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Question 1

(a) Scientists measure the length of sperm cells from different animals.
The animals are placed in size from animal A (the smallest) to animal F (the largest).
Fig. 1.1 shows the results.
(i) Using Fig. 1.1, state the length of a sperm cell in animal B.
(ii) State the animal with the shortest length of sperm.
(iii) State evidence from Fig. 1.1 that shows that length of sperm does not increase with size of animal.
(b) Plant and animal cells both contain a nucleus.
Name two other structures seen in both plant and animal cells.
(c) The boxes on the left show some of the parts of a plant cell.
The boxes on the right show the functions of the parts.
Draw lines to link each part with its function.
(d) Substances enter and leave cells.
Name the process by which substances enter the cells.
(e) Explain why muscle cells need high rates of respiration.

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

• Topic B15.4 — Reproduction (Part (a))
• Topic B2.1 — Cell structure (Parts (b), (c))
• Topic B3.1 — Movement of substances (Part (d))
• Topic B12.1 — Respiration (Part (e))

▶️ Answer/Explanation

(a)(i) 0.12 mm

The height of the bar for animal B on Fig. 1.1 is read directly from the y-axis.
This corresponds to a value of 0.12 mm.

(a)(ii) Animal A

Animal A has the shortest bar on the graph, at 0.04 mm.
This is the smallest value shown for any animal.

(a)(iii)

Animal F is the biggest animal but does not have the longest sperm length.
Animal D has the longest sperm length but is not the biggest animal.
This shows sperm length is not correlated with body size.

(b) Cell membrane; cytoplasm

Both plant and animal cells share a cell membrane, which controls what enters and leaves the cell.
Both also contain cytoplasm, the jelly-like substance where chemical reactions occur.
Other accepted answers include ribosomes or mitochondria.

(c)

(d) Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
It occurs down a concentration gradient and does not require energy input.
This allows small molecules such as oxygen and glucose to enter cells.

(e)

Respiration releases the energy that muscle cells need.
This energy is required for muscle contraction and movement.
Because muscles are constantly active, they need a continuous, high supply of ATP from respiration.

Question 2

(a) The thermal decomposition of calcium carbonate makes calcium oxide and carbon dioxide.
(i) Write the word equation for the thermal decomposition of calcium carbonate.
(ii) The production of lime (calcium oxide) from limestone (calcium carbonate) is one use of limestone.
State one other use of limestone.
(iii) Calcium carbonate has the formula \(\text{CaCO}_3\).
State the number of different elements present in calcium carbonate.
(iv) Explain why the thermal decomposition of calcium carbonate is a chemical change and not a physical change.
(v) The thermal decomposition of calcium carbonate is an endothermic reaction.
State what is meant by an endothermic reaction.
(b)(i) Carbon dioxide is a compound but carbon is an element.
State the difference between an element and a compound.
(b)(ii) Carbon is a solid and carbon dioxide is a gas.
Complete Table 2.1 to describe the differences between a solid and a gas.
One difference has been done for you.
(c) Carbon exists in many forms including diamond and graphite.
Diamond and graphite are described as giant covalent structures.
Fig. 2.1 shows three covalent structures, A, B and C.
(i) State which structure represents diamond. Choose from A, B or C.
(ii) State which structure represents graphite. Choose from A, B or C.
(iii) State why the bonding between the carbon atoms in diamond is covalent and not ionic.

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

• Topic C6.1 — Chemical reactions/thermal decomposition (Part (a))
• Topic C1.1 — States of matter/elements and compounds (Part (b))
• Topic C2.6 — Covalent bonding, giant structures (Part (c))

▶️ Answer/Explanation

(a)(i) calcium carbonate → calcium oxide + carbon dioxide

Heating calcium carbonate breaks it down into two simpler substances.
This word equation shows the reactant on the left and the two products on the right.

(a)(ii) Neutralising acidic soil / building material

Limestone is widely used in construction and in agriculture.
It is also used to make cement and to reduce soil acidity.

(a)(iii) 3

\(\text{CaCO}_3\) contains calcium, carbon, and oxygen.
That is three different elements chemically combined together.

(a)(iv) New substances are produced

A chemical change always produces one or more new substances with different properties.
Since calcium oxide and carbon dioxide are chemically different from calcium carbonate, this is a chemical change.

