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Question 1

(a) Fig. 1.1 is a diagram of the female reproductive system.
(i) State the letter from Fig. 1.1 that shows the part where:
  • fertilisation occurs,
  • gametes are produced.
(ii) Identify the part labelled B in Fig. 1.1.
(b) Fig. 1.2 shows a student’s description of fertilisation in humans.
The description of fertilisation in humans is incorrect.
Circle the two incorrect words in the description in Fig. 1.2.
(c) Fig. 1.3 shows how the thickness of the uterus lining changes during the menstrual cycle.
Table 1.1 represents the days during an average menstrual cycle.
(i) Place ticks (✓) in Table 1.1 to show the days when the uterus lining is shed.
(ii) Place crosses (✗) in Table 1.1 to show the days when the uterus lining is at its thickest.
(d) Complete the sentence to define the term reproduction.
Reproduction is the processes that make ……………… of the same ……………… of organism.

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

• Topic B15.4 — Sexual reproduction in humans (Parts (a), (b), (c))
• Topic B15.2 — Sexual reproduction (Part (d))

▶️ Answer/Explanation

(a)(i) Fertilisation occurs at A; gametes are produced at E

Fertilisation occurs in the oviduct (fallopian tube), labelled A.
Gametes (eggs) are produced in the ovaries, labelled E.

(a)(ii) Uterus

Part B is the uterus.
This is the organ where a fertilised egg implants and develops during pregnancy.

(b) “separation” and “ovary” are incorrect

Fertilisation is the fusion, not separation, of nuclei.
It involves a sperm cell and an egg cell, not an ovary cell.

(c)(i) Ticks under days 1–4

The uterus lining is shed during menstruation.
Fig. 1.3 shows the lining thickness falling sharply over days 1–4.

(c)(ii) Crosses under days 24–26

The lining reaches its maximum thickness just before the next menstrual period begins.
This corresponds to days 24–26 on the graph.

(d) More; kind

Reproduction is the process that makes more of the same kind of organism.
This ensures the continuation of a species across generations.

Question 2

(a) Magnesium is in Period 3 of the Periodic Table.
(i) Use the Periodic Table to name the noble gas in Period 3 of the Periodic Table.
(ii) Describe the change in metallic character across Period 3.
(b) Magnesium reacts with carbon dioxide.
Magnesium oxide and carbon are made.
(i) Write the word equation for this reaction.
…………………. + …………………. → …………………. + ………………….
(ii) The reaction between magnesium and carbon dioxide is exothermic. Explain what is meant by an exothermic reaction.
(c) An atom of magnesium has a proton number (atomic number) of 12 and a nucleon number (mass number) of 24.
Complete Table 2.1 to show the names and numbers of the two particles contained in the nucleus of this magnesium atom.
(d) Magnesium carbonate reacts with dilute hydrochloric acid to make aqueous magnesium chloride.
(i) Magnesium carbonate is insoluble in water.
State the separation technique used to remove magnesium carbonate from a mixture of magnesium carbonate and water.
(ii) Magnesium chloride dissolves in water.
State the separation technique used to obtain solid magnesium chloride from a solution of magnesium chloride.
(iii) Magnesium carbonate reacts with dilute hydrochloric acid.
Complete the balanced symbol equation for this reaction.
\( \text{MgCO}_3 + \text{……HCl} \rightarrow \text{MgCl}_2 + \text{CO}_2 + \text{H}_2\text{O} \)
(iv) Dilute hydrochloric acid contains aqueous chloride ions. State the test for aqueous chloride ions and give the observation for a positive result.
(e) Table 2.2 shows the composition of a magnesium alloy.
(i) Calculate the mass of magnesium in 500 kg of the alloy.
(ii) The alloy is used in aircraft bodies. Suggest why the alloy of magnesium is used rather than pure magnesium for aircraft bodies.

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

• Topic C8.1 — Arrangement of elements (Part (a))
• Topic C6.3 — Redox (Part (b))
• Topic C2.2 — Atomic structure and the Periodic Table (Part (c))
• Topic C12.4 — Separation and purification (Part (d)(i), (ii))
• Topic C7.1 — The characteristic properties of acids and bases (Part (d)(iii))
• Topic C12.5 — Qualitative analysis (Part (d)(iv))
• Topic C9.3 — Alloys and their properties (Part (e))

▶️ Answer/Explanation

(a)(i) Argon

Noble gases are found in Group 18 (Group VIII) of the Periodic Table.
In Period 3, the noble gas is argon (Ar).

