ………………………………
produces sperm ………………………………
transfers sperm to the urethra. ………………………………
(ii) Draw an X on Fig. 1.1 to identify the position of the prostate gland.
(b) Sperm cells are the male gametes in humans.
Sperm cells are 0.05 mm in length.
Female gametes are 0.1 mm in length.
(i) State the name of the female gametes in humans.
(ii) Calculate how many times longer female gametes are than male gametes.
(c) Sperm cells have a high rate of respiration.
(i) State the word equation for aerobic respiration.
(ii) Complete the sentence about respiration.
Respiration releases the ……………………………… needed for cell division and movement.
(d) Sperm cells are involved in fertilisation. Describe the process of fertilisation in humans. Include the site of fertilisation in your answer.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B15.4 — Sexual reproduction in humans (Parts (a), (b), (d))
• Topic B12.1 — Respiration (Part (c))

▶️ Answer/Explanation

(a)(i) carries urine and semen = B; produces sperm = D; transfers sperm to urethra = A

The urethra (B) is the shared tube for urine and semen.
The testis (D) is the site of sperm production.
The sperm duct (A) carries sperm from the testis to the urethra.

(a)(ii) X drawn on the prostate gland (the structure labelled E, below the bladder)

The prostate gland surrounds the urethra just below the bladder.
It adds fluid to semen to nourish and help transport sperm.

(b)(i) egg cell / ovum

The female gamete in humans is called the egg cell, ovum, or oocyte.

(b)(ii) 2 times

Using \( \dfrac{0.1}{0.05} = 2 \).
So the female gamete is twice as long as the male gamete.

(c)(i) glucose + oxygen → carbon dioxide + water

Aerobic respiration requires oxygen to fully break down glucose.
The products are carbon dioxide and water, with energy released.

(c)(ii) energy

Respiration releases the energy needed for cell division (mitosis/meiosis) and for the movement of the flagellum/tail of the sperm.

(d) Fusion of the nucleus of the sperm with the nucleus of the egg cell; this occurs in the oviduct

Fertilisation is the fusion of the male gamete (sperm) nucleus with the female gamete (egg) nucleus.
This forms a single diploid nucleus in the resulting zygote.
In humans, fertilisation takes place in the oviduct (fallopian tube).

Question 2

(a) Choose from the following substances to answer the questions.
Each substance may be used once, more than once or not at all.
aluminium  
 chlorine  
copper(II) sulfate
  ethanol
  limestone
  sulfur
(i) Used as a chemical test for water.
(ii) Used as a solvent.
(iii) Used in aircraft parts.
(iv) Used in the manufacture of sulfuric acid.
(v) Used in the treatment of the water supply.
(vi) Used in the treatment of acidic soil.
(b) Sulfuric acid has the formula \( \text{H}_2\text{SO}_4 \).
(i) State the total number of atoms in one molecule of sulfuric acid.
(ii) State the total number of different elements found in one molecule of sulfuric acid.
(c) A student reacts magnesium with dilute sulfuric acid.
The equation for the reaction is shown.
\( \text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)} \)
(i) State the names of the two products of the reaction.
(ii) State the separation technique used by the student to remove any unreacted solid magnesium from the reaction mixture.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C10.1 — Water (Part (a)(i), (a)(v))
• Topic C9.2 — Uses of metals (Part (a)(iii))
• Topic C2.1/C3.1 — Elements, compounds and formulas (Part (a)(ii), (a)(iv), (a)(vi), (b))
• Topic C9.4 — Reactivity series (Part (c))
• Topic C12 — Experimental techniques (Part (c)(ii))

▶️ Answer/Explanation

(a)(i) copper(II) sulfate

Anhydrous (white) copper(II) sulfate turns blue when water is added.
This colour change is the standard chemical test for water.

(a)(ii) ethanol

Ethanol is a common organic solvent used to dissolve substances that do not dissolve well in water.

(a)(iii) aluminium

Aluminium has a low density and good strength, making it ideal for aircraft parts.

(a)(iv) sulfur

Sulfur is burned to produce sulfur dioxide, which is converted to sulfur trioxide and then sulfuric acid (Contact Process).

(a)(v) chlorine

Chlorine is added to water supplies to kill microbes (pathogens), making water safe to drink.