(a)(v) Thermal energy is taken in

An endothermic reaction absorbs energy from the surroundings, usually as heat.
This energy is used to break the bonds in calcium carbonate.

(b)(i)

An element contains only one type of atom.
A compound contains two or more different elements chemically combined together.

(b)(ii)

(c)(i) A

Structure A shows each carbon atom bonded to four others in a rigid tetrahedral network.
This 3D lattice structure is characteristic of diamond.

(c)(ii) B

Structure B shows carbon atoms arranged in flat hexagonal layers.
These layers are weakly bonded to each other, which matches graphite’s structure.

(c)(iii) 2 non-metal atoms bonding / electrons are shared

Carbon atoms are non-metal atoms of the same element.
Bonding between non-metal atoms involves sharing electrons, which is covalent bonding, not the transfer of electrons seen in ionic bonding.

Question 3

(a) A student constructs a circuit containing two lamps in parallel connected across two cells in series. Each lamp is controlled by a separate switch.
(i) Draw a circuit diagram for this circuit using standard electrical symbols.
(ii) State one advantage of connecting the lamps in parallel rather than in series.
(b) Fig. 3.1 shows a filament lamp.
(i) When the lamp is switched on, thermal energy is transferred from the filament through the metal base. Name the process that transfers the thermal energy.
(ii) Suggest one part of the electromagnetic spectrum emitted by the lamp.
(c) Fig. 3.2 shows a ray of light from the lamp passing through a rectangular glass block.
(i) On Fig. 3.2, label the angle of incidence with the letter i and the angle of refraction with the letter r.
(ii) Explain why the ray of light changes direction as it is refracted through the glass block.

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

• Topic P4.3.2 — Electrical circuits (Part (a))
• Topic P2.3.1 / P3.3 — Thermal energy transfer / electromagnetic spectrum (Part (b))
• Topic P3.2.2 — Refraction of light (Part (c))

▶️ Answer/Explanation

(a)(i)

The diagram should show two cells connected in series.
Two lamps should be connected in parallel across the cells.
Each lamp should have its own switch in series with it, in its own branch.

(a)(ii) If one lamp fails, the other still lights

In a parallel circuit, each lamp has its own separate current path.
So each lamp also receives the full battery voltage, and one lamp failing does not stop the other from working.

(b)(i) Conduction

Conduction transfers thermal energy through a solid material without the material itself moving.
Here, heat passes from the hot filament through the solid metal base.

(b)(ii) Visible light / infrared

A hot filament emits electromagnetic radiation as visible light, which lets us see it glow.
It also emits infrared radiation, which is felt as heat.

(c)(i)

The angle of incidence, \(i\), is marked between the incident ray and the normal at the point of entry into the glass.
The angle of refraction, \(r\), is marked between the refracted ray and the normal at the same point.

(c)(ii) Change of speed

Light travels at a different speed in glass compared with air.
This change in speed as light crosses the boundary causes it to change direction, i.e. it refracts.

Question 4

(a) Fig. 4.1 is a diagram of the male reproductive system in humans.
Identify the letter from Fig. 4.1 that represents the:
part where sperm is produced,
part which secretes the fluid that sperm swim in,
tube which carries both semen and urine.
(b) State the name of the part of the female reproductive system that releases gametes.
(c) The statements in Table 4.1 describe either asexual or sexual reproduction.
Complete Table 4.1 to identify the type of reproduction each statement describes.
(d) Fig. 4.2 is a diagram representing the reproduction of a type of organism called a hydra.
Identify if this organism is reproducing asexually or sexually.
Use evidence from Fig. 4.2 to give a reason for your answer.
(e) Reproduction and respiration are characteristics of living things.
State two other characteristics of living things.

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

• Topic B15.4 — Reproduction in humans (Parts (a), (b))
• Topic B15.1 — Sexual vs asexual reproduction (Parts (c), (d))
• Topic B1.1 — Characteristics of living organisms (Part (e))

▶️ Answer/Explanation

(a) E; C; A

E (testis) is where sperm cells are produced.
C (seminal vesicle) secretes the fluid in which sperm swim.
A (urethra) is the shared tube that carries both semen and urine out of the body.

(b) Ovary

The ovary is the female reproductive organ that releases an egg (gamete) during ovulation.

