Question 1
(a) (i)-(ii) Subtopic: B6.1 Photosynthesis
(b) Subtopic: B5 Enzymes
(c) Subtopic: B6.1 Photosynthesis
(d) Subtopic: B6.1 Photosynthesis
A scientist investigates the effect of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis on the same plant.
Fig. 1.1 is a graph of the results.
(a) (i) Describe how light intensity affects the rate of photosynthesis as shown in Fig. 1.1.
▶️Answer/Explanation
As light intensity increases, the rate of photosynthesis initially increases. Then at higher light intensities, the rate of photosynthesis becomes constant (plateaus). This shows that light intensity is a limiting factor at lower intensities but not at higher intensities.
(ii) State the factor that is limiting the rate of photosynthesis at
- X
- Y
- Z
▶️Answer/Explanation
X: Light intensity (at low light intensity, increasing light increases photosynthesis)
Y: Carbon dioxide concentration (different CO₂ concentrations give different plateau levels)
Z: Temperature (different temperatures give different plateau levels)
(b) Photosynthesis is an enzyme-controlled reaction. The investigation is repeated at a temperature of 80 °C. State and explain how this will affect the rate of photosynthesis.
▶️Answer/Explanation
Photosynthesis would stop at 80°C. This is because the high temperature would denature the enzymes involved in photosynthesis. When enzymes are denatured, their active site changes shape so substrates can no longer bind, stopping the reaction.
(c) State the balanced equation for photosynthesis.
▶️Answer/Explanation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(d) Explain why chlorophyll is needed for photosynthesis.
▶️Answer/Explanation
Chlorophyll is needed because it absorbs light energy and transfers it to chemical energy. This energy is used to synthesize carbohydrates (glucose) from carbon dioxide and water during photosynthesis.
Question 2
(a) Subtopic: C3.2 Relative masses of atoms and molecules
(b) Subtopic: C7.3 Preparation of salts
(c) (i)-(ii) Subtopic: C3.3 The mole and the Avogadro constant
(d) Subtopic: C6.2 Rate of reaction
Ammonium sulfate is a fertiliser. The formula of ammonium sulfate is (NH4)2SO4.
(a) Calculate the relative formula mass, Mr, of ammonium sulfate.
[Ar; H, 1; N, 14; O, 16; S, 32].
▶️Answer/Explanation
Mr = (2×N) + (8×H) + (1×S) + (4×O)
= (2×14) + (8×1) + (1×32) + (4×16)
= 28 + 8 + 32 + 64
= 132
(b) Ammonium sulfate is made by reacting ammonia with sulfuric acid. Write a balanced symbol equation for this reaction.
▶️Answer/Explanation
2NH3 + H2SO4 → (NH4)2SO4
(c) Potassium sulfate, K2SO4, is another fertiliser. In an experiment, 22.4 g of potassium hydroxide, KOH, dissolved in distilled water, reacts with 19.6 g of sulfuric acid to make potassium sulfate.
(i) Calculate the number of moles of potassium hydroxide and the number of moles of sulfuric acid that react.
[Ar; H, 1; K, 39; O, 16; S, 32]
▶️Answer/Explanation
Moles of KOH = mass/Mr = 22.4 ÷ (39+16+1) = 22.4 ÷ 56 = 0.40 mol
Moles of H2SO4 = mass/Mr = 19.6 ÷ (2+32+64) = 19.6 ÷ 98 = 0.20 mol
(ii) Using your answers from part (c)(i), deduce the balanced symbol equation for the reaction. Show your working.
▶️Answer/Explanation
From (i), mole ratio KOH:H2SO4 = 0.40:0.20 = 2:1
Therefore equation is:
2KOH + H2SO4 → K2SO4 + 2H2O
(d) Ammonia is used in the manufacture of some fertilisers. Ammonia is made in the Haber process.
nitrogen + hydrogen ⇌ ammonia
Fig. 2.1 shows the percentage of ammonia made using different conditions of temperature and pressure.