(a)(vi) limestone

Limestone (calcium carbonate) is a base; it neutralises excess acid in soil.

(b)(i) 7 atoms

Counting atoms in \( \text{H}_2\text{SO}_4 \): 2 hydrogen + 1 sulfur + 4 oxygen = 7 atoms in total.

(b)(ii) 3 different elements

The three different elements present are hydrogen (H), sulfur (S), and oxygen (O).

(c)(i) magnesium sulfate and hydrogen

From the equation, the two products formed are magnesium sulfate (\( \text{MgSO}_4 \)) and hydrogen gas (\( \text{H}_2 \)).

(c)(ii) filtration

Filtration separates the insoluble unreacted solid magnesium from the magnesium sulfate solution.
The solid remains as residue on the filter paper while the solution passes through as filtrate.

Question 3

A person climbs a mountain.
(a) The person is exposed to infrared and ultraviolet radiation from the Sun. Infrared and ultraviolet radiation are part of the electromagnetic spectrum.
(i) Fig. 3.1 shows an incomplete electromagnetic spectrum.
On Fig. 3.1, write infrared and ultraviolet in their correct places.
(ii) Ultraviolet waves travel at 300 000 000 m/s in a vacuum.
State the speed of infrared waves in a vacuum.
(b) The person makes a loud sound and then hears an echo.
State what is meant by an echo.
(c) The person takes a photograph with a camera.
The camera contains a thin converging lens.
Fig. 3.2 shows an incomplete ray diagram for a thin converging lens forming an image.
(i) State the name of the points labelled F on Fig. 3.2.
(ii) On Fig. 3.2:
  • draw a line to complete the path of ray 2
  • draw the image formed
  • label the image.
(d) The person drops the camera from the top of the mountain. The camera falls down the mountain.
Fig. 3.3 shows the distance–time graph for the motion of the camera over the first 6.0 s.
Use Fig. 3.3 to determine the average speed of the camera over 6.0 s.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic P3.3 — Electromagnetic spectrum (Part (a))
• Topic P3.4 — Sound (Part (b))
• Topic P3.2.3 — Thin converging lens (Part (c))
• Topic P1.2 — Motion (Part (d))

▶️ Answer/Explanation

(a)(i) 

Frequency increases left to right, so infrared sits just before visible light.
Ultraviolet sits just after visible light, before γ-rays.

(a)(ii) 300 000 000 m/s

All electromagnetic waves, including infrared, travel at the same speed in a vacuum: \( 3.0 \times 10^8 \, \text{m/s} \).

(b) An echo is the reflection of a sound wave

Sound waves bounce off a hard surface and return to the source.
This reflected sound is heard as a delayed repeat of the original sound.

(c)(i) principal focus / focal point

F marks the point where parallel rays converge after passing through a converging lens.

(c)(ii) Ray 2 travels straight through the centre of the lens; image formed is inverted and real, located where rays 1 and 2 cross

Ray 2 (through the optical centre) continues in a straight line without bending.
The image forms where the two rays intersect, and is real, inverted, and diminished.
Label this intersection point as the “image”.

(d) 30 m/s

From the graph, total distance travelled in 6.0 s = 180 m.
Average speed \( = \dfrac{\text{distance}}{\text{time}} = \dfrac{180}{6.0} = 30 \, \text{m/s} \).

Question 4

(a) Fig. 4.1 is a photomicrograph of a cross-section through a root.
The boxes on the left show the letters of the parts labelled X and Y in Fig. 4.1.
The boxes on the right show some functions.
Draw lines to link each part with its main function(s).
Draw three lines.
(b) State the function of phloem.
(c) Mineral ions are necessary for plant health.
(i) Explain why nitrate ions are required for growth.
(ii) Magnesium ions are used to make a substance required for photosynthesis. State the name of this substance.
(d) Table 4.1 shows some of the requirements for the processes of germination and photosynthesis.
Place ticks (✓) in the boxes to show the requirements for each process.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B8.1 — Xylem and phloem (Part (a), (b))
• Topic B6.1 — Photosynthesis (Part (c), (d))

▶️ Answer/Explanation

(a) 

X labels the xylem vessels in the centre of the root, which transport water and dissolved mineral ions upward.
Y labels the root hair, which absorbs water and mineral ions from the soil due to its large surface area.