(c)

Offspring genetically identical → asexual reproduction, since no fusion of gametes occurs.
Produces zygotes → sexual reproduction.
Involves fusion of nuclei → sexual reproduction.

(d) Asexual; offspring produced by only one parent

Fig. 4.2 shows a hydra producing a bud that grows and detaches to form a new individual.
Only one parent is involved, with no fusion of gametes, which is characteristic of asexual reproduction.

(e) Any two from: movement, sensitivity, growth, excretion, nutrition

Movement allows an organism to respond to its environment, for example to find food or avoid danger.
Growth is an increase in size or dry mass over time, another defining characteristic of living organisms.

Question 5

(a) Iron is an element in Period 4 of the Periodic Table.
(i) Name the collection of metals in Period 4 that contains iron.
(ii) An atom of iron has a proton number of 26 and a nucleon number of 56.
State the number of electrons in this atom.
(iii) Iron is obtained from iron oxide by reaction with carbon.
The word equation for the reaction is shown.
iron oxide + carbon → iron + carbon dioxide
Name the substance that is reduced in this reaction. Explain your answer.
(b) Steel is an alloy of iron.
(i) State what is meant by an alloy.
(ii) State why steel is used rather than pure iron for making cars.
(iii) Table 5.1 shows the percentage composition of an alloy.
Calculate the percentage of iron in the alloy.
(c) A student investigates the rusting of iron nails.
Fig. 5.1 shows the student’s experiments.
Table 5.2 shows the student’s observations after one week.
Complete Table 5.2 to explain the student’s observations after one week.

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

• Topic C9.4 — Transition metals, extraction of iron (Parts (a), (b))
• Topic C9.4 — Rusting (Part (c))

▶️ Answer/Explanation

(a)(i) Transition elements / transition metals

Iron sits in the central block of the Periodic Table, which is the collection of metals known as the transition elements.

(a)(ii) 26

In a neutral atom, the number of electrons equals the proton number.
Since iron has a proton number of 26, it has 26 electrons.

(a)(iii) Iron oxide; loses oxygen

Reduction is the loss of oxygen from a substance.
Iron oxide loses its oxygen to form iron, so iron oxide is the substance reduced.

(b)(i) Mixture of metals / mixture of a metal and other element

An alloy is formed by mixing a metal with one or more other elements to alter its properties.

(b)(ii) Stronger

Pure iron is relatively soft and prone to bending.
Steel is much stronger, making it more suitable for car bodies and structural parts.

(b)(iii) 7%

The total percentage composition must add up to 100%.
\( \text{percentage of iron} = 100 – (33 + 60) = 7\% \)

(c)

Tube X (does not rust): no water is present, since the drying agent removes water vapour from the dry air.
Tube Y (rusts): both oxygen (air) and water are present, and rusting needs both.
Tube Z (does not rust): the nail is painted, so air and water cannot reach the iron surface.

Question 6

(a) Fig. 6.1 shows a copper wire connected to a battery and placed between the poles of a strong magnet.
When the switch is closed the wire moves upwards.
(i) Describe two ways to change the apparatus so that the wire moves downwards when the switch is closed.
(ii) State the term that describes a flow of electrons in a metal conductor.
(b) A β-particle is an electron.
α-particles and β-particles are radioactive emissions released during the radioactive decay of isotopes.
(i) State the meaning of the terms: radioactive decay; isotopes.
(ii) Complete the sentences below using the symbols α and β:
…… particles are less ionising than …… particles.
…… particles are less penetrating than …… particles.
(c) A plastic ruler and a piece of cloth are both uncharged.
A student rubs the plastic ruler with the cloth.
The plastic ruler becomes positively charged.
(i) Describe in terms of electrons how the plastic ruler becomes positively charged.
(ii) State which row from Table 6.1 shows the charge on the cloth compared to the charge on the plastic ruler.

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

• Topic P4.5.4 — Electromagnetic effects/force on a current-carrying conductor (Part (a)(i))
• Topic P4.2.2 — Electric current (Part (a)(ii))
• Topic P5.2.2 — Radioactivity (Part (b))
• Topic P4.2.1 — Electrostatics (Part (c))

▶️ Answer/Explanation

(a)(i) Reverse the battery polarity / current; reverse the magnet polarity

The direction of the force on the wire depends on the directions of the current and the magnetic field.
Reversing either the current direction or the magnetic field direction reverses the direction of the force, moving the wire downwards instead of upwards.