(a)(ii) Metallic character decreases across Period 3 (metal to non-metal)

Moving left to right, elements become less metallic and more non-metallic.
The tendency to lose electrons decreases while the tendency to gain electrons increases.

(b)(i) Magnesium + carbon dioxide → magnesium oxide + carbon

Magnesium displaces carbon from carbon dioxide because magnesium is more reactive.
The word equation shows reactants converting to the two named products.

(b)(ii) Thermal energy is released to the surroundings

An exothermic reaction releases energy, usually as heat, to the surroundings.
This causes the temperature of the surroundings to rise.

(c) Protons = 12; Neutrons = 12

Proton number = number of protons = 12.
Nucleon number − proton number = neutrons: \( 24 – 12 = 12 \).

(d)(i) Filtration

Filtration separates an insoluble solid from a liquid.
Magnesium carbonate remains on the filter paper while water passes through.

(d)(ii) Crystallisation

Crystallisation obtains a solid from a solution by evaporating the solvent.
Solid magnesium chloride crystals form as the water evaporates.

(d)(iii) \( \text{MgCO}_3 + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{CO}_2 + \text{H}_2\text{O} \)

Balancing chlorine and hydrogen atoms requires 2 moles of HCl.
This balances both sides of the equation for all elements.

(d)(iv) Test: add acidified (aqueous) silver nitrate; Observation: white precipitate forms

Silver nitrate reacts with chloride ions to form insoluble silver chloride.
The white precipitate confirms the presence of chloride ions.

(e)(i) 470 kg

Mass of magnesium = 94% of 500 kg.
\( 0.94 \times 500 = 470 \, \text{kg} \).

(e)(ii) The alloy is stronger and/or less dense than pure magnesium

Alloys are generally stronger and more durable than pure metals.
A lighter, stronger material is ideal for aircraft bodies, improving strength while reducing weight.

Question 3

(a) A torch (flashlight) contains four cells, a lamp and a switch connected in series.
(i) Draw a circuit diagram for the torch using standard electrical symbols.
(ii) The potential difference (p.d.) across the lamp is \(6.0 \, \text{V}\) when the switch is closed.
The resistance of the lamp is \(5.0 \, \Omega\) when lit.
Calculate the current in the lamp.
(iii) Two lamps each of resistance \(5.0 \, \Omega\) are connected together in a series circuit.
Calculate the combined resistance of the two lamps in series.
(b) Choose words from the list to complete the sentences to describe the energy transfers that occur when the torch is switched on. You may use each word once, more than once or not at all.
chemical     elastic     electrical     gravitational     light     nuclear    sound     thermal
The energy stored in the cells is ……………… potential energy.
This energy is transferred into ……………… energy which passes through the lamp.
The useful energy from the lamp is ……………… energy.
Some energy is wasted as ……………… energy.
(c) Solar energy is sometimes used to power torches. Solar energy is a renewable energy resource.
State two other renewable energy resources.
(d) Fig. 3.1 shows a ray of light from a torch shining on a mirror.
(i) State the name of line XZ.
(ii) State what happens to the ray of light at point X on Fig. 3.1.

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

• Topic P4.3.1 — Circuit diagrams and symbols (Part (a)(i))
• Topic P4.2.4 — Ohm’s law (Part (a)(ii))
• Topic P4.3.2 — Series circuits (Part (a)(iii))
• Topic P1.6.1 — Energy stores and transfers (Part (b))
• Topic P1.6.3 — Renewable energy resources (Part (c))
• Topic P3.2.1 — Reflection of light (Part (d))

▶️ Answer/Explanation

(a)(i) Circuit diagram: 4 cells, 1 lamp, 1 switch, all in series

Standard symbols are used for cells, lamp, and switch.
All four cells, the lamp, and the switch must be connected in a single loop (series).

(a)(ii) 1.2 A

Using Ohm’s law, \( I = \dfrac{V}{R} \).
\( I = \dfrac{6.0}{5.0} = 1.2 \, \text{A} \).

(a)(iii) 10.0 Ω

In series, total resistance is the sum of individual resistances.
\( R_{total} = 5.0 + 5.0 = 10.0 \, \Omega \).

(b) chemical; electrical; light; thermal

Energy stored in cells is chemical potential energy.
It converts to electrical energy in the circuit, producing useful light energy and wasted thermal energy.

(c) Any two: wind, hydroelectric power, tidal, geothermal, wave

Wind energy is harnessed using wind turbines.
Hydroelectric power is generated from the movement of water, such as in dams.

(d)(i) Normal

Line XZ is the normal.
It is an imaginary line drawn perpendicular to the mirror’s surface at the point of incidence.

(d)(ii) Reflection

At point X, the light ray is reflected off the mirror.
The angle of incidence equals the angle of reflection, both measured from the normal.

Question 4

(a) Fig. 4.1 is a diagram of the heart.
(i) Identify the parts labelled B, C and D in Fig. 4.1.
(ii) Describe the function of the structure labelled E.
(iii) The blood vessel labelled A is an artery.
Describe the role of arteries in the circulatory system.
(b) Name two of the main components of blood.
(c) A student measures their heart rate over a 24-hour period.
Fig. 4.2 shows a graph of the results.
(i) Calculate the difference in heart rate between the minimum and maximum heart rate seen in Fig. 4.2.
(ii) At 10:00 hours the student attends a fitness class.
Estimate the length of time of the fitness class.
(iii) At 18:00 hours the student was scared by a snake.
Describe and explain the result seen at 18:00 hours in Fig. 4.2.
Use ideas about hormones in your answer.

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

• Topic B9.2 — Heart (Part (a)(i), (ii))
• Topic B9.3 — Blood vessels (Part (a)(iii))
• Topic B9.4 — Blood (Part (b))
• Topic B13.2 — Hormones (Part (c))

▶️ Answer/Explanation

(a)(i) B: septum; C: muscle/muscle wall; D: ventricles

The septum (B) divides the left and right sides of the heart.
The muscle wall (C) contracts to pump blood, and the ventricles (D) are the lower chambers of the heart.

(a)(ii) Ensures one-way flow of blood

Structure E is a valve.
Valves prevent the backflow of blood, keeping it moving in one direction through the heart.

(a)(iii) Transport blood away from the heart

Arteries carry oxygenated blood away from the heart to the rest of the body (except the pulmonary artery).
They have thick, muscular, elastic walls to withstand high pressure.

(b) Any two: red blood cells, white blood cells, platelets, plasma

Red blood cells transport oxygen.
White blood cells are involved in immunity, while plasma is the liquid component that carries dissolved substances.

(c)(i) 110 bpm

Maximum heart rate on the graph is approximately 180 bpm.
Minimum heart rate is approximately 70 bpm, giving a difference of \( 180 – 70 = 110 \, \text{bpm} \).

(c)(ii) 4 hours

The heart rate rises sharply at 10:00 and remains elevated before falling back to resting level.
This elevated period lasts approximately 4 hours, from around 10:00 to 14:00.

(c)(iii) Rapid increase in heart rate (by about 70 bpm, to around 140 bpm) due to adrenaline release

Fear triggers the release of adrenaline from the adrenal glands.
Adrenaline increases heart rate rapidly, preparing the body for a “fight or flight” response.

Question 5

(a) Potassium is a very reactive metal.
Potassium is stored under oil as shown in Fig. 5.1.
Suggest two reasons why potassium is stored under oil.
(b) Potassium has a proton number of 19.
Complete Fig. 5.2 to show the electronic structure of a potassium atom.
The inner shell electrons have been drawn for you.
(c) Solid potassium reacts with chlorine gas to make solid potassium chloride.
(i) Describe the differences between the structures of a solid and a gas in terms of particle separation and particle arrangement.
(ii) When potassium reacts with chlorine, potassium atoms become potassium ions. Describe what happens to a potassium atom when it becomes a potassium ion.
(iii) Potassium and chlorine are elements. Potassium chloride is a compound. Describe the difference between an element and a compound.
(d) The maximum mass of potassium chloride that dissolves in 100 cm\(^3\) of water is called the solubility of potassium chloride.
Fig. 5.3 shows the solubility of potassium chloride in water at different temperatures.
(i) Describe the trend in solubility shown in Fig. 5.3.
(ii) State the mass of potassium chloride that dissolves in 100 cm\(^3\) of water at \(45\,^\circ\text{C}\).