(b) Transport of dissolved sugars (sucrose) and amino acids

Phloem moves the products of photosynthesis, mainly sucrose and amino acids, from leaves to other parts of the plant (translocation).

(c)(i) Nitrate ions are needed to make amino acids, which make up proteins required for growth

Nitrate ions provide the nitrogen needed to synthesise amino acids.
These amino acids are then built into proteins, which are essential for cell growth and repair.

(c)(ii) chlorophyll

Magnesium is a key component of the chlorophyll molecule, the green pigment needed to absorb light energy for photosynthesis.

(d) 

Photosynthesis requires light energy and carbon dioxide as raw materials to produce glucose.
Germination requires oxygen for aerobic respiration to release energy for the growing seedling; it does not require light or carbon dioxide.

Question 5

(a) Chlorine reacts with hydrogen to make hydrogen chloride.
(i) Construct the word equation for this reaction.
……………………….. + ……………………….. → ……………………….. 
(ii) Hydrogen chloride gas is a covalent compound.
Explain why hydrogen chloride gas is a covalent compound and not an ionic compound.
(iii) Fig. 5.1 shows the electronic structure in atoms of hydrogen and chlorine.
Draw a dot-and-cross diagram to show the arrangement of outer-shell electrons in a molecule of hydrogen chloride gas.
(iv) Hydrogen chloride dissolves in water to make dilute hydrochloric acid.
The pH of the hydrochloric acid is found using a pH probe.
Describe one other way of finding the pH of dilute hydrochloric acid.
(v) Suggest the pH of the dilute hydrochloric acid.
(b) Table 5.1 shows information about three Group VII elements.
(i) Complete Table 5.1.
(ii) State the name given to Group VII of the Periodic Table.
(iii) Explain why chlorine, bromine and iodine are all in Group VII of the Periodic Table.
Use ideas about electrons in your answer.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C2.5 — Simple molecules and covalent bonds (Part (a)(i)–(iii))
• Topic C7.1 — Characteristic properties of acids and bases (Part (a)(iv), (v))
• Topic C8.3 — Group VII properties (Part (b))

▶️ Answer/Explanation

(a)(i) hydrogen + chlorine → hydrogen chloride

Hydrogen gas reacts directly with chlorine gas to form hydrogen chloride gas.

(a)(ii) Both hydrogen and chlorine are non-metals

Ionic bonds form between metals and non-metals through electron transfer.
Since both atoms here are non-metals, they share electrons instead, forming a covalent bond.

(a)(iii) One shared (bonding) pair of electrons between H and Cl, with Cl retaining its three lone pairs

H contributes one electron and Cl contributes one electron to the shared pair.
Chlorine’s remaining six outer electrons are shown as three lone pairs around the Cl symbol.

(a)(iv) Use universal indicator solution (or paper) and match the colour produced to a pH colour chart

Universal indicator changes colour depending on the pH of the solution.
Comparing the resulting colour to a standard chart gives an estimate of the pH.

(a)(v) Any value from 1 to 3

Dilute hydrochloric acid is a strong acid, so it has a low pH, typically between 1 and 3.

(b)(i) 

Like chlorine and bromine, iodine exists as a diatomic molecule, \( \text{I}_2 \).
Since its melting point (113°C) is above 20°C, iodine is a solid at room temperature.

(b)(ii) halogens

Group VII elements are collectively known as the halogens.

(b)(iii) They all have seven electrons in their outer shell

Elements are grouped together in the Periodic Table based on having the same number of outer-shell electrons, which gives them similar chemical properties.