(a)(ii) Current

In a metal conductor, a current is the flow of free electrons through the material.

(b)(i)

Radioactive decay is the process by which an unstable nucleus loses energy by emitting radiation.
Isotopes are atoms of the same element that have the same proton number but a different nucleon number (different number of neutrons).

(b)(ii) β particles are less ionising than α particles; α particles are less penetrating than β particles

α-particles are large and highly charged, so they ionise strongly but are stopped easily, giving them low penetrating power.
β-particles are much smaller and less strongly charged, so they ionise less but penetrate further.

(c)(i) The ruler loses electrons

When the ruler is rubbed with the cloth, electrons transfer from the ruler to the cloth.
This leaves the ruler with fewer electrons than protons, giving it an overall positive charge.

(c)(ii) Row 4

Since electrons move from the ruler to the cloth, the cloth gains a negative charge.
The magnitude of charge gained by the cloth equals the magnitude of the positive charge on the ruler, which corresponds to row 4 (negative, equal).

Question 7

(a) A student has two peas.
Pea A is green.
Pea B is yellow.
The colour of peas is controlled by a single gene.
  • The allele for green-coloured peas is g.
  • The allele for yellow-coloured peas is G.
(i) Circle two words that can be used to describe the genotype of pea A.
allele      dominant      heterozygous      homozygous
phenotype      recessive
(ii) State the two possible genotypes of pea B.
(b) Two pea plants are crossed.
Fig. 7.1 is an incomplete genetic diagram showing a genetic cross of two pea plants.
(i) Complete the genetic diagram in Fig. 7.1 to show the expected genotypes of the offspring.
(ii) State the percentage chance of the parent plants in Fig. 7.1 producing offspring that have the genotype gg.
(c) Chromosomes, DNA and genes are involved in inheritance.
(i) List these structures in order of size from smallest to largest.

(ii) State the phenotype of a person with the sex chromosomes XX.

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

• Topic B16.3 — Monohybrid inheritance (Parts (a), (b))
• Topic B16.1 — Chromosomes, genes, and DNA (Part (c))

▶️ Answer/Explanation

(a)(i) Homozygous; recessive

Green is the recessive phenotype, so pea A’s genotype must be gg.
An individual with two identical alleles (gg) is described as homozygous recessive.

(a)(ii) GG and Gg

Yellow is the dominant phenotype.
A yellow pea could be homozygous dominant (GG) or heterozygous (Gg), since only one dominant allele is needed to show the dominant trait.

(b)(i)

Crossing G and g gametes from each parent gives offspring genotypes: GG, Gg, Gg, and gg.

(b)(ii) 25%

Out of the four possible genotype combinations in the Punnett square, only one is gg.
This gives a probability of \( \frac{1}{4} = 25\% \).

(c)(i) DNA → genes → chromosomes

DNA is the molecule that carries genetic information.
A gene is a short section of DNA, and many genes coiled together with proteins form a chromosome, making chromosomes the largest of the three.

(c)(ii) Female

In humans, the XX sex chromosome combination results in a female phenotype.

Question 8

(a) Petroleum is a fossil fuel.
(i) Name one other fossil fuel.
(ii) Petroleum is separated into fractions to make it more useful.
Complete the sentences using words from the list.
Each word may be used once, more than once or not at all.
compound      distillation      electrolysis
filtration      mixture      molecule
Petroleum is a ………….. of different hydrocarbons.
Petroleum is separated by fractional ………….. .
(iii) Gasoline is obtained from petroleum.
State the two products of the complete combustion of gasoline.
(b) Ethane, \(\text{C}_2\text{H}_6\), is an alkane. Ethene, \(\text{C}_2\text{H}_4\), is an alkene.
(i) Explain why ethene is described as an unsaturated hydrocarbon.
(ii) Describe the chemical test and the positive result for an unsaturated hydrocarbon.
(c) Ethene molecules react together to form a polymer.
(i) Describe what happens to the ethene molecules when they form a polymer.
(ii) Name the polymer made from ethene.

