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Question 1Natural Sciences (Biology, Chemistry, Physics)

In a human somatic cell, which process directly ensures that each daughter cell receives an identical set of chromosomes during mitosis?

  1. Homologous chromosomes pair and exchange segments during prophase I.
  2. Cytokinesis divides the cytoplasm equally between the two daughter cells.
  3. Sister chromatids separate and move to opposite poles after metaphase alignment. Correct answer.
  4. The nuclear envelope breaks down and reforms around each chromosome set.

Explanation

Mitosis preserves chromosome number because sister chromatids, which are exact copies formed during S phase, are pulled to opposite poles at anaphase. Homologous pairing is meiosis-specific, cytokinesis only partitions cytoplasm, and envelope breakdown does not itself segregate chromosomes.

Question 2Natural Sciences (Biology, Chemistry, Physics)

A solution of hydrochloric acid has a pH of 2 at 25 °C. What is the concentration of hydrogen ions, [H⁺], in this solution?

  1. 1 × 10⁻² mol dm⁻³ Correct answer.
  2. 0.5 mol dm⁻³
  3. 1 × 10⁻¹² mol dm⁻³
  4. 2 mol dm⁻³

Explanation

By definition, pH = −log₁₀[H⁺], so [H⁺] = 10^(−pH). Substituting pH 2 gives 10⁻² mol dm⁻³, which is 0.01 mol dm⁻³. The value 10⁻¹² would correspond to pOH 2 or pH 12, not pH 2.

Question 3Natural Sciences (Biology, Chemistry, Physics)

A car accelerates uniformly from rest to 20 m s⁻¹ in 5.0 s. What distance does it travel during this time?

  1. 50 m Correct answer.
  2. 20 m
  3. 100 m
  4. 4.0 m

Explanation

For uniform acceleration from rest, average velocity is half the final velocity, so v_avg = (0 + 20)/2 = 10 m s⁻¹. Distance = v_avg × t = 10 × 5.0 = 50 m. Using s = ut + ½at² with a = 4.0 m s⁻² gives the same result.

Question 4Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly describes the role of NADH in aerobic respiration?

  1. It directly phosphorylates ADP to ATP in the mitochondrial matrix.
  2. It transports pyruvate across the inner mitochondrial membrane.
  3. It donates electrons to the electron transport chain, which uses their energy to pump protons. Correct answer.
  4. It accepts electrons at the end of the chain and is reduced to water.

Explanation

NADH is a reduced coenzyme that carries electrons from glycolysis and the Krebs cycle to complex I of the electron transport chain. The energy released as electrons pass down the chain drives proton pumping, creating the gradient used by ATP synthase. Oxygen, not NADH, is the final electron acceptor.

Question 5Natural Sciences (Biology, Chemistry, Physics)

Which trend is observed across Period 3 from sodium to chlorine?

  1. Atomic radius increases because more electron shells are added.
  2. Metallic character increases because the elements lose electrons more readily.
  3. First ionisation energy generally increases because effective nuclear charge increases. Correct answer.
  4. Electronegativity decreases because the atoms become larger.

Explanation

Across a period, protons are added to the same shell, so effective nuclear charge rises and the outermost electron is held more tightly. This generally increases first ionisation energy and electronegativity while decreasing atomic radius and metallic character.

Question 6Natural Sciences (Biology, Chemistry, Physics)

Which structure is the primary site of absorption of digested nutrients in the human alimentary canal?

  1. The oesophagus
  2. The stomach lining
  3. The large intestine
  4. The small intestine Correct answer.

Explanation

The small intestine has a large surface area created by villi and microvilli, plus a rich blood supply, making it the main site of nutrient absorption. The stomach mainly stores and begins protein digestion, the oesophagus only transports food, and the large intestine mainly absorbs water and salts.

Question 7Natural Sciences (Biology, Chemistry, Physics)

A block of mass 2.0 kg is pulled along a rough horizontal surface by a horizontal force of 12 N. If the frictional force is 4.0 N, what is the acceleration of the block?