The highest percentage of ammonia is made at 200 °C and 300 atmospheres pressure. However, in an ammonia factory, a temperature of 450 °C and 200 atmospheres pressure are used. Explain why. Use ideas about the percentage of ammonia made and the rate of reaction.
▶️Answer/Explanation
1. 450°C increases the rate of reaction (higher temperature gives particles more kinetic energy)
2. 200 atmospheres is safer and less expensive than 300 atmospheres
3. It’s a compromise – while 200°C gives higher yield, the rate is too slow for industrial production
4. At 450°C, the equilibrium yield is lower but is reached much faster
Question 3
(a) Subtopic: P3.2.2 Refraction of light
(b) (i)-(iii) Subtopic: P1.2 Motion
(a) A flea is a small insect. A student uses a magnifying glass to observe a flea. The magnifying glass produces a virtual image. Describe the difference between a real image and a virtual image.
▶️Answer/Explanation
Real Image: – Formed when light rays actually converge at a point – Can be projected onto a screen – Formed on the opposite side of the lens from the object Virtual Image: – Formed when light rays appear to diverge from a point (but don’t actually meet) – Cannot be projected onto a screen – Seen on the same side of the lens as the object
(b) (i) The flea jumps upwards from rest. The speed of the flea increases to 1.2 m/s in 0.001 s. State the difference between the terms speed and velocity.
▶️Answer/Explanation
Speed: – Scalar quantity (magnitude only) – Measures how fast an object is moving regardless of direction Velocity: – Vector quantity (magnitude AND direction) – Measures speed in a specific direction
(ii) Calculate the acceleration of the flea.
▶️Answer/Explanation
Using the formula: a = (v – u)/t Where: v = final velocity = 1.2 m/s u = initial velocity = 0 m/s (from rest) t = time = 0.001 s a = (1.2 – 0)/0.001 = 1.2/0.001 = 1200 m/s²
(iii) The flea has a mass of 0.0005 g. Calculate the force causing this acceleration.
▶️Answer/Explanation
1. Convert mass to kg: 0.0005 g = 0.0000005 kg 2. Use Newton’s Second Law: F = m × a = 0.0000005 kg × 1200 m/s² = 0.0006 N This extremely small force is typical for insect-scale movements.
Question 4
(a) (i) Subtopic: B9.4 Blood
(a) (ii) Subtopic: B3.1 Diffusion
(a) (iii) Subtopic: B11 Gas exchange in humans
(a) (iv) Subtopic: B15.4 Sexual reproduction in humans
(b) (i)-(ii) Subtopic: B16.2 Cell division
(a) Fig. 4.1 is a diagram of the blood vessels in the placenta of the mother and her fetus.
(i) Oxygen moves across the placenta from the mother’s blood to the blood of the fetus. Suggest the name of one substance that moves in the opposite direction.
▶️Answer/Explanation
Carbon dioxide (CO2) is the main waste product that moves from the fetus to the mother’s blood for removal. Other possible answers include urea (nitrogenous waste) or water.
(ii) Oxygen moves across the placenta by diffusion. Define diffusion.
▶️Answer/Explanation
Diffusion is the net movement of particles (molecules or ions) from a region of their higher concentration to a region of their lower concentration, down a concentration gradient. This process occurs due to the random movement of particles and does not require energy (it’s a passive process).
(iii) The placenta is a gas exchange surface. Suggest two features of the placenta that enable efficient gas exchange.
▶️Answer/Explanation
1) Large surface area: The placental villi provide extensive surface area for maximum diffusion
2) Thin membrane: The placental barrier is only 3.5-5μm thick, allowing rapid gas exchange
Other acceptable answers: Good blood supply (maintains concentration gradient), semi-permeable membrane (selective to gases), moist surface (allows gases to dissolve)
(iv) Name the part of the female reproductive system where
- the fetus develops
- fertilisation occurs
- the female gametes are released.
▶️Answer/Explanation
– Uterus (womb): Where the fetus develops during pregnancy
– Oviduct (Fallopian tube): Where fertilisation occurs (sperm meets egg)
– Ovary: Releases female gametes (ova/eggs) during ovulation
(b) The process of mitosis is used for growth.
