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LGCSE Physical Science (0181) study materials
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LGCSE Physical Science (0181) practice questions

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Question 1Scientific enquiry, practical skills and the nature of science

A student measures the mass of a metal block three times and obtains 24.8 g, 24.9 g and 24.8 g, while the balance reads 25.3 g when a 25.0 g standard mass is placed on it. Which statement best describes the student's results?

  1. The readings are neither accurate nor precise
  2. The readings are precise but not accurate Correct answer.
  3. The readings are both accurate and precise
  4. The readings are accurate but not precise

Explanation

Precision refers to how closely repeated measurements agree with each other, and these three values differ by only 0.1 g, so they are precise. Accuracy refers to closeness to the true value; the balance reads 0.3 g high against a standard, so the results are systematically offset and therefore not accurate.

Question 2Scientific enquiry, practical skills and the nature of science

In a fair test of how light intensity affects the rate of photosynthesis in pondweed, which variable should be controlled by keeping it constant?

  1. The intensity of the light falling on the plant
  2. The temperature of the water Correct answer.
  3. The distance of the lamp from the plant
  4. The number of bubbles counted per minute

Explanation

The independent variable is light intensity, changed by moving the lamp, and the dependent variable is the bubble count per minute. Temperature affects enzyme activity and so must be held constant as a control variable, otherwise it could confound the result.

Question 3Scientific enquiry, practical skills and the nature of science

A student writes a plan stating: 'I think that increasing the temperature will make the reaction faster.' In the structure of a scientific investigation, what is this statement?

  1. A hypothesis Correct answer.
  2. A measurement
  3. A control variable
  4. A conclusion

Explanation

A hypothesis is a testable prediction about the relationship between variables, and this statement predicts that raising temperature will increase reaction rate. It is proposed before data are collected and can be supported or rejected by the results.

Question 4Scientific enquiry, practical skills and the nature of science

Which instrument and unit pair is most appropriate for measuring the volume of a liquid sample of about 25 cm³ in a school laboratory?

  1. A thermometer read in °C
  2. A metre rule read in cm
  3. A top-pan balance read in g
  4. A measuring cylinder read in cm³ Correct answer.

Explanation

Volume of a liquid is measured with a graduated vessel such as a measuring cylinder, and the cubic centimetre (cm³) is the appropriate unit for a sample of this size. The other instruments measure length, mass and temperature respectively.

Question 5Scientific enquiry, practical skills and the nature of science

When reading the volume of clear water in a measuring cylinder, where should the observer's eye be placed, and which part of the meniscus should be read?

  1. At the same level as the meniscus, reading the bottom of the curve Correct answer.
  2. Above the meniscus, reading the top of the curve
  3. At the same level as the meniscus, reading the top of the curve
  4. Below the meniscus, reading the top of the curve

Explanation

Reading at eye level avoids parallax error, which would otherwise make the value appear different from different viewpoints. For water the meniscus curves downwards, so the bottom of the curve gives the correct volume.

Question 6Scientific enquiry, practical skills and the nature of science

A table of results shows a column headed 'Time / s' with entries 10, 20, 30 and 40. What does the '/ s' in the column heading indicate?

  1. The quantity is multiplied by its unit in seconds
  2. The quantity is divided by its unit, so the numbers are in seconds Correct answer.
  3. The values are all estimates rather than measurements
  4. The values have been averaged over several readings

Explanation

In scientific tables the column heading is written as quantity divided by unit, so a heading of 'Time / s' means the numbers listed are time values expressed in seconds. This convention keeps units out of the body of the table.

Question 7Scientific enquiry, practical skills and the nature of science

A student measures the extension of a spring for increasing loads and plots a graph of extension against load. The points lie on a straight line through the origin up to 6 N, after which the line curves. What is the best interpretation of the straight section?

  1. The load is independent of the extension in that range
  2. Extension is directly proportional to load in that range Correct answer.
  3. The spring has permanently deformed from the start
  4. Extension is inversely proportional to load in that range

Explanation

A straight line through the origin shows that the two quantities increase in proportion, so extension is directly proportional to load, which is Hooke's law behaviour. The curve beyond 6 N marks the limit of proportionality where the relationship breaks down.