  1. 8.0 m s⁻²
  2. 6.0 m s⁻²
  3. 2.0 m s⁻²
  4. 4.0 m s⁻² Correct answer.

Explanation

The resultant horizontal force is 12 N − 4.0 N = 8.0 N. Newton's second law gives a = F_net/m = 8.0/2.0 = 4.0 m s⁻². Friction must be subtracted because it acts opposite to the applied force.

Question 8Natural Sciences (Biology, Chemistry, Physics)

Which molecule is the immediate energy currency used by most cellular processes?

  1. Glucose
  2. ATP Correct answer.
  3. Pyruvate
  4. NADPH

Explanation

ATP is the universal energy currency because hydrolysis of its terminal phosphate bond releases free energy that can be coupled to endergonic reactions. Glucose is a fuel, NADPH is a reducing agent in biosynthesis, and pyruvate is an intermediate of glycolysis.

Question 9Natural Sciences (Biology, Chemistry, Physics)

In a cross between two heterozygous pea plants (Tt × Tt), where T is tall and t is dwarf, what fraction of the offspring is expected to be dwarf?

  1. 3/4
  2. 1/4 Correct answer.
  3. 1/2
  4. 0

Explanation

A Tt × Tt cross gives a 1 TT : 2 Tt : 1 tt genotypic ratio. Only tt plants are dwarf, so one quarter of the offspring are expected to be dwarf. The dominant tall phenotype accounts for the remaining three quarters.

Question 10Natural Sciences (Biology, Chemistry, Physics)

A gas occupies 200 cm³ at 300 K and constant pressure. What volume will it occupy at 600 K?

  1. 100 cm³
  2. 200 cm³
  3. 400 cm³ Correct answer.
  4. 600 cm³

Explanation

Charles's law states that volume is directly proportional to absolute temperature at constant pressure, so V₂ = V₁ × T₂/T₁. Substituting gives 200 × 600/300 = 400 cm³. Doubling the absolute temperature doubles the volume.

Question 11Natural Sciences (Biology, Chemistry, Physics)

Which observation provides evidence that a chemical reaction has occurred when dilute hydrochloric acid is added to calcium carbonate?

  1. The mixture becomes warmer and the solid disappears.
  2. The calcium carbonate dissolves to form a clear solution.
  3. The pH of the solution rises above 7.
  4. Effervescence occurs and a gas that turns limewater milky is produced. Correct answer.

Explanation

Calcium carbonate reacts with hydrochloric acid to produce carbon dioxide, which is observed as effervescence and turns limewater milky. Dissolving alone is a physical change, warming is not specific to this reaction, and adding acid lowers rather than raises pH.

Question 12Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly explains why a transformer can change an alternating voltage but not a direct voltage?

  1. A direct current cannot flow through the primary coil because it has too much resistance.
  2. An alternating current has a higher frequency than a direct current, so it carries more energy.
  3. A direct current produces a constant magnetic flux, so no induced e.m.f. is generated in the secondary coil. Correct answer.
  4. A transformer only works when the secondary coil has more turns than the primary coil.

Explanation

Faraday's law requires a changing magnetic flux to induce an e.m.f. An alternating current continuously changes the flux in the core, whereas a steady direct current produces a constant flux and therefore no induced voltage in the secondary.

Question 13Natural Sciences (Biology, Chemistry, Physics)

A student measures the rate of photosynthesis in an aquatic plant by counting oxygen bubbles at different light intensities. Which change would most improve the reliability of the results?

  1. Change the temperature to see whether the trend persists.
  2. Count the bubbles for a longer time at each intensity.
  3. Repeat each light intensity several times and calculate a mean. Correct answer.
  4. Use a brighter lamp to increase the number of bubbles counted.

Explanation

Reliability concerns the consistency of repeated measurements. Taking several readings at each light intensity and averaging them reduces the effect of random variation. Increasing light or counting longer may improve precision but does not test repeatability.

Question 14Natural Sciences (Biology, Chemistry, Physics)

Which property of water is most directly responsible for its ability to dissolve many ionic compounds?