(i) State two other uses of mitosis.
▶️Answer/Explanation
1) Tissue repair: Replaces damaged or dead cells (e.g., skin healing after cut)
2) Asexual reproduction: Some organisms reproduce by mitosis (e.g., bacteria, some plants)
Other possible answers: Replacement of worn-out cells (e.g., gut lining), embryonic development
(ii) The chromosomes inside the nuclei of cells produced by mitosis are different to those produced by meiosis. Describe two of these differences.
▶️Answer/Explanation
1) Chromosome number: – Mitosis produces diploid cells (2n – identical to parent cell) – Meiosis produces haploid cells (n – half the chromosome number) 2) Genetic variation: – Mitosis produces genetically identical cells – Meiosis produces genetically unique cells due to crossing over and independent assortment
Question 5
(a) (i)-(iii) Subtopic: C2.2 Atomic structure and the Periodic Table
(b) (i)-(iii) Subtopic: C2.3 Isotopes
(c) Subtopic: C2.4 Ions and ionic bonds
(a) Look at the list of atomic symbols.
Br Cu K Mg
N Ne Zn
Answer the following questions choosing from the list of atomic symbols. Each symbol can be used once, more than once or not at all.
(i) State the symbol for an element with a full outer shell of electrons.
▶️Answer/Explanation
Ne (Neon) – As a noble gas in Group 0, neon has a full outer electron shell (2,8 configuration).
(ii) State the symbol of the element with the electronic structure 2,8,2.
▶️Answer/Explanation
Mg (Magnesium) – Atomic number 12, with electron configuration 2,8,2 (Group 2 element).
(iii) State the symbols of two elements that form basic oxides.
▶️Answer/Explanation
K (Potassium) and Mg (Magnesium) – Metals form basic oxides (e.g., K2O, MgO). Other possible answers: Cu, Zn (transition metals that form basic oxides).
(b) The symbol of an isotope of oxygen is 188O. Fig. 5.1 shows the nucleus of an atom of 188O.
State the name of the particle shown by
▶️Answer/Explanation
(i) Proton – Positively charged subatomic particle that determines the atomic number (8 for oxygen).
(ii) Neutron – Neutral subatomic particle that contributes to mass number (18 – 8 = 10 neutrons in this isotope).
(iii) Draw a similar diagram to Fig. 5.1 to show a different isotope of oxygen.
▶️Answer/Explanation
Example: 168O would have: – 8 protons (●) – 8 neutrons (○)
Any isotope with proton number 8 but different neutron number is acceptable (e.g., 178O with 9 neutrons).
(c) Fig. 5.2 shows the electronic structures of a lithium atom and of a chlorine atom.
When lithium reacts with chlorine a lithium ion and a chloride ion are made. Draw dot-and-cross diagrams to show the electronic structures of a lithium ion and of a chloride ion.
Include the charge on each ion.
▶️Answer/Explanation
Lithium ion (Li+): – Single shell with 2 electrons (no outer electron) – Positive charge shown
Chloride ion (Cl–): – 2,8,8 configuration (gains 1 electron) – Negative charge shown
Note: Diagrams would show electron transfer from Li to Cl, with square brackets and charges.
Question 6
(a) (i)-(ii) Subtopic: P1.6.1 Energy
(b) Subtopic: P3.1 General properties of waves
(c) Subtopic: P2.2.2 Melting, boiling and evaporation
(d) Subtopic: P1.7 Pressure
(a) A boat has a mass of 2000 kg.
(i) State the kinetic energy of the boat when the boat is not moving.
▶️Answer/Explanation
0 J – Kinetic energy (KE) = ½mv². When stationary (v=0), KE=0.
(ii) Calculate the kinetic energy of the boat when it moves at a constant speed of 11 m/s.