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

• Topic C9.4 — Reactivity series (Part (a))
• Topic C2.2 — Atomic structure and the Periodic Table (Part (b))
• Topic C1.1 — Solids, liquids and gases (Part (c)(i))
• Topic C2.4 — Ions and ionic bonds (Part (c)(ii))
• Topic C2.1 — Elements, compounds and mixtures (Part (c)(iii))
• Topic C12.4 — Separation and purification (Part (d))

▶️ Answer/Explanation

(a) To prevent potassium reacting with air/oxygen; to prevent potassium reacting with water vapour

Potassium is highly reactive and reacts readily with oxygen and moisture in the air.
Storing it under oil forms a barrier that prevents contact with these substances.

(b) Electronic structure: 2, 8, 8, 1

Potassium has 19 electrons in total, with 2 already drawn in the inner shells.
The remaining electrons fill the shells as 8, 8, 1 (outer shell has 1 electron).

(c)(i) Particle separation: close/touching in a solid, far apart in a gas. Particle arrangement: regular in a solid, random in a gas

In a solid, particles are closely packed and arranged in a regular, fixed pattern.
In a gas, particles are far apart and move randomly with no fixed arrangement.

(c)(ii) The potassium atom loses one electron

A potassium atom has one electron in its outer shell.
It loses this electron to form a positively charged potassium ion, \( \text{K}^+ \).

(c)(iii) An element contains only one type of atom; a compound contains two or more elements/different atoms chemically combined

Elements consist of a single type of atom, such as potassium or chlorine alone.
Compounds form when different elements chemically bond together, such as potassium chloride.

(d)(i) Solubility increases as temperature rises

The graph in Fig. 5.3 shows an upward trend.
As temperature increases, more potassium chloride can dissolve in the same volume of water.

(d)(ii) 41 g (±1)

Reading from the graph at \( 45\,^\circ\text{C} \) gives a solubility value of approximately 41 g.
This is read by drawing a vertical line up from 45 °C to the curve, then horizontally to the y-axis.

Question 6

(a) Fig. 6.1 shows the speed-time graph for the journey of a train travelling between two stations.
(i) Use letters from the graph in Fig. 6.1 to identify the two stations on the train’s journey.
(ii) On Fig. 6.1, label with a cross (X) a part of the journey when the train is accelerating.
(iii) Use Fig. 6.1 to calculate the total distance travelled by the train on this journey.
(b) Fig. 6.2 shows the forces acting on the train when it travels at a constant speed.
(i) State which force, P, Q, R or S, is the weight of the train.
(ii) Compare the magnitude and the direction of forces Q and S when the train is travelling at a constant speed.

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

• Topic P1.2 — Speed, distance and speed-time graphs (Part (a))
• Topic P1.2 — Forces and motion at constant speed (Part (b))

▶️ Answer/Explanation

(a)(i) A and D

The train’s speed is zero at point A, the starting station.
The speed returns to zero again at point D, the final station.

(a)(ii) X placed anywhere between A and B, or between C and D

Acceleration occurs where the speed is increasing on the graph.
This happens during the rising sections A–B and C–D.

(a)(iii) 2500 m

Total distance is the area under the speed-time graph.
Area of triangle A–B \( = \frac{1}{2} \times 100 \times 20 = 1000 \, \text{m} \), area of rectangle B–C \( = 50 \times 20 = 1000 \, \text{m} \), and area of triangle C–D \( = \frac{1}{2} \times 50 \times 20 = 500 \, \text{m} \), giving a total of \( 1000 + 1000 + 500 = 2500 \, \text{m} \).

(b)(i) R

Weight always acts vertically downward due to gravity.
In Fig. 6.2, this direction corresponds to force R.

(b)(ii) Equal in magnitude, opposite in direction

At constant speed, the resultant force on the train is zero.
So the driving force Q must be equal in size but opposite in direction to the frictional/resistive force S.

Question 7

(a) A scientist investigates the effect of immersion of plant cells in different concentrations of sugar solution.
Fig. 7.1 shows the appearance of the plant cells before immersion.
Beakers A–D contain different concentrations of sugar solution.
Some plant cells are placed in each solution and left for an hour.
Table 7.1 shows the results.
(i) Describe the change seen to the plant cells placed in beaker A in Table 7.1.
(ii) Suggest which beaker contains the solution with the same concentration as the plant cells. Give a reason for your answer.
(b) Water enters the plant through the root hair cells. Complete the flow chart to show the pathway of water through the cells of a plant.
root hair cell → ……………………………… → xylem → ………………………………
(c) Xylem transports water through the plant stem.
Name the tissue that transports sugars through the plant stem.
(d) Water and carbon dioxide are raw materials required for photosynthesis.
Name two other requirements for photosynthesis.
(e) Protecting land from deforestation helps stop the loss of soil.
This is because tree roots hold the soil together, stopping it being washed away.
Suggest two other benefits of protecting land from deforestation.