Question 6

(a) α-particles, β-particles and γ-rays are all used in hospitals to treat cancer.
Table 6.1 gives information about the nature and charge of these three radiations.
(i) Complete Table 6.1.
(ii) Place the three radiations in order of their ionising ability, from most ionising to least ionising.
most ionising ………………..
least ionising ………………..
(iii) State which radiation is the most penetrating.
(b) X-rays and ultrasound waves are also used in hospitals.
(i) State one use of X-rays in hospitals.
(ii) Ultrasound waves have a frequency that is too high for a human to hear.
Use your knowledge of the range of audible frequencies for a human to suggest the frequency of ultrasound waves.
(c) A doctor in the hospital uses some sanitising hand liquid. The liquid contains ethanol, which evaporates from the skin of the doctor and cools the doctor’s hands.
Explain why the evaporation of the ethanol causes the doctor’s skin to cool.
Use ideas about molecules in your answer.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic P5.2.2 — The three types of nuclear emission (Part (a))
• Topic P3.3 — Electromagnetic spectrum / Topic P3.4 — Sound (Part (b))
• Topic P2.1 — Kinetic particle model of matter (Part (c))

▶️ Answer/Explanation

(a)(i) 

α-particles are helium nuclei (2 protons + 2 neutrons), giving them a positive charge.
β-particles are fast-moving electrons, giving them a negative charge.
γ-rays are electromagnetic waves and therefore carry no electrical charge.

(a)(ii) most ionising: α-particles; then β-particles; least ionising: γ-rays

α-particles are large and slow-moving, causing many ionising collisions.
γ-rays are pure energy waves with no mass, so they ionise far less per unit path length.

(a)(iii) γ-rays

γ-rays are the most penetrating because they are uncharged and pass through most materials with little interaction.

(b)(i) Medical imaging/scanning of bones, or security scanning

X-rays pass more easily through soft tissue than dense bone, producing images used to diagnose fractures and other internal conditions.

(b)(ii) Any value above 20 000 Hz

The human audible range is approximately 20 Hz to 20 000 Hz.
Ultrasound is defined as sound above this range, so any frequency greater than 20 000 Hz is acceptable.

(c) The most energetic molecules escape from the surface of the liquid, leaving behind molecules with lower average kinetic energy, which lowers the temperature

Evaporation occurs when the fastest-moving (most energetic) ethanol molecules escape from the liquid surface.
This removes energy from the remaining liquid.
The average kinetic energy of the remaining molecules decreases, lowering the temperature of the liquid and the skin it touches.

Question 7

(a) Table 7.1 shows some information about five different enzymes.
(i) Identify the enzyme(s) from Table 7.1 that:
  • have the widest pH range of activity …………………….. and ……………………..
  • is active at pH 5.5 …………………………………………
  • is most active in alkaline conditions. …………………………………………
(ii) State one other factor, apart from pH, that affects the activity of enzymes.
(b) Circle two words that are used to describe enzymes.
carbohydrates       catalysts       fats
hormones       proteins       solvents
(c) The list shows some parts of the alimentary canal and associated organs.
anus       gall bladder       mouth
oesophagus        pancreas       small intestine
Choose words from the list to identify where each process occurs. Each word may be used once, more than once or not at all.
mechanical digestion …………………..
egestion ……………………
ingestion …………………….
(d) Describe one similarity between absorption and assimilation.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B5.1 — Enzymes (Part (a), (b))
• Topic B7.2 — Digestive system (Part (c), (d))

▶️ Answer/Explanation

(a)(i) widest range: A and C; active at pH 5.5: A; most active in alkaline: C

Enzyme A (4.5–8.5) and enzyme C (7.0–11.0) both span 4.0 pH units, the widest ranges in the table.
Only enzyme A’s range (4.5–8.5) includes pH 5.5.
Enzyme C has the highest “most active” pH value (9.0), which is alkaline.

(a)(ii) Temperature (or substrate concentration)

Enzyme activity is also affected by temperature, since kinetic energy and denaturation depend on it.

(b) catalysts and proteins

Enzymes are biological catalysts that speed up reactions without being used up.
Structurally, enzymes are made of protein.

(c) mechanical digestion: mouth; egestion: anus; ingestion: mouth

Mechanical digestion (chewing) occurs in the mouth, physically breaking food into smaller pieces.
Egestion (removal of undigested waste as faeces) occurs at the anus.
Ingestion (taking food into the body) occurs at the mouth.

(d) Both involve the movement of digested food / nutrient molecules

Absorption is the movement of digested nutrients from the intestine into the blood, while assimilation is their uptake and use by cells — both processes involve the movement of soluble nutrient molecules.