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

• Topic C11.3 — Fossil fuels, fractional distillation of petroleum, combustion (Part (a))
• Topic C11.5 — Alkanes and alkenes (Part (b))
• Topic C11.7 — Polymers (Part (c))

▶️ Answer/Explanation

(a)(i) Coal / natural gas

Coal, natural gas, and petroleum are all fossil fuels formed from ancient organic matter.

(a)(ii) Mixture; distillation

Petroleum is a mixture of different hydrocarbon molecules, not chemically bonded together.
Because they have different boiling points, they can be separated by fractional distillation.

(a)(iii) Carbon dioxide; water

Complete combustion occurs when there is enough oxygen for the hydrocarbon to fully react.
The products are carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)).

(b)(i) Contains a carbon–carbon double bond

Ethene has the structure \(\text{H}_2\text{C}=\text{CH}_2\), containing one C=C double bond.
This double bond means not all carbon atoms are bonded to the maximum number of other atoms, making it unsaturated.

(b)(ii) Test: aqueous bromine (bromine water); result: decolourises

Bromine water is orange-brown in colour.
When shaken with an unsaturated hydrocarbon, the double bond reacts with the bromine and the colour disappears (turns colourless).

(c)(i) The molecules join together (into long chains)

In addition polymerisation, the C=C double bonds in ethene open up.
The carbon atoms then bond to each other in sequence, forming one long-chain molecule.

(c)(ii) Polyethene

The polymer formed by joining many ethene monomers together is called polyethene (polythene).

Question 9

Fig. 9.1 shows a refrigerator.
(a) The freezing compartment at the top of the refrigerator cools all the air in the refrigerator.
State the main method of thermal energy transfer used in this cooling process.
(b) The volume of air in the refrigerator is \(210\,000\,\text{cm}^3\).
The density of air is \(0.00126\,\text{g/cm}^3\).
Calculate the mass of air in the refrigerator.
Show your working.
(c) A liquid-in-glass thermometer is placed inside the freezing compartment to measure a temperature of \(-20\,^\circ\text{C}\).
(i) Name a suitable liquid to use in the thermometer.
(ii) State the physical property of the liquid that varies with temperature in a liquid-in-glass thermometer.
(d) The refrigerator emits a quiet sound with a low pitch.
(i) Describe the amplitude and frequency of this sound.
(ii) State the unit of frequency.
(e) The refrigerator contains two lamps connected in series.
Lamp A has a resistance of \(4000\,\Omega\) and lamp B has a resistance of \(5000\,\Omega\).
(i) Calculate the combined resistance of the two lamps connected in series.
(ii) The potential difference across the lamps is 240 V.
Use your answer to (e)(i) to calculate the current in the lamps.
Show your working.

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

• Topic P2.3.2 — Convection (Part (a))
• Topic P1.4 — Density (Part (b))
• Topic P2.1.1 — States of matter (Part (c))
• Topic P3.4 — Sound (Part (d))
• Topic P4.2.4 — Resistance (Part (e))

▶️ Answer/Explanation

(a) Convection

Cold, dense air from the freezing compartment sinks, while warmer, less dense air rises to replace it.
This circulating movement of air is convection, and it cools the whole refrigerator.

(b) 265 g

\( \text{mass} = \text{density} \times \text{volume} \)
\( \text{mass} = 0.00126 \times 210\,000 = 264.6 \approx 265\,\text{g} \)

(c)(i) Mercury / alcohol

Mercury and alcohol both stay liquid at low temperatures like \(-20\,^\circ\text{C}\), unlike water, which would freeze.

(c)(ii) Volume

As temperature changes, the volume of the liquid expands or contracts, and this change in volume is shown by the liquid level in the narrow tube.

(d)(i) Small amplitude; low frequency

A quiet sound corresponds to a small amplitude, since amplitude relates to loudness.
A low-pitched sound corresponds to a low frequency, since pitch relates to frequency.

(d)(ii) Hertz (Hz)

Frequency is measured in hertz, which represents the number of complete wave cycles per second.