Question 8Scientific enquiry, practical skills and the nature of science

Two students each measure the boiling point of a liquid. Student A records 78.2 °C, 78.1 °C and 78.2 °C. Student B records 77.0 °C, 79.5 °C and 78.0 °C. Both obtain a mean of about 78.1 °C. Which statement is correct?

  1. Both sets are equally reliable because the means are the same
  2. Student B's results are more reliable because the values are more spread out
  3. Student A's results are more reliable because they are more consistent Correct answer.
  4. Student B's results are more accurate because the spread is larger

Explanation

Reliability is judged by how consistent repeated measurements are, and Student A's values differ by only 0.1 °C, showing good repeatability. Student B's wide spread of 2.5 °C suggests random error, so those readings are less reliable even though the mean happens to be similar.

Question 9Scientific enquiry, practical skills and the nature of science

In an experiment to find the density of an irregular stone, which sequence of steps is most appropriate?

  1. Measure mass with a balance, then find volume by displacement in a measuring cylinder, then divide mass by volume Correct answer.
  2. Measure mass with a balance, then find volume with a ruler, then multiply mass by volume
  3. Measure the stone's weight with a spring balance, then divide by its length
  4. Measure volume with a ruler, then find mass by displacement in water, then divide volume by mass

Explanation

Density is mass per unit volume, so the mass is found with a balance and the volume of an irregular solid is found by the volume of water it displaces. Dividing mass by volume then gives the density in g/cm³.

Question 10Scientific enquiry, practical skills and the nature of science

A student wants to investigate whether the type of surface affects how far a toy car travels after leaving a ramp. Which is the independent variable?

  1. The height of the ramp
  2. The distance travelled by the car
  3. The type of surface Correct answer.
  4. The mass of the toy car

Explanation

The independent variable is the one deliberately changed by the experimenter, which here is the surface type being compared. The distance travelled is the dependent variable, while ramp height and car mass must be controlled.

Question 11Scientific enquiry, practical skills and the nature of science

A graph of gas volume against temperature is plotted from data and a best-fit straight line is drawn. One data point lies well above the line while the others lie close to it. What is the most appropriate action?

  1. Remove the point and redraw the line without recording the change
  2. Check the reading for that point and consider repeating it as a possible anomalous result Correct answer.
  3. Move the point onto the line so the graph looks neater
  4. Ignore all the data and start the experiment again

Explanation

A point far from an otherwise good straight-line trend is likely to be an anomalous result caused by a measurement or recording error. The correct response is to recheck the original reading and, where possible, repeat that measurement before deciding whether to exclude it, recording any decision made.

Question 12Scientific enquiry, practical skills and the nature of science

A student tests whether a fertiliser increases plant growth by growing five plants with fertiliser and five without, keeping light, water and temperature the same. What is the main purpose of the group grown without fertiliser?

  1. It provides a second independent variable
  2. It acts as a control for comparison with the treated group Correct answer.
  3. It makes the results less reliable
  4. It increases the total number of plants measured

Explanation

A control group is treated identically except for the variable under test, so any difference in growth can be attributed to the fertiliser. Without it, other factors could explain the results.

Question 13Scientific enquiry, practical skills and the nature of science

A table records the temperature of a cooling liquid every minute. Which type of graph is most suitable for displaying these data, and why?

  1. A scatter graph with no joined points, because time cannot be ordered
  2. A pie chart, because it shows the proportion of each temperature
  3. A line graph, because both variables are continuous and show a trend Correct answer.
  4. A bar chart, because temperature is a category

Explanation

Both time and temperature are continuous quantities, and a line graph shows how temperature changes with time, revealing the cooling trend. Pie charts and bar charts are suited to proportions or categories rather than continuous change.

Question 14Scientific enquiry, practical skills and the nature of science

A pupil claims that a plant grows faster when spoken to because the spoken-to plant grew 2 cm taller. The plants were on different windowsills and one received more sunlight. What is the main weakness of the conclusion?

  1. Only one plant was used in each condition
  2. A confounding variable was not controlled, so the cause cannot be identified Correct answer.
  3. The claim cannot be tested by experiment
  4. The growth was measured in centimetres rather than metres

Explanation

Because the plants were kept in different light conditions, sunlight could explain the difference in growth just as well as the speaking. When variables other than the one under test are allowed to vary, the conclusion is not valid.