▶️Answer/Explanation
Using KE = ½mv²:
= ½ × 2000 kg × (11 m/s)²
= 1000 × 121
= 121,000 J (or 121 kJ)
(b) The boat reaches the entrance to a harbour. Fig. 6.1 shows five wavefronts approaching the narrow harbour entrance.
On Fig. 6.1, draw two wavefronts after they pass through the harbour entrance.
▶️Answer/Explanation
Wavefronts should:
1) Curve outward in semicircles from the entrance (diffraction pattern)
2) Maintain equal wavelength between wavefronts
3) Show increased spacing as they spread out
Note: This demonstrates diffraction – waves spreading when passing through a gap.
(c) There is water on the deck of the boat. The water slowly evaporates. State two conditions that could change so that the water evaporates faster.
▶️Answer/Explanation
1) Increase temperature – More kinetic energy for water molecules to escape
2) Increase air flow – Removes water vapor, maintaining concentration gradient
Other valid answers: Reduce humidity, increase surface area (spread the water)
(d) A seabird of mass 1.2 kg lands on the deck of the boat. The total area of the seabird’s two feet in contact with the deck is 5.4 cm². Calculate the pressure exerted by the seabird on the deck when it is standing on two feet. The gravitational field strength g is 10 N/kg.
▶️Answer/Explanation
1) Calculate weight (force):
F = mg = 1.2 kg × 10 N/kg = 12 N
2) Calculate pressure:
P = F/A = 12 N / 5.4 cm²
= 2.22 N/cm² (accept 2.2)
Question 7
(a) (i)-(ii) Subtopic: B7.1 Diet
(b) Subtopic: B7.1 Diet
(c) Subtopic: B7.1 Diet
(d) Subtopic: B7.1 Diet
(e) Subtopic: B10 Diseases and immunity
(a) Table 7.1 shows the deficiency diseases of children of different ages admitted to a hospital.
(i) Describe the general trends seen in Table 7.1.
▶️Answer/Explanation
Key trends:
1) Both diseases show decreasing prevalence with increasing age
2) Marasmus is more common than kwashiorkor in all age groups
3) Cases drop sharply after 24 months (transition to solid foods)
4) By 37-48 months, both diseases become rare (only 1 case of kwashiorkor)
(ii) A total of 212 children were admitted to the hospital. Calculate the percentage of these children with kwashiorkor.
▶️Answer/Explanation
1) Total kwashiorkor cases = 19+16+3+1+1 = 40
2) Percentage = (40/212) × 100
= 18.87% (accept 19%)
(b) Treatment of kwashiorkor and marasmus includes an increase of one particular nutrient in the diet. State the name of this nutrient.
▶️Answer/Explanation
Protein – Both conditions are protein-energy malnutrition disorders:
• Kwashiorkor: Protein deficiency with some energy intake
• Marasmus: Severe deficiency of both protein and energy
(c) Fig. 7.1 is a photograph of a person with a deficiency disease called rickets.
State and explain the cause of the appearance of the person shown in Fig. 7.1.
▶️Answer/Explanation
Cause: Vitamin D deficiency
Effect:
1) Vitamin D is needed for calcium absorption
2) Without it, bones become soft and malformed
3) Weight-bearing causes characteristic bowing of legs
4> May also show swollen wrists/ankles and delayed growth
(d) Describe the importance of fibre in the diet.
▶️Answer/Explanation
Key functions:
1) Prevents constipation by adding bulk to feces
2) Helps maintain healthy gut microbiome
3) May reduce risk of bowel cancer
4) Can help control blood sugar levels
5) Contributes to satiety (feeling full)
(e) A diet containing too much fat can cause coronary heart disease. State two other risk factors for coronary heart disease.
▶️Answer/Explanation
Any two from:
• Smoking
• Lack of exercise
• High blood pressure
• Diabetes
• Obesity
• Family history/genetics
• High cholesterol
• Stress
• Age (risk increases with age)
Question 8
(a) Subtopic: C11.5 Alkenes
(b) Subtopic: C11.1 Formulas and terminology
(c) (i)-(ii) Subtopic: C11.6 Alcohols
Fig. 8.1 shows the structures of ethene and of ethanol.