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

• Topic B3.2 — Osmosis in plant cells (Part (a))
• Topic B8.1 — Transport in plants (Part (b), (c))
• Topic B6.1 — Photosynthesis (Part (d))
• Topic B19.1 — Human influences on ecosystems (Part (e))

▶️ Answer/Explanation

(a)(i) The cell swells/increases in size

Beaker A contains a dilute sugar solution, less concentrated than the cell contents.
Water moves into the cell by osmosis, causing it to swell.

(a)(ii) Beaker B; the cells have not changed

In beaker B, the plant cells show no visible change in Table 7.1.
This indicates no net movement of water, meaning the solution concentration equals the cell’s internal concentration.

(b) root hair cell → root cortex cell → xylem → mesophyll cell

Water travels from the root hair cell into the root cortex cells.
It then moves into the xylem for transport up the plant, finally reaching the mesophyll cells in the leaf.

(c) Phloem

Phloem tissue transports dissolved sugars produced during photosynthesis.
This process is called translocation.

(d) Light; chlorophyll

Light provides the energy needed to drive photosynthesis.
Chlorophyll is the pigment that absorbs this light energy within chloroplasts.

(e) Any two: prevents extinction of animals; conserves habitats; protects food sources; maintains food chains/webs; maintains wood/fuel resources; protects natural beauty

Deforestation destroys habitats, threatening animal species with extinction.
Protecting forests also maintains food chains and preserves natural resources such as wood.

Question 8

(a) Fig. 8.1 shows the electrolysis of molten lead(II) bromide.
(i) State the name of the electrode where lead is made.
(ii) Explain why an orange gas is seen above the molten lead(II) bromide.
(b) Lead is extracted from lead oxide by reaction with carbon.
The equation for the reaction is shown.
lead oxide + carbon → lead + carbon dioxide
Name the substance that is oxidised and the substance that is reduced in this reaction.
(c) Lead is a metal.
Suggest one test to show that lead is a metal.
(d) Lead reacts very slowly with dilute sulfuric acid.
State two ways of increasing the rate of reaction between lead and dilute sulfuric acid.

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

• Topic C4.1 — Electrolysis (Part (a))
• Topic C6.3 — Redox (Part (b))
• Topic C9.1 — Properties of metals (Part (c))
• Topic C6.2 — Rate of reaction (Part (d))

▶️ Answer/Explanation

(a)(i) Cathode

Positive lead ions (\( \text{Pb}^{2+} \)) are attracted to the negative electrode.
They gain electrons there and are reduced to lead metal.

(a)(ii) Bromine (vapour) is released

Bromide ions (\( \text{Br}^- \)) lose electrons at the anode, forming bromine gas (\( \text{Br}_2 \)).
Bromine gas has a distinctive orange colour.

(b) Substance oxidised: carbon; substance reduced: lead oxide

Carbon gains oxygen to form carbon dioxide, so it is oxidised.
Lead oxide loses oxygen to form lead metal, so it is reduced.

(c) Test for electrical conductivity

Metals are typically good conductors of electricity.
If lead conducts electricity, this supports the conclusion that it is a metal.

(d) Increase temperature; increase surface area of lead (or increase concentration of acid)

Raising the temperature gives particles more energy, increasing collision frequency and rate.
Increasing the surface area of the lead exposes more particles to the acid, also increasing the rate.

Question 9

Fig. 9.1 shows a tumble dryer.
In a tumble dryer, wet clothes are warmed and dried. Water on the clothes evaporates.
(a)(i) Describe the process of evaporation.
Use ideas about molecules in your answer.
(ii) During evaporation the water does not boil.
State the boiling point of water.
(iii) Inside the tumble dryer, water vapour changes into liquid water.
State the term used to describe a gas changing into a liquid.
(iv) Thermal energy passes through the metal casing of the tumble dryer.
State the method of thermal energy transfer through metals.
(b)(i) The tumble dryer is noisy and emits loud sound waves with a low pitch.
Describe the sound waves in terms of their amplitude and frequency.
(ii) State the lowest audible frequency for a healthy human ear.
State the unit of your answer.
(iii) The speed of sound in air is \(340 \, \text{m/s}\).
Calculate the time taken for a sound wave to travel \(85 \, \text{m}\).