Question 8

(a) Copper is a metal in high demand that is now often recycled.
To recycle copper, it must be melted down into a liquid.
(i) Suggest one reason other than cost why copper and other metals are recycled.
(ii) Describe two differences between the arrangement and motion of atoms in molten copper and the arrangement and motion of atoms in solid copper.
(b) A metal coin is covered with a layer of copper by electroplating.
(i) The coin is used as one of the electrodes. State for which electrode the coin is used.
(ii) Suggest a suitable solution to be used as the electrolyte for electroplating the coin.
(iii) Electroplating uses the process of electrolysis; a definition is provided.
Electrolysis is the breakdown of a covalent compound when solid or in aqueous solution by the passage of electricity.
This definition of electrolysis is not correct.
Circle the two words in the definition that are not correct.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C9.6 — Extraction of metals / recycling (Part (a)(i))
• Topic C1 — States of matter (Part (a)(ii))
• Topic C4.1 — Electrolysis (Part (b))

▶️ Answer/Explanation

(a)(i) Copper ore is a finite/limited resource

Recycling conserves the limited supply of copper ore, reducing the need for new mining.

(a)(ii) Arrangement: solid is regular, liquid is irregular; Motion: solid atoms only vibrate about a fixed point, liquid atoms can move/slide past each other

In solid copper, atoms are arranged in a fixed, regular lattice.
In molten (liquid) copper, the arrangement becomes irregular/random.
Solid atoms vibrate in place, whereas liquid atoms have enough energy to move around each other.

(b)(i) cathode

In electroplating, the object to be coated (the coin) is made the negative electrode (cathode), so positive copper ions are attracted to and deposited on it.

(b)(ii) copper sulfate solution (or copper chloride solution)

A solution containing copper ions is needed so that copper can be deposited onto the coin during electrolysis.

(b)(iii) “covalent” and “solid” should be circled

Electrolysis breaks down an ionic compound, not a covalent one.
It must be molten or in aqueous solution (so ions are free to move), not solid, since ions cannot move in a solid lattice.

Question 9

(a) The two boxes on the left each contain the name of an energy resource.
The four boxes on the right contain advantages or disadvantages of the energy resources.
Draw two lines from each energy resource to the advantages or disadvantages of the energy resource.
(b) The statements, A to E, describe processes in a coal-burning power station. They are not in the correct order.
A Coal burns to produce thermal energy.
B A turbine drives a generator.
C Steam is produced.
D Steam turns a turbine.
E Thermal energy boils water.
(i) Use the letters B to E to complete the sequence to describe how the power station generates electricity.
A → ……. → ……. → ……. → ……. → electricity is generated
(ii) State the boiling point of water.
(c) When coal is burned in a power station, the energy in the coal is transferred to the energy outputs shown in Table 9.1.
The energy outputs are shown as percentages of the total energy from the coal.
(i) Explain why Table 9.1 shows that energy is conserved in the processes in the power station.
(ii) The power station in Table 9.1 produces a lot of thermal energy. Explain why the power station is not efficient.
(d) The electricity generated from the power station is supplied to a house to power the lighting circuit.
Lamps for lighting in the house are connected in a parallel circuit and not in a series circuit.
(i) Complete the circuit diagrams to show the difference between a series circuit and a parallel circuit.
Use two lamps in each circuit.
(ii) State one advantage of connecting lamps in parallel in a lighting circuit.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic P1.6.3 — Energy resources (Part (a))
• Topic P1.6.3 — Energy resources / efficiency (Part (b), (c))
• Topic P4.3.2 — Series and parallel circuits (Part (d))

▶️ Answer/Explanation

(a) coal → non-renewable AND polluting; geothermal → renewable AND non-polluting

Coal is a fossil fuel that cannot be replaced once used up, and burning it releases pollutants.
Geothermal energy comes from the Earth’s heat, which is naturally replenished and produces no pollution.

(b)(i) A → E → C → D → B → electricity is generated

Coal burns (A), releasing thermal energy that boils water (E), producing steam (C).
The steam turns a turbine (D), which drives a generator (B) to produce electricity.

(b)(ii) 100 °C

Water boils at 100 °C at standard atmospheric pressure.

(c)(i) The percentages add up to 100%

Since \( 25 + 15 + 60 = 100 \), all the input energy is accounted for in the listed outputs, showing none is lost overall.