  1. Its low density when frozen
  2. Its high specific heat capacity
  3. Its high surface tension
  4. Its polarity and ability to form hydrogen bonds Correct answer.

Explanation

Water is a polar molecule with partial charges that attract and surround ions, hydrating them and keeping them in solution. Its high specific heat, surface tension and ice density are consequences of hydrogen bonding but do not directly explain ion dissolution.

Question 15Natural Sciences (Biology, Chemistry, Physics)

A particle with charge 3.0 × 10⁻⁶ C moves at 2.0 × 10⁵ m s⁻¹ perpendicular to a uniform magnetic field of 0.50 T. What is the magnitude of the magnetic force on the particle?

  1. 3.0 × 10⁻¹ N
  2. 1.5 × 10⁻¹ N
  3. 6.0 × 10⁻¹ N
  4. 0.30 N Correct answer.

Explanation

The magnetic force on a moving charge is F = qvB sin θ. With θ = 90°, sin θ = 1, so F = (3.0 × 10⁻⁶)(2.0 × 10⁵)(0.50) = 0.30 N. The perpendicular condition maximises the force for these values.

Question 16Natural Sciences (Biology, Chemistry, Physics)

Which feature of the mammalian heart ensures that blood flows in one direction through the circulatory system?

  1. The thick muscular wall of the left ventricle
  2. The presence of valves between atria and ventricles and at the exits of the ventricles Correct answer.
  3. The branching network of capillaries in the lungs
  4. The high oxygen concentration in arterial blood

Explanation

Valves prevent backflow by closing when pressure gradients reverse, so blood moves from atria to ventricles and then into arteries. The muscular wall provides force, and capillary networks allow exchange, but neither prevents reverse flow.

Question 17Natural Sciences (Biology, Chemistry, Physics)

When ethanoic acid reacts with ethanol in the presence of a sulfuric acid catalyst, the organic product is ethyl ethanoate. Which type of reaction is this?

  1. Oxidation
  2. Polymerisation
  3. Esterification Correct answer.
  4. Addition

Explanation

A carboxylic acid reacting with an alcohol to form an ester and water is esterification, a condensation reaction. Addition and polymerisation would change the carbon skeleton differently, and oxidation would require a change in oxidation state.

Question 18Natural Sciences (Biology, Chemistry, Physics)

A population of insects develops resistance to an insecticide over several generations. Which mechanism best explains this observation?

  1. Insects learn to avoid the insecticide and pass this behaviour to their offspring.
  2. The insecticide is diluted over time, so insects survive at lower doses.
  3. Individuals with pre-existing resistance survive and reproduce, increasing the frequency of resistance alleles. Correct answer.
  4. The insecticide causes mutations that make individual insects resistant.

Explanation

Natural selection acts on existing genetic variation: resistant individuals survive insecticide exposure and pass resistance alleles to their offspring. The insecticide selects rather than creates resistance, and acquired behaviour is not inherited in this way.

Question 19Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly describes the difference between ionic and covalent bonding?

  1. Ionic bonds are always weaker than covalent bonds.
  2. Ionic bonds result from electrostatic attraction between ions, while covalent bonds result from shared electron pairs. Correct answer.
  3. Ionic bonds occur only between metals, while covalent bonds occur only between non-metals.
  4. Ionic bonds involve sharing of electron pairs, while covalent bonds involve transfer of electrons.

Explanation

Ionic bonding is the electrostatic attraction between oppositely charged ions formed by electron transfer, while covalent bonding involves shared electron pairs. The other statements misdescribe the mechanisms or make absolute claims that are not generally true.

Question 20Natural Sciences (Biology, Chemistry, Physics)

A ray of light passes from air into glass and bends towards the normal. Which quantity remains unchanged as the light crosses the boundary?

  1. Frequency Correct answer.
  2. Wavelength
  3. Direction of propagation
  4. Speed

Explanation

The frequency of light is determined by the source and does not change when light enters a different medium. Speed and wavelength both decrease in glass, and the direction changes because of refraction.

Question 21Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly describes the effect of a competitive inhibitor on an enzyme-catalysed reaction?