(a) Ethene is an unsaturated hydrocarbon. Explain the meanings of the underlined words.
▶️Answer/Explanation
Unsaturated: Contains at least one carbon-carbon double bond (C=C) capable of undergoing addition reactions
Hydrocarbon: Compound consisting only of hydrogen and carbon atoms (no other elements present)
(b) Ethene has a simple molecular structure. Ethene does not dissolve in water. State one other physical property of substances with a simple molecular structure.
▶️Answer/Explanation
Any one of:
• Low boiling/melting points (weak intermolecular forces)
• Poor electrical conductivity (no free electrons/ions)
• Volatile (evaporate easily at room temperature)
• Usually gases/liquids at room temp (small molecules)
(c) Ethanol is an alcohol made by fermentation.
(i) Ethanol is used in alcoholic drinks. State another use for ethanol.
▶️Answer/Explanation
Any one of:
• Fuel (e.g., bioethanol in cars)
• Solvent (e.g., in perfumes, medicines)
• Antiseptic/disinfectant (kills microorganisms)
• Chemical feedstock (for making other compounds)
(ii) Describe how ethanol is made by fermentation.
▶️Answer/Explanation
Process:
1) Raw material: Sugar (e.g., glucose from crops) dissolved in water
2) Microorganism: Yeast added (contains enzyme zymase)
3) Conditions: Anaerobic (no oxygen), warm (20-40°C optimum)
4) Reaction: Yeast converts sugar → ethanol + CO2
5) Equation: C6H12O6 → 2C2H5OH + 2CO2
6) Purification: Distillation to concentrate the ethanol
Question 9
(a) (i)-(vi) Subtopic: P4.3.2 Series and parallel circuits
(b) (i)-(iii) Subtopic: P3.4 Sound
(a) Fig. 9.1 shows a bicycle with a front lamp F and a rear lamp R, powered by a 9V generator (dynamo).
Fig. 9.2 shows the circuit diagram for the lamps.
Lamp F has a resistance of 12 Ω and lamp R has a resistance of 6 Ω.
(i) Calculate the combined resistance of the two lamps in this circuit.
▶️Answer/Explanation
For parallel circuit:
1/Rtotal = 1/R1 + 1/R2
= 1/12 + 1/6
= 1/12 + 2/12 = 3/12 = 1/4
Rtotal = 4Ω
(ii) Lamp R is switched on. Show that the current in lamp R is 1.5A.
▶️Answer/Explanation
Using Ohm’s Law (V=IR):
I = V/R = 9V/6Ω = 1.5A
Note: In parallel circuits, each branch gets full voltage
(iii) Calculate the charge that passes through lamp R in 300 seconds. State the unit of your answer.
▶️Answer/Explanation
Q = I × t
= 1.5A × 300s
= 450 C (coulombs)
(iv) The generator supplies an alternating current to light the lamps. Describe the difference between alternating current (a.c.) and direct current (d.c.).
▶️Answer/Explanation
DC:
• Flows in one direction only
• Constant voltage (e.g., batteries)
AC:
• Regularly reverses direction (typically 50-60Hz)
• Voltage varies sinusoidally (e.g., mains electricity)
(v) State the useful energy transformation that occurs in the generator.
▶️Answer/Explanation
Kinetic energy → Electrical energy
(Bicycle motion turns the generator coil in a magnetic field)
(vi) State the useful energy transformation that occurs in the lamp.
▶️Answer/Explanation
Electrical energy → Light energy
(Plus some heat energy as byproduct)
(b) The generator is noisy and emits sound waves that pass through the air. Sound waves are longitudinal waves and visible light waves are transverse waves.
(i) Give one other example of a transverse wave.
▶️Answer/Explanation
Any of:
• Light/EM waves
• Water surface waves
• S-waves (seismic)
• Waves on a string
(ii) The sound waves pass through the air as a series of compressions (C) and rarefactions (R). Fig. 9.3 shows the positions of the compressions and rarefactions as the sound wave passes through the air.