(c)(ii) Most of the energy output is wasted as thermal energy, with only 25% converted into useful electrical energy

A power station is inefficient because the majority (75%) of energy ends up as unwanted heat rather than being converted into useful electrical output.

(d)(i) Parallel circuit: two lamps each on separate branches across the battery; Series circuit: two lamps in a single loop one after another

In a parallel circuit, each lamp is on its own branch, connected independently across the supply.
In a series circuit, both lamps share the same single current path, one after the other.

(d)(ii) Each lamp can be switched on/off independently, or if one lamp fails the others continue to work, or each lamp gets the full mains voltage

In a parallel circuit, the failure of one lamp does not break the circuit for the others, and each lamp receives the same full voltage as the supply.

Question 10

(a) Scientists record the mass of each baby at birth in one country.
The results are shown in Fig. 10.1
(i) Complete the sentences about the mass of babies at birth as shown in Fig. 10.1.
The lowest mass at birth was …………………. g.
The most frequent mass at birth was ………………. g.
Fig. 10.1 shows that mass of babies at birth is an example of continuous variation. This is because there is a …………………. of phenotypes between two extremes.
A phenotype is defined as the …………………. features of an organism.
(ii) State one other example of continuous variation in humans.
(b) The statements A–E describe stages of natural selection. They are not in the correct order.
A There is competition for resources.
B Those with adaptations best suited to their environment survive and reproduce.
C This process is repeated over several generations.
D There is variation in a population.
E Alleles are passed on to the next generation.
Complete the answer spaces to show the correct order. One has been done for you.
……. → A → ……. → ……. → …….
(c) Alleles are made of DNA.
(i) Define the term allele.
(ii) State where alleles are found in a cell.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B17.1 — Variation (Part (a))
• Topic B17 — Variation and selection / natural selection (Part (b))
• Topic B16.1/B16.3 — Chromosomes, genes and alleles (Part (c))

▶️ Answer/Explanation

(a)(i) lowest mass = 800 g; most frequent mass = 2400 g; range; observable

Reading the graph, the curve starts at approximately 800 g (lowest value plotted).
The peak of the curve (highest frequency) occurs at approximately 2400 g.
Continuous variation shows a full range of values between two extremes, with no distinct categories.
A phenotype refers to the observable features of an organism.

(a)(ii) Height (or another example such as foot size, hand span)

Height in humans varies continuously across a population, with no fixed discrete categories — making it a valid example.

(b) D → A → B → C → E

There must first be variation (D) in a population for competition (A) for resources to matter.
Those best adapted survive and reproduce (B), and this is repeated over generations (C), with alleles passed on (E).

(c)(i) An allele is an alternative form of a gene

Different alleles of the same gene can produce different versions of a characteristic (e.g., different alleles for eye colour).

(c)(ii) In the nucleus

Alleles are sections of DNA found on chromosomes, which are located in the nucleus of the cell.

Question 11

(a) The proton numbers and nucleon numbers of carbon and hydrogen are shown in Table 11.1.
(i) State the number of neutrons in a carbon atom and the number of neutrons in a hydrogen atom.
(ii) State the number of electrons in a carbon atom.
(b)(i) State the difference between a saturated hydrocarbon and an unsaturated hydrocarbon. 
(ii) Aqueous bromine is used to show the difference between a saturated hydrocarbon and an unsaturated hydrocarbon.
Describe the result of the test with a saturated hydrocarbon.
(c) Fig. 11.1 shows the apparatus used for the catalytic cracking of a saturated hydrocarbon.
(i) State the purpose of the catalyst in the cracking of a saturated hydrocarbon.
(ii) Cracking is an endothermic reaction. State what is meant by an endothermic reaction.
(d) The complete combustion of hydrocarbons produces the greenhouse gas carbon dioxide.
(i) Name one other greenhouse gas.
(ii) Identify the gas in the air that reacts with hydrocarbons during combustion.
(iii) Carbon dioxide is one of the gases found in clean air.
State the name of the two gases found in clean air in the greatest proportions.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C2.2 — Atomic structure and the Periodic Table (Part (a))
• Topic C11.1/C11.4 — Formulas and terminology / Alkanes (Part (b))
• Topic C11 — Cracking / Topic C5.1 — Exothermic and endothermic reactions (Part (c))
• Topic C10.2 — Air quality and climate (Part (d))

▶️ Answer/Explanation

(a)(i) carbon = 6 neutrons; hydrogen = 0 neutrons

Number of neutrons = nucleon number − proton number.
Carbon: \( 12 – 6 = 6 \). Hydrogen: \( 1 – 1 = 0 \).