Question 15Scientific enquiry, practical skills and the nature of science

A student records the following lengths for a wire: 12.4 cm, 12.5 cm, 12.4 cm, 12.6 cm. What is the mean length, expressed to an appropriate number of significant figures?

  1. 12.48 cm
  2. 12.5 cm Correct answer.
  3. 12 cm
  4. 12.475 cm

Explanation

The mean is (12.4 + 12.5 + 12.4 + 12.6) ÷ 4 = 12.475 cm. Since the readings are given to one decimal place, the mean should be quoted to one decimal place as 12.5 cm, matching the precision of the measurements.

Question 16Scientific enquiry, practical skills and the nature of science

Why do scientists usually repeat an experiment several times and calculate a mean, rather than relying on a single measurement?

  1. To guarantee that the result is accurate
  2. To make the independent variable easier to change
  3. To reduce the effect of random errors on the result Correct answer.
  4. To eliminate all systematic errors from the apparatus

Explanation

Random errors vary unpredictably between readings, and averaging several values reduces their overall influence, improving reliability. Systematic errors, however, shift every reading in the same direction and are not removed by repetition.

Question 17Scientific enquiry, practical skills and the nature of science

A student investigates how the period of a pendulum depends on its length. The period is measured with a stopwatch that has a reaction-time uncertainty of about 0.2 s, and one swing takes about 1.4 s. Which improvement would most reduce the percentage uncertainty in the period?

  1. Use a heavier pendulum bob of the same size
  2. Time one swing but read the stopwatch more carefully
  3. Time twenty swings and divide the total time by twenty Correct answer.
  4. Measure the length of the pendulum with a metre rule

Explanation

The percentage uncertainty is the uncertainty divided by the measured value, so making the timed interval longer reduces it. Timing many swings and dividing gives a much larger total time for the same reaction-time uncertainty, greatly improving precision.

Question 18Scientific enquiry, practical skills and the nature of science

A student plots a graph of the extension of a rubber band against applied force and obtains a curved line that does not pass through the origin. Which conclusion is best supported?

  1. The force is directly proportional to the extension
  2. The experiment contains no anomalous data
  3. The rubber band does not obey Hooke's law over the range tested Correct answer.
  4. The rubber band has a constant spring constant throughout

Explanation

Hooke's law requires extension to be directly proportional to force, which gives a straight line through the origin. A curve that misses the origin shows the relationship is non-linear, so the band does not obey the law in this range.

Question 19Scientific enquiry, practical skills and the nature of science

Two methods are used to measure the density of the same metal. Method 1 gives 8.9, 9.0 and 8.9 g/cm³; Method 2 gives 7.6, 8.4 and 9.2 g/cm³. The accepted value is 8.9 g/cm³. Which comparison is correct?

  1. Both methods are equally precise and equally accurate
  2. Method 1 is more precise and more accurate than Method 2 Correct answer.
  3. Method 2 is more precise and more accurate than Method 1
  4. Method 1 is more precise but less accurate than Method 2

Explanation

Method 1's readings are tightly clustered and centred on 8.9 g/cm³, so they are both precise and accurate. Method 2's readings are widely spread and straddle the accepted value, showing lower precision and, on average, lower accuracy.

Question 20Scientific enquiry, practical skills and the nature of science

A student concludes that a catalyst increases the rate of a reaction because gas was produced faster with the catalyst present. During the trials the water bath temperature rose from 20 °C to 30 °C. What is the strongest criticism of the conclusion?

  1. The volume of gas produced was not measured with a gas syringe
  2. Temperature changed during the trials, so its effect cannot be separated from the catalyst's effect Correct answer.
  3. Only the rate of gas production was recorded
  4. A catalyst should not be used in a school laboratory

Explanation

Because temperature also affects reaction rate, allowing it to rise during the trials means the faster gas production could be due to heating rather than the catalyst. To make a valid conclusion, temperature must be controlled while only the catalyst is changed.

Question 21States of matter, particle theory and separation techniques

Which statement best describes the arrangement and motion of particles in a gas?

  1. Particles are far apart and move randomly at high speed. Correct answer.
  2. Particles are held in a regular lattice and cannot change position.
  3. Particles are close together but slide past one another in a random arrangement.
  4. Particles are closely packed in fixed positions and only vibrate.

Explanation

In a gas the particles are separated by large distances compