Biology
A plant is placed in bright light but its stomata close. Explain why this could reduce the rate of photosynthesis.
Less CO₂ can enter the leaf, so CO₂ becomes a limiting factor.
Magnesium reacts with hydrochloric acid. Explain, in terms of particles, why increasing the concentration of hydrochloric acid increases the rate of reaction.
More acid particles per unit volume → more frequent successful collisions with Mg.
A car travels at a constant speed of 20 m/s for 30 seconds. How far does it travel?
600 m
A student repeats an experiment three times and gets 21.1, 21.2 and 27.8. What should they do with the 27.8 result?
Identify it as anomalous, investigate/check it, and calculate the mean without it if justified.
A red blood cell is placed in pure water. What happens and why?
Water enters by osmosis; the cell swells and may burst because there is no cell wall.
A person has a mutation that prevents their cells from producing functional haemoglobin. Explain two effects this would have on their body.
Less oxygen transported → less aerobic respiration → less ATP released; muscles may respire anaerobically more, producing lactic acid.
A student adds excess insoluble copper(II) oxide to warm dilute sulfuric acid. Explain how they could obtain pure, dry copper(II) sulfate crystals.
Filter off excess CuO → evaporate some water → cool to crystallise → filter crystals → dry.
A 2 kg object accelerates from 3 m/s to 11 m/s in 4 seconds. Calculate the resultant force.
Acceleration = 2 m/s²; force = 4 N.
A solution contains either sulfate ions or carbonate ions. Describe a test that could distinguish between them.
Add dilute nitric acid. Carbonate produces bubbles of CO₂, whereas sulfate does not. The gas can be confirmed as CO₂ using limewater, which turns milky.
Why does a metal conduct electricity but solid sodium chloride does not?
Metals have delocalised electrons that can move; solid NaCl has ions fixed in a lattice. Molten/aqueous NaCl can conduct.
An enzyme-controlled reaction is carried out at 20°C and then repeated at 40°C. The rate increases initially but then decreases at temperatures above 40°C. Explain why the rate eventually decreases.
Higher temperatures cause enzyme to denature. The active site changes shape, so the substrate is no longer complementary and fewer enzyme–substrate complexes form.
A mixture contains sodium chloride and silver nitrate. A precipitate forms. Write the ionic equation and identify the spectator ions.
Ag⁺ + Cl⁻ → AgCl. Spectator ions: Na⁺ and NO₃⁻.
A student says: "An object moving at constant velocity has no forces acting on it." Explain why this is incorrect.
Forces can act but must be balanced, so resultant force = 0 and acceleration = 0.
A student has four colourless solutions and needs to determine which contains reducing sugar. Describe the test and the positive result.
Add Benedict's solution and heat in a hot water bath. A positive result changes from blue to green/yellow/orange/brick-red, depending on the concentration.
An object has a mass of 500 g and a volume of 200 cm³. Calculate its density in g/cm³.
2.5 g/cm³.
A person has a blocked bile duct. Explain how this would affect the digestion and absorption of fats in the small intestine.
Less bile reaches the small intestine, so fats are not emulsified as effectively. This gives a smaller surface area for lipase to act on, so fat digestion is slower. Less fatty acid and glycerol may therefore be available for absorption.
An electrolysis experiment uses molten lead(II) bromide. What happens at each electrode and why?
Pb²⁺ gains electrons at the cathode: Pb²⁺ + 2e⁻ → Pb. Br⁻ loses electrons at the anode: 2Br⁻ → Br₂ + 2e⁻.
A lamp is rated 12 V and 24 W. Calculate the current through the lamp.
2 A.
A student investigates how concentration affects the rate of reaction between hydrochloric acid and calcium carbonate. Give two variables that should be controlled and explain why one of them needs to be controlled.
Possible controls include mass/surface area of calcium carbonate, volume of acid, temperature, or total volume. They must be controlled so that any change in reaction rate can be attributed to the concentration of hydrochloric acid rather than another variable.
A student says: "If an object is moving, there must be a resultant force acting on it." Explain why this statement is incorrect, giving an example.
An object can move at constant velocity when the resultant force is zero. For example, a car travelling at constant speed in a straight line has balanced forces.
During vigorous exercise, a person's breathing rate and depth both increase. Explain how these changes help maintain the rate of aerobic respiration in muscle cells.
More air reaches the alveoli, increasing oxygen available for diffusion into the blood. More oxygen is transported to the muscles, allowing aerobic respiration to continue at a higher rate and releasing more energy.
Two hydrocarbons have molecular formulae C₄H₁₀ and C₄H₈. One is an alkane and one could be an alkene. Explain how you could distinguish between them experimentally.
Add bromine water. Alkene decolourises bromine water; alkane does not under normal conditions.
Two identical resistors are connected first in series and then in parallel to the same battery. In which arrangement is the total current greater? Explain why.
Parallel. Total resistance is lower, so for the same voltage, current is greater.
student measures the diameter of a wire using a ruler to the nearest millimetre. They then calculate its cross-sectional area. Explain why using a micrometer would improve the experiment.
The diameter is small, so ruler measurements have relatively large percentage uncertainty; a micrometer gives a smaller measurement uncertainty, making the calculated area more reliable.
A student heats hydrated copper(II) sulfate crystals strongly. The blue crystals turn white, and water droplets form on the cooler part of the test tube. The student says, "The copper(II) sulfate has decomposed because it has changed colour." Explain why this conclusion is not necessarily correct, and describe how the original blue substance could be obtained again.
The crystals have lost water of crystallisation, rather than necessarily decomposing the copper(II) sulfate itself. Add water to the white anhydrous copper(II) sulfate; it becomes blue hydrated copper(II) sulfate again.