(a)(ii) 6 electrons

In a neutral atom, the number of electrons equals the number of protons, so carbon has 6 electrons.

(b)(i) A saturated hydrocarbon has only single carbon–carbon bonds, while an unsaturated hydrocarbon has at least one carbon–carbon double bond

This is the key structural difference that defines whether a hydrocarbon is saturated or unsaturated.

(b)(ii) Aqueous bromine remains orange/brown (no colour change)

Saturated hydrocarbons do not react with bromine water, so it stays orange.
In contrast, unsaturated hydrocarbons decolourise it.

(c)(i) To speed up the reaction (without being used up)

The catalyst lowers the activation energy needed for cracking, allowing the reaction to proceed faster at a lower temperature.

(c)(ii) An endothermic reaction is one that takes in thermal energy from the surroundings, causing a decrease in temperature

Energy must be absorbed to break the strong carbon–carbon and carbon–hydrogen bonds during cracking.

(d)(i) Methane (or water vapour)

Methane is a greenhouse gas released from decomposing vegetation and animal digestion.

(d)(ii) Oxygen

Oxygen in the air reacts with hydrocarbons during combustion to produce carbon dioxide and water.

(d)(iii) Nitrogen and oxygen

Clean, dry air is approximately 78% nitrogen and 21% oxygen, making these the two most abundant gases.

Question 12

(a) Fig. 12.1 shows an electric heater used in a classroom in a school.
The air around the heater is warmed.
(i) On Fig. 12.1 draw three more arrows to show how the warmed air moves around the classroom.
One arrow has been drawn for you.
(ii) State the name of the method of thermal energy transfer you have drawn in (a)(i).
(b) The teacher in the classroom measures the temperature in the room with a thermometer.
Fig. 12.2 shows the thermometer.
(i) State the name of the temperature scale used on the thermometer.
(ii) State the name of a liquid that is used in thermometers.
(iii) State the physical property of the liquid that varies with temperature.
(c)(i) In the school an electric bell rings to show that the lesson has ended.
The bell makes sound waves that travel through the air.
Sound waves cannot travel through a vacuum.
Explain why sound cannot travel through a vacuum.
(ii) Inside the electric bell there is a solenoid.
The solenoid coil is shown in Fig. 12.3.
On Fig. 12.3, draw the pattern of the magnetic field that is produced when an electric current passes through the solenoid as shown.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic P2.3.2 — Convection (Part (a))
• Topic P2.2 — Thermal properties and temperature (Part (b))
• Topic P3.4 — Sound (Part (c)(i))
• Topic P4.5.3 — Magnetic effect of current (Part (c)(ii))

▶️ Answer/Explanation

(a)(i) Arrow rising from the heater, moving across the ceiling, dropping down the far wall, and moving back along the floor toward the heater

Warm air rises because it becomes less dense.
It cools as it moves along the ceiling and across, then sinks and returns along the floor, forming a complete convection current loop.

(a)(ii) Convection

Convection is the transfer of thermal energy through the bulk movement of a fluid (here, warm air rising and cool air sinking).

(b)(i) Celsius

The scale shown (with markings such as −10 to 110 °C) is the Celsius temperature scale.

(b)(ii) Alcohol (or mercury)

Both alcohol and mercury are commonly used liquids in liquid-in-glass thermometers.

(b)(iii) Volume (or density)

As temperature increases, the liquid expands, increasing its volume (and decreasing its density), which is used to indicate temperature change.

(c)(i) There is no medium for sound to travel through in a vacuum

Sound is a mechanical wave that requires particles to vibrate and pass on the wave.
A vacuum contains no particles, so sound has nothing to propagate through.

(c)(ii) Field lines run parallel and close together inside the solenoid (like a bar magnet), curving out and around the outside from one end to the other

Inside the solenoid, the magnetic field is strong and uniform, with lines running along the axis.
Outside, the field lines curve from the north end to the south end, similar to a bar magnet’s field pattern.

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