(e)(i) 9000 Ω

In series, resistances simply add together.
\( R_{total} = 4000 + 5000 = 9000\,\Omega \)

(e)(ii) 0.027 A

\( I = \dfrac{V}{R} = \dfrac{240}{9000} \approx 0.027\,\text{A} \)

Question 10

Fig. 10.1 is a diagram showing part of the carbon cycle.
(a) Name the process occurring at X in Fig. 10.1.
(b) When deforestation occurs there is a build-up of carbon dioxide in the atmosphere.
(i) List two other negative effects of deforestation.
(ii) Stopping deforestation will reduce the build-up of carbon dioxide in the atmosphere.
Use the information in Fig. 10.1 to suggest two other ways we can reduce the build-up of carbon dioxide in the atmosphere.
(c) Table 10.1 shows some features of respiration and photosynthesis.
Place ticks (✓) in the boxes to show the features of each process.
(d) Water is one of the substances required by plants and is also lost by plants during transpiration.
(i) State the name of the vessels that transport water through a plant.
(ii) Complete the sentences to define the term transpiration.
Transpiration is loss of water vapour from plant ………….. by evaporation of water at the surfaces of the mesophyll ………….. followed by diffusion of water vapour through the ………….. .

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

• Topic B18.3 — Carbon cycle (Part (a))
• Topic B19.1 — Human influences on ecosystems/deforestation (Part (b))
• Topic B6.1 / B12.1 — Photosynthesis and respiration (Part (c))
• Topic B8.3 — Transpiration (Part (d))

▶️ Answer/Explanation

(a) Fossilisation

Dead organisms that are buried and subjected to heat and pressure over millions of years are gradually converted into fossil fuels.
This long-term process is called fossilisation.

(b)(i) Any two from: habitat destruction, extinction, loss of soil, flooding

Removing trees destroys habitats, which can lead to species extinction.
It also exposes soil, leading to erosion and an increased risk of flooding.

(b)(ii) Replant/plant more trees; reduce combustion of fossil fuels/emissions by industry

Planting more trees increases the rate of photosynthesis, removing more \(\text{CO}_2\) from the atmosphere.
Reducing fossil fuel combustion in industry lowers the amount of \(\text{CO}_2\) released in the first place.

(c)

Photosynthesis uses light energy to convert \(\text{CO}_2\) and water into carbohydrates and oxygen.
Respiration breaks down glucose using oxygen, releasing carbon dioxide as a waste product, and does not require light.

(d)(i) Xylem

Xylem vessels carry water and dissolved minerals from the roots up through the plant to the leaves.

(d)(ii) Leaves; cells; stomata

Water vapour is lost from the leaves.
It evaporates from the surfaces of the mesophyll cells inside the leaf.
It then diffuses out of the leaf through the stomata.

Question 11

(a) Table 11.1 shows the melting points of some Group I elements.
Predict the melting point of potassium.
(b) Potassium reacts with chlorine to make potassium chloride.
Potassium chloride is an ionic compound.
Fig. 11.1 shows the electronic structure of a potassium atom and of a chlorine atom.
(i) Complete the diagrams in Fig. 11.2 to show the ions in potassium chloride.
(ii) Balance the symbol equation for the reaction between potassium and chlorine.
\(\ldots\text{K} + \text{Cl}_2 \rightarrow \ldots\text{KCl}\).
(c) Universal indicator is added to water.
The water is neutral.
(i) State the pH of the water.
(ii) A teacher reacts potassium with the water.
Potassium hydroxide solution is made in the reaction.
Suggest the pH of the resulting potassium hydroxide solution and the colour of the universal indicator.
(iii) The teacher reacts lithium with water.
Compare the reactivity of lithium with the reactivity of potassium.

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

• Topic C8.2 — Group I elements, trends, ionic bonding, acids and alkalis (Parts (a), (b), (c))

▶️ Answer/Explanation

(a) Any value between 45 °C and 90 °C (e.g. approx. 63 °C)

Melting points decrease steadily down Group I: lithium (181) → sodium (98) → rubidium (39) → caesium (28).
Potassium’s melting point must lie between sodium’s (98 °C) and rubidium’s (39 °C), so any value in the range 45–90 °C is accepted.

(b)(i) Potassium ion: 2.8.8, charge +; chloride ion: 2.8.8, charge −

Potassium loses its one outer electron to form a \(\text{K}^+\) ion with a stable 2.8.8 electron arrangement.
Chlorine gains one electron to form a \(\text{Cl}^-\) ion, also with a stable 2.8.8 arrangement.