On Fig. 9.3, mark one wavelength with a double headed arrow (↔).
▶️Answer/Explanation
Wavelength is:
Distance between two consecutive compressions (C to C)
OR
Distance between two consecutive rarefactions (R to R)
(iii) Describe how the distance between two compressions changes if the frequency of the sound wave increases.
▶️Answer/Explanation
Distance decreases
Because wavelength (λ) and frequency (f) are inversely related by v = fλ (where v is constant in same medium)
Question 10
(a)-(b) Subtopic: C10.1 Water
Increasing the concentration of nitrate ions in freshwater can cause eutrophication.
(a) State two sources of nitrate ions which cause water pollution.
▶️Answer/Explanation
Primary sources:
1) Agricultural fertilizers (washed into waterways)
2) Sewage/animal waste (contains nitrogen compounds)
Other acceptable answers:
• Industrial discharge
• Decomposing organic matter
• Atmospheric deposition (from burning fossil fuels)
(b) Complete the sentences to explain how an increase in nitrate ions in water causes eutrophication.
The increased availability of nitrate ions increases the growth of …… on the surface of the water. Underwater plants cannot receive light and so cannot photosynthesise. Underwater plants die and are broken down by …… . This causes an increase in …… respiration. Dissolved …… concentration in the water decreases, which causes aquatic organisms to die.
▶️Answer/Explanation
Eutrophication sequence:
1) Algae/phytoplankton (explosive growth called algal bloom)
2) Decomposers/bacteria/fungi (break down dead plants)
3) Aerobic (oxygen-using) respiration by decomposers
4) Oxygen depletion creates dead zones
Final result: Ecosystem collapse as fish and other organisms suffocate
Question 11
(a) (i)-(iii) Subtopic: C6.3 Redox
(b) Subtopic: C6.3 Redox
(c) Subtopic: C9.6 Extraction of metals
(a) The equations A, B, C and D represent four possible reactions.
equation A: Zn + Cu2+ → Zn2+ + Cu
equation B: Cu – 2e– → Cu2+
equation C: Cu2+ + 2e– → Cu
equation D: Cu2+ + 2OH– → Cu(OH)2
(i) State which equation A, B, C or D represents only oxidation.
▶️Answer/Explanation
Equation B – Shows copper losing electrons (oxidation is loss of electrons – OIL RIG). The copper atom (Cu) becomes a copper ion (Cu2+) by losing 2 electrons.
(ii) State which equation A, B, C or D represents both oxidation and reduction.
▶️Answer/Explanation
Equation A – This is a redox reaction where:
• Zn is oxidized (loses electrons to become Zn2+)
• Cu2+ is reduced (gains electrons to become Cu)
(iii) Copper is purified by electrolysis of copper sulfate solution. Fig. 11.1 shows the apparatus used.
State which equation A, B, C or D represents the reaction that takes place at the cathode.
▶️Answer/Explanation
Equation C – At the cathode (negative electrode), copper ions gain electrons (reduction) to form pure copper metal: Cu2+ + 2e– → Cu
(b) Copper is extracted from copper oxide by heating with carbon
2CuO + C → 2Cu + CO2
CuO acts as an oxidizing agent in this reaction. Define the term oxidizing agent.
▶️Answer/Explanation
Oxidizing agent: A substance that:
1) Causes oxidation of another substance
2) Gains electrons (is itself reduced)
3) In this reaction, CuO oxidizes carbon (C → CO2) while being reduced (Cu2+ → Cu)
(c) Aluminium is extracted from its purified ore by electrolysis. Fig. 11.2 shows the equipment that is used.
Describe how aluminium is extracted from its purified ore.
Include in your answer:
- the name of the compound containing aluminium
- what is made at each electrode.
▶️Answer/Explanation
Process details:
1) Compound: Molten aluminium oxide (Al2O3) dissolved in cryolite (lowers melting point)
2) At cathode (negative): Aluminium ions gain electrons → molten aluminium metal (Al3+ + 3e– → Al)
3) At anode (positive): Oxide ions lose electrons → oxygen gas (2O2- → O2 + 4e–)
4) Note: Graphite anodes gradually burn away (react with O2 to form CO2)
Question 12
(a) Subtopic: P2.3.2 Convection
(b) (i)-(ii) Subtopic: P5.2.2 The three types of nuclear emission
(c) Subtopic: P4.5.6 The transformer
(a) Fig. 12.1 shows a hot water storage tank in a house.
The water is heated by an electric heater placed near the bottom of the tank. Cold water enters at the bottom of the tank and hot water leaves at the top of the tank.
Explain why all the water in the tank is heated by convection.
▶️Answer/Explanation
Convection process:
1) Heated water at bottom expands, becoming less dense
2) Less dense warm water rises to the top
3) Cooler, denser water sinks to replace it
4) Creates convection current that circulates heat through entire tank
5) Hot water accumulates at top (where outlet is positioned)
(b) The house is fitted with a smoke detector. The smoke detector contains a radioactive isotope of americium-241. Americium-241 decays by α-particle emission.
(i) Explain why it’s safe to use this isotope near people.
▶️Answer/Explanation
Safety reasons:
1) Alpha particles have low penetration (stopped by few cm of air or paper)
2) Americium is sealed in detector (no inhalation risk)
3) Very small quantity used (typically 0.9 micrograms)
4) α-radiation doesn’t penetrate skin
(ii) Complete the α-decay equation for americium-241:
▶️Answer/Explanation
Balanced nuclear equation:
²⁴¹₉₅Am → ²³⁷₉₃Np + ⁴₂He
Key points:
• Mass number decreases by 4 (241→237)
• Atomic number decreases by 2 (95→93)
• Alpha particle is helium nucleus (⁴₂He)
(c) There is a rechargeable electric toothbrush in the bathroom of the house. Fig. 12.2 shows the electric toothbrush and the charger.
In the charger, there is a transformer that steps down the voltage from 220 V to 2.4 V. The primary coil of the transformer has 5000 turns.
Calculate the number of turns on the secondary coil.
▶️Answer/Explanation
Using transformer equation:
Vp/Vs = Np/Ns
220/2.4 = 5000/Ns
Ns = (5000 × 2.4)/220
= 12000/220
= 54.5 turns (accept 54 or 55)
Question 13
(a)-(b) Subtopic: C2.6 Giant covalent structures
(c) Subtopic: C9.3 Alloys and their properties
(d) Subtopic: C9.5 Corrosion of metals
Fig. 13.1 shows the structures of graphite and silicon(IV) oxide.
(a) Graphite is used as a lubricant. Explain why. Use ideas about the structure and bonding in graphite.
▶️Answer/Explanation
Lubrication properties due to:
1) Layered structure with weak London forces between layers
2) Layers can slide easily over one another
3) Strong covalent bonds within layers maintain structural integrity
4) No cross-links between layers allows smooth movement
(b) State one similarity between the structure of graphite and the structure of silicon(IV) oxide.
▶️Answer/Explanation
Both have:
• Giant covalent/network structures
• Strong covalent bonds throughout
• High melting points
• Atoms arranged in regular patterns
(c) Look at Fig. 13.2.
State the type of mixture shown by Fig. 13.2.
▶️Answer/Explanation
Alloy – A homogeneous mixture of:
• Two or more metals
• Or metal with non-metal (e.g., steel = iron + carbon)
Properties: Often stronger/harder than pure metals due to disrupted crystal structure
(d) Galvanised steel is made of steel which is coated in a layer of zinc. Explain why a galvanised steel gate does not rust.
▶️Answer/Explanation
Protection mechanisms:
1) Barrier protection: Zinc coating physically blocks air/water from reaching steel
2) Sacrificial protection: Zinc is more reactive than iron:
• Zinc oxidizes preferentially (Zn → Zn2+ + 2e–)
• Electrons flow to iron, preventing its oxidation
3) Zinc oxide layer is adherent and protective