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

• Topic P2.2.2 — Melting, boiling and evaporation (Part (a)(i), (ii), (iii))
• Topic P2.3.1 — Conduction (Part (a)(iv))
• Topic P3.4 — Sound (Part (b)(i), (ii), (iii))

▶️ Answer/Explanation

(a)(i) Faster-moving/more energetic molecules escape/leave the surface

Molecules in a liquid have a range of energies.
The most energetic molecules at the surface overcome intermolecular forces and escape as vapour, even below boiling point.

(a)(ii) 100 °C

This is the standard boiling point of water at atmospheric pressure.
At this temperature, water changes rapidly from liquid to gas throughout the bulk of the liquid.

(a)(iii) Condensation

Condensation describes a gas changing into a liquid.
Water vapour loses energy and molecules slow down enough to form liquid water.

(a)(iv) Conduction

Thermal energy transfer through a solid metal occurs by conduction.
Vibrating particles and free electrons transfer energy through the metal casing.

(b)(i) Large amplitude; low frequency

Amplitude relates to loudness, so loud sound has a large amplitude.
Frequency relates to pitch, so a low pitch corresponds to a low frequency.

(b)(ii) 20 Hz

The lowest audible frequency for a healthy human ear is approximately 20 Hz.
The unit for frequency is the hertz (Hz).

(b)(iii) 0.25 s

Time = distance ÷ speed.
\( t = \dfrac{85}{340} = 0.25 \, \text{s} \).

Question 10

(a) Fig. 10.1 is a diagram of the human alimentary canal and associated organs.
(i) Label Fig. 10.1 on the answer lines provided.
Choose words from the list.
  • anus
  • large intestine
  • mouth
  • pancreas
  • small intestine
  • stomach
(ii) Different processes occur in the alimentary canal.
Complete Table 10.1 using words from the list in (a)(i).
(b) Describe the role of chemical digestion in the alimentary canal.
(c) The boxes contain the beginnings and the endings of some sentences.
Join one sentence beginning to one sentence ending to define the term assimilation.

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

• Topic B7.2 — Digestive system (Part (a))
• Topic B7.3 — Digestion (Part (b))
• Topic B7.3 — Digestion, assimilation (Part (c))

▶️ Answer/Explanation

(a)(i) Labels: stomach; pancreas; small intestine

Fig. 10.1 shows three unlabelled organs pointed to by lines.
These correspond to the stomach, pancreas, and small intestine respectively.

(a)(ii) ingestion → mouth; egestion → anus; mechanical digestion → mouth/stomach

Ingestion is the taking in of food, which occurs at the mouth.
Egestion is the removal of undigested waste from the anus, while mechanical digestion (physical breakdown) occurs at the mouth or stomach.

(b) Large/insoluble food molecules are broken down into small/soluble molecules; ref. to absorption

Chemical digestion uses enzymes to break down large, insoluble molecules into smaller, soluble ones.
This allows the small molecules to be absorbed into the bloodstream through the walls of the small intestine.

(c) Assimilation is the movement of digested food molecules into the cells of the body, where they are used, becoming part of the cells

Only digested (soluble) molecules can be assimilated, not undigested food.
These molecules move into body cells and are used to build new cell material or release energy.

Question 11

Fig. 11.1 shows the structures of five compounds A, B, C, D and E.
(a) Use the letters A–E to identify all the hydrocarbon molecules.
(b) Use the letters A–E to identify the two products of the complete combustion of a hydrocarbon.
(c) Use Fig. 11.1 to name one greenhouse gas.
(d) Draw a dot-and-cross diagram to show the bonding in molecule E. Only show the outer shell electrons.
(e) Compound D is an unsaturated compound.
(i) State what is meant by the term unsaturated compound.
(ii) Name the process that produces smaller unsaturated hydrocarbon molecules from large saturated hydrocarbon molecules.
(iii) State the chemical test for an unsaturated hydrocarbon and give the observation for a positive result.