(b)(ii) \(2\text{K} + \text{Cl}_2 \rightarrow 2\text{KCl}\)

Two potassium atoms are needed to react with one chlorine molecule (\(\text{Cl}_2\)), producing two formula units of KCl, balancing the atoms on both sides.

(c)(i) pH 7

Neutral water has a pH of exactly 7.

(c)(ii) Any value between 7 and 14; purple

Potassium hydroxide is a strong alkali, giving the solution a high pH (up to 14).
Universal indicator turns purple in strongly alkaline solutions.

(c)(iii) Lithium reacts less violently / lithium is less reactive than potassium

Reactivity of Group I metals increases down the group.
Since lithium is above potassium, it is less reactive and reacts with water less vigorously.

Question 12

(a) An astronaut travels to the Moon in a spacecraft.
The weight of the spacecraft at take-off is \(25\,000\,000\,\text{N}\). When the spacecraft blasts off from Earth, it is pushed upwards by a force of \(32\,000\,000\,\text{N}\).
Calculate the resultant upward force on the spacecraft.
(b) The spacecraft has solar panels to gather energy from the Sun. This energy is stored in batteries on the spacecraft.
(i) Complete the sentences to describe the energy conversion that takes place.
The Sun’s light energy is transformed into ………… energy by the solar panels.
This energy is stored as ………… energy in the batteries.
(ii) Solar energy is a renewable energy source.
State one other renewable energy source.
(c) The spacecraft travels \(386\,000\,\text{km}\) from Earth to the Moon in 72 hours.
Calculate the average speed of the spacecraft in km/s. Show your working.
(d) On Earth, the astronaut has a mass of 80 kg and a weight of 800 N. On the Moon the astronaut has a mass of 80 kg and a weight of 135 N.
Describe the difference between mass and weight.
(e) The astronaut communicates with Earth using radio waves.
(i) Fig. 12.1 shows an incomplete electromagnetic spectrum. Place radio waves in the correct place.
(ii) Explain why it is not possible for the astronaut to communicate with Earth using sound waves.
(iii) Fig. 12.2 shows a sound wave. Label with a double-headed arrow one wavelength of the sound wave.

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

• Topic P1.5.1 — Resultant forces (Part (a))
• Topic P1.6.1/P1.6.3 — Energy conversion and renewable energy (Part (b))
• Topic P1.2 — Speed (Part (c))
• Topic P1.3 — Mass and weight (Part (d))
• Topic P3.3/P3.4 — Electromagnetic spectrum and sound (Part (e))

▶️ Answer/Explanation

(a) 7,000,000 N

\( \text{Resultant force} = \text{upward force} – \text{weight} \)
\( \text{Resultant force} = 32\,000\,000 – 25\,000\,000 = 7\,000\,000\,\text{N} \)

(b)(i) Electrical; chemical potential

Solar panels convert the Sun’s light energy directly into electrical energy via the photovoltaic effect.
This electrical energy is then stored as chemical potential energy in the batteries.

(b)(ii) Wind / hydroelectric power / tidal / geothermal

These are all renewable sources because they are naturally replenished and do not deplete finite resources.

(c) 1.49 km/s

\( 72\,\text{hours} = 72 \times 60 \times 60 = 259\,200\,\text{s} \)
\( \text{speed} = \dfrac{386\,000}{259\,200} \approx 1.49\,\text{km/s} \)

(d)

Mass is the actual amount of matter contained in a body, measured in kg, and it does not change with location.
Weight is the force exerted on that mass due to gravity, so it varies depending on the gravitational field strength — hence the astronaut weighs less on the Moon despite having the same mass.

(e)(i) Radio waves placed in the right-hand box

The electromagnetic spectrum is arranged by increasing frequency from radio waves to gamma rays.
Radio waves have the longest wavelength and lowest frequency, so they belong at the far right (opposite end from X-rays/gamma rays).

(e)(ii) Sound waves need a medium and cannot travel through a vacuum

Sound is a mechanical wave that requires particles of a medium (solid, liquid, or gas) to transmit vibrations.
Space is a vacuum, containing no particles, so sound cannot travel between the spacecraft and Earth.

(e)(iii) Double-headed arrow from peak to peak (or trough to trough)

One wavelength is the distance covered by one complete wave cycle.
This is shown by marking the distance between two consecutive peaks, or equivalently, two consecutive troughs.

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