  1. It binds to the active site and its effect can be reduced by increasing the substrate concentration. Correct answer.
  2. It binds to an allosteric site and permanently changes the enzyme's shape.
  3. It increases the activation energy of the reaction.
  4. It binds to the enzyme-substrate complex and prevents product release.

Explanation

A competitive inhibitor resembles the substrate and binds reversibly to the active site. Increasing substrate concentration outcompetes the inhibitor, so the inhibition can be overcome. Non-competitive inhibitors bind elsewhere and are not reversed by more substrate.

Question 22Natural Sciences (Biology, Chemistry, Physics)

A 0.10 mol dm⁻³ solution of a weak monoprotic acid has a pH of 3.0 at 25 °C. What is the acid dissociation constant, Ka, of the acid?

  1. 1.0 × 10⁻⁵ mol dm⁻³ Correct answer.
  2. 1.0 × 10⁻⁴ mol dm⁻³
  3. 1.0 × 10⁻⁶ mol dm⁻³
  4. 1.0 × 10⁻³ mol dm⁻³

Explanation

At pH 3.0, [H⁺] = 1.0 × 10⁻³ mol dm⁻³. For a weak acid, Ka ≈ [H⁺]²/[HA] = (1.0 × 10⁻³)²/0.10 = 1.0 × 10⁻⁵ mol dm⁻³. The approximation is valid because the degree of dissociation is small.

Question 23Natural Sciences (Biology, Chemistry, Physics)

In a photoelectric experiment, light of frequency f is incident on a metal surface and electrons are emitted. Which graph of maximum kinetic energy of emitted electrons against frequency would be a straight line?

  1. A line with a positive gradient and a positive y-intercept
  2. A line with a positive gradient and a negative y-intercept Correct answer.
  3. A horizontal line
  4. A line with a negative gradient and a positive y-intercept

Explanation

Einstein's photoelectric equation is K_max = hf − φ, which has the form y = mx + c with gradient h and intercept −φ. Because the work function φ is positive, the intercept on the kinetic-energy axis is negative and the gradient is positive.

Question 24Natural Sciences (Biology, Chemistry, Physics)

A dihybrid cross is carried out between two pea plants heterozygous for both seed shape (Rr) and seed colour (Yy), where R and Y are dominant. What proportion of the offspring is expected to show both recessive traits?

  1. 9/16
  2. 1/16 Correct answer.
  3. 3/16
  4. 1/4

Explanation

The two genes assort independently, so the probability of both recessive phenotypes is the product of the individual probabilities: (1/4) × (1/4) = 1/16. The 9:3:3:1 ratio confirms that only one sixteenth shows both recessive traits.

Question 25Natural Sciences (Biology, Chemistry, Physics)

A buffer solution is made from ethanoic acid and sodium ethanoate. Which statement best explains how this buffer resists a decrease in pH when a small amount of strong acid is added?

  1. The buffer dilutes the added acid so its effect is negligible.
  2. Sodium ions neutralise the added acid.
  3. Ethanoic acid dissociates further to release hydrogen ions.
  4. Ethanoate ions react with added hydrogen ions to form more ethanoic acid. Correct answer.

Explanation

Added H⁺ ions are removed by reaction with the conjugate base, ethanoate, forming weakly dissociated ethanoic acid. This shifts the equilibrium and keeps the hydrogen ion concentration nearly constant. Dilution alone would not maintain pH against added acid.

Question 26Natural Sciences (Biology, Chemistry, Physics)

Which organelle is the primary site of aerobic respiration in eukaryotic cells?

  1. Chloroplast
  2. Mitochondrion Correct answer.
  3. Golgi apparatus
  4. Ribosome

Explanation

Mitochondria house the electron transport chain and ATP synthase, which generate most of the cell's ATP under aerobic conditions. Chloroplasts perform photosynthesis, ribosomes synthesise proteins, and the Golgi apparatus modifies and packages proteins.

Question 27Natural Sciences (Biology, Chemistry, Physics)

Which type of chemical bond involves the sharing of electron pairs between atoms?