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

• Topic C11.4 — Alkanes (Part (a), (b), (c))
• Topic C2.5 — Simple molecules and covalent bonds (Part (d))
• Topic C11.4 — Alkanes (Part (e)(i))
• Topic C11.3 — Fuels (Part (e)(ii))
• Topic C11.5 — Alkenes (Part (e)(iii))

▶️ Answer/Explanation

(a) B, C and D

Hydrocarbons contain only carbon and hydrogen atoms.
B (methane), C (ethane), and D (ethene) fit this description, while A contains oxygen and E contains oxygen too.

(b) A and E

Complete combustion of a hydrocarbon produces carbon dioxide and water.
A represents \( \text{CO}_2 \) and E represents \( \text{H}_2\text{O} \).

(c) Carbon dioxide (or methane)

Carbon dioxide (compound A) is a well-known greenhouse gas that traps heat in the atmosphere.
Methane (compound B) is also a potent greenhouse gas.

(d) One shared pair of electrons between O and each H; oxygen has an octet

Oxygen has six outer shell electrons and shares one electron with each hydrogen atom.
This forms two single covalent bonds, giving oxygen a full outer shell of eight electrons.

(e)(i) Contains a carbon-carbon double (or triple) bond

Unsaturated compounds have at least one C=C double bond.
This allows them to undergo addition reactions, unlike saturated compounds.

(e)(ii) Cracking

Cracking breaks down large saturated hydrocarbon molecules using heat and a catalyst.
This produces smaller, more useful molecules, often including unsaturated hydrocarbons.

(e)(iii) Test: add aqueous bromine (bromine water); Observation: orange to colourless

Bromine water reacts with the double bond in an unsaturated hydrocarbon via an addition reaction.
This causes the bromine water to change from orange to colourless.

Question 12

(a) Electricity is generated in a nuclear power station by the nuclear fission of plutonium-239.
Describe what happens to a nucleus of plutonium-239 during nuclear fission.
(b) Plutonium-239 is an isotope of plutonium.
(i) State the meaning of isotope.
(ii) Plutonium-239 emits β-radiation.
Suggest how a small sample of plutonium-239 can be stored safely.
(iii) Plutonium-239 has a half-life of \(24\,000\) years.
A sample of plutonium-239 has a mass of \(800 \, \text{g}\).
Calculate the mass of plutonium-239 remaining after \(48\,000\) years.
(iv) State two differences between β-radiation and γ-radiation.
(v) Fig. 12.1 shows an incomplete electromagnetic spectrum.
Place γ-radiation in its correct place in Fig. 12.1.

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

• Topic P5.1 — Nuclear fission (Part (a))
• Topic C2.3 — Isotopes (Part (b)(i))
• Topic P5.2.5 — Safe storage and use of radioactive sources (Part (b)(ii))
• Topic P5.2.4 — Half-life calculations (Part (b)(iii))
• Topic P5.2.2 — Properties of radiation (Part (b)(iv))
• Topic P3.3 — Electromagnetic spectrum (Part (b)(v))

▶️ Answer/Explanation

(a) The nucleus splits

A neutron is absorbed by the plutonium-239 nucleus, making it unstable.
The unstable nucleus splits into two smaller nuclei, releasing energy and further neutrons.

(b)(i) Atoms of the same element with the same number of protons but different numbers of neutrons

Isotopes share the same atomic number since they have identical proton numbers.
They differ in mass number because they contain different numbers of neutrons.

(b)(ii) Store in a lead-lined container

Although the sample emits β-radiation, lead provides effective shielding.
This prevents the radiation from escaping and harming people nearby.

(b)(iii) 200 g

\( 48\,000 \) years represents 2 half-lives (\( 48\,000 \div 24\,000 = 2 \)).
Mass remaining: \( 800 \to 400 \) g (1st half-life) \( \to 200 \) g (2nd half-life).

(b)(iv) β is more ionising; γ is more penetrating (β is charged, γ is uncharged)

β-particles are charged and interact more strongly with matter, making them more ionising.
γ-rays are electromagnetic waves with no charge, allowing them to penetrate much further through materials.

(b)(v) γ-radiation placed in the left-hand box, before ultraviolet

The electromagnetic spectrum is ordered by increasing wavelength (decreasing frequency) from gamma to radio waves.
γ-radiation has the shortest wavelength and highest frequency, so it sits at the far left, before ultraviolet.

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