  1. Metallic bond
  2. Covalent bond Correct answer.
  3. Ionic bond
  4. Hydrogen bond

Explanation

A covalent bond forms when two atoms share one or more pairs of electrons, typically between non-metals. Ionic bonds involve transfer of electrons, hydrogen bonds are weaker attractions between polar molecules, and metallic bonds involve delocalised electrons in a lattice.

Question 28Natural Sciences (Biology, Chemistry, Physics)

Which quantity is defined as the rate of doing work or transferring energy?

  1. Power Correct answer.
  2. Pressure
  3. Momentum
  4. Force

Explanation

Power is the rate at which work is done or energy is transferred, measured in watts (joules per second). Force is a push or pull, momentum is mass times velocity, and pressure is force per unit area.

Question 29Natural Sciences (Biology, Chemistry, Physics)

Which process produces two identical daughter cells from one parent cell?

  1. Fertilisation
  2. Binary fission
  3. Mitosis Correct answer.
  4. Meiosis

Explanation

Mitosis is a type of cell division that results in two genetically identical daughter cells, each with the same number of chromosomes as the parent. Meiosis produces four genetically different gametes, fertilisation combines gametes, and binary fission is used by prokaryotes.

Question 30Natural Sciences (Biology, Chemistry, Physics)

Which gas is produced when dilute hydrochloric acid reacts with calcium carbonate?

  1. Carbon dioxide Correct answer.
  2. Chlorine
  3. Oxygen
  4. Hydrogen

Explanation

Calcium carbonate reacts with hydrochloric acid to form calcium chloride, water, and carbon dioxide gas. Hydrogen is produced when acids react with metals, oxygen is not produced in this reaction, and chlorine is not a product.

Question 31Natural Sciences (Biology, Chemistry, Physics)

Which component of the human blood transports oxygen from the lungs to respiring tissues?

  1. Haemoglobin in red blood cells Correct answer.
  2. Platelets
  3. Plasma proteins
  4. White blood cells

Explanation

Haemoglobin, a protein found in red blood cells, binds oxygen in the lungs and releases it in respiring tissues. Platelets are involved in clotting, white blood cells in immunity, and plasma proteins in transport and immunity but not oxygen carriage.

Question 32Natural Sciences (Biology, Chemistry, Physics)

What is the SI unit of electric current?

  1. Coulomb
  2. Volt
  3. Ohm
  4. Ampere Correct answer.

Explanation

The ampere (A) is the SI base unit of electric current, defined as one coulomb of charge passing a point per second. The volt is the unit of potential difference, the ohm is the unit of resistance, and the coulomb is the unit of charge.

Question 33Natural Sciences (Biology, Chemistry, Physics)

Which type of electromagnetic radiation has the shortest wavelength?

  1. Infrared
  2. Radio waves
  3. Ultraviolet
  4. Gamma rays Correct answer.

Explanation

Gamma rays have the shortest wavelengths and highest frequencies in the electromagnetic spectrum. Ultraviolet has longer wavelengths than gamma, infrared longer still, and radio waves have the longest wavelengths.

Question 34Natural Sciences (Biology, Chemistry, Physics)

A car travels 150 m in 10 s. What is its average speed?

  1. 0.15 m s⁻¹
  2. 1500 m s⁻¹
  3. 1.5 m s⁻¹
  4. 15 m s⁻¹ Correct answer.

Explanation

Average speed is distance divided by time: 150 m ÷ 10 s = 15 m s⁻¹. The other values result from incorrect division or multiplication.

Question 35Natural Sciences (Biology, Chemistry, Physics)

In a DNA molecule, adenine pairs with which base?

  1. Cytosine
  2. Uracil
  3. Guanine
  4. Thymine Correct answer.

Explanation

In DNA, adenine forms two hydrogen bonds with thymine, and guanine pairs with cytosine. Uracil replaces thymine in RNA, not DNA.

Question 36Natural Sciences (Biology, Chemistry, Physics)

What is the oxidation state of manganese in potassium manganate(VII), KMnO₄?

  1. +2
  2. +7 Correct answer.
  3. +4
  4. +6

Explanation

In KMnO₄, potassium is +1 and each oxygen is −2. Let x be the oxidation state of Mn: +1 + x + 4(−2) = 0, so x = +7. The other values do not satisfy the overall neutral charge.

Question 37Natural Sciences (Biology, Chemistry, Physics)

A resistor of 12 Ω is connected across a 6.0 V battery. What is the current through the resistor?

  1. 72 A
  2. 0.50 A Correct answer.
  3. 2.0 A
  4. 0.20 A

Explanation

Using Ohm's law, I = V / R = 6.0 V / 12 Ω = 0.50 A. The other values arise from multiplying or inverting the quantities incorrectly.

Question 38Natural Sciences (Biology, Chemistry, Physics)

Which statement best explains why the enzyme catalase increases the rate of decomposition of hydrogen peroxide?

  1. It increases the activation energy of the reaction.
  2. It lowers the activation energy of the reaction. Correct answer.
  3. It provides extra substrate molecules.
  4. It raises the temperature of the reaction mixture.

Explanation

Enzymes are biological catalysts that provide an alternative reaction pathway with a lower activation energy, increasing the proportion of collisions that result in reaction. They are not consumed and do not supply substrate or significantly raise temperature.

Question 39Natural Sciences (Biology, Chemistry, Physics)

Which separation technique is most suitable for separating ethanol from a mixture of ethanol and water?

  1. Filtration
  2. Crystallisation
  3. Fractional distillation Correct answer.
  4. Chromatography

Explanation

Fractional distillation separates miscible liquids with different boiling points, such as ethanol (78 °C) and water (100 °C). Filtration separates solids from liquids, chromatography separates dissolved substances, and crystallisation purifies solids.

Question 40Natural Sciences (Biology, Chemistry, Physics)

A wave has a frequency of 50 Hz and a wavelength of 4.0 m. What is its speed?

  1. 200 m s⁻¹ Correct answer.
  2. 0.08 m s⁻¹
  3. 54 m s⁻¹
  4. 12.5 m s⁻¹

Explanation

Wave speed is the product of frequency and wavelength: v = f × λ = 50 Hz × 4.0 m = 200 m s⁻¹. The other values result from dividing or adding the given quantities.

Question 41Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly describes the role of the placenta in mammalian pregnancy?

  1. It produces all of the fetus's red blood cells throughout pregnancy.
  2. It stores waste products until birth instead of removing them.
  3. It mixes maternal and fetal blood to supply oxygen directly.
  4. It allows exchange of nutrients and gases between maternal and fetal blood without mixing the two bloods. Correct answer.

Explanation

The placenta is a specialised exchange organ where maternal and fetal blood flow close together but do not mix, allowing diffusion of oxygen, nutrients, and waste products. It does not mix the two circulations or store waste until birth.

Question 42Natural Sciences (Biology, Chemistry, Physics)

Which observation would show that a chemical reaction has taken place when an iron nail is placed in copper(II) sulfate solution?

  1. The nail becomes shinier and the solution turns deeper blue.
  2. The nail dissolves completely and the solution remains unchanged in colour.
  3. A reddish-brown solid forms on the nail and the blue colour of the solution fades. Correct answer.
  4. The temperature of the solution drops sharply and a gas is evolved.

Explanation

Iron is more reactive than copper, so it displaces copper from copper(II) sulfate. Copper metal deposits as a reddish-brown solid on the nail, and the blue colour of the solution fades as copper ions are removed.

Question 43Natural Sciences (Biology, Chemistry, Physics)

A gas occupies 300 cm³ at a pressure of 1.0 × 10⁵ Pa. What volume will it occupy at 2.0 × 10⁵ Pa, assuming constant temperature?

  1. 300 cm³
  2. 150 cm³ Correct answer.
  3. 600 cm³
  4. 75 cm³

Explanation

Boyle's law states that pressure and volume are inversely proportional at constant temperature: P₁V₁ = P₂V₂. Thus V₂ = (1.0 × 10⁵ × 300) / (2.0 × 10⁵) = 150 cm³.

Question 44Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly describes the difference between a scalar quantity and a vector quantity?

  1. A scalar can be negative, but a vector cannot.
  2. A scalar is always larger than a vector.
  3. A scalar has magnitude only, while a vector has both magnitude and direction. Correct answer.
  4. A scalar has both magnitude and direction, while a vector has magnitude only.

Explanation

Scalars, such as mass and speed, are fully described by magnitude alone. Vectors, such as displacement and velocity, require both magnitude and direction for a complete description.

Question 45Natural Sciences (Biology, Chemistry, Physics)

In a cross between a homozygous dominant tall pea plant (TT) and a homozygous recessive dwarf pea plant (tt), what proportion of the F₁ offspring will be tall?

  1. All of them Correct answer.
  2. None of them
  3. Half of them
  4. One quarter of them

Explanation

All F₁ offspring receive a T allele from the tall parent and a t allele from the dwarf parent, so their genotype is Tt. Since T is dominant, every offspring shows the tall phenotype.

Question 46Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly explains why a solid ionic compound does not conduct electricity, while its molten form does?

  1. In the solid, electrons are free to move; when molten, electrons are fixed.
  2. In the solid, the compound is covalent; when molten, it becomes ionic.
  3. In the solid, ions are free to move; when molten, ions are fixed in a lattice.
  4. In the solid, ions are fixed in a lattice; when molten, ions are free to move. Correct answer.

Explanation

Ionic solids conduct electricity only when their ions are free to move. In the solid state, ions are held in fixed positions in a lattice, but melting releases them to move and carry charge.

Question 47Natural Sciences (Biology, Chemistry, Physics)

A 2.0 kg object is lifted vertically through a height of 5.0 m at constant speed. Taking g = 10 m s⁻², how much work is done against gravity?

  1. 10 J
  2. 100 J Correct answer.
  3. 50 J
  4. 20 J

Explanation

Work done against gravity equals the gain in gravitational potential energy: W = mgh = 2.0 kg × 10 m s⁻² × 5.0 m = 100 J. The other values come from using only one of the factors or dividing instead of multiplying.

Question 48Natural Sciences (Biology, Chemistry, Physics)

Which statement best explains why the pH of a buffer solution changes very little when small amounts of acid or base are added?

  1. The buffer contains a weak acid and its conjugate base, which neutralise added acid or base. Correct answer.
  2. The buffer prevents any reaction from occurring when acid or base is added.
  3. The buffer has a very high concentration of hydrogen ions that resists change.
  4. The buffer contains a strong acid and a strong base, which react completely with each other.

Explanation

A buffer typically consists of a weak acid and its conjugate base (or a weak base and its conjugate acid). Added acid is neutralised by the conjugate base, and added base is neutralised by the weak acid, so the pH remains nearly constant.

Question 49Natural Sciences (Biology, Chemistry, Physics)

In a photoelectric experiment, the maximum kinetic energy of emitted electrons is plotted against the frequency of incident light. Which feature of the graph gives the work function of the metal?

  1. The magnitude of the intercept on the kinetic energy axis Correct answer.
  2. The intercept on the frequency axis
  3. The gradient of the line
  4. The area under the line

Explanation

Einstein's photoelectric equation is KEmax = hf − φ. On a graph of KEmax against f, the gradient is Planck's constant h, and the negative intercept on the KEmax axis equals −φ, so its magnitude gives the work function.

Question 50Natural Sciences (Biology, Chemistry, Physics)

Which statement correctly explains why a population of bacteria can become resistant to an antibiotic over many generations?

  1. The antibiotic causes bacteria to mutate in a direction that makes them resistant.
  2. Random mutations create resistant variants, which survive and reproduce when the antibiotic is present. Correct answer.
  3. Resistance arises because bacteria learn to avoid the antibiotic.
  4. Individual bacteria gradually become resistant during their lifetime and pass this on.

Explanation

Variation arises from random mutation before exposure to the antibiotic. When the antibiotic is applied, resistant individuals survive and reproduce, increasing the frequency of resistance alleles in the population by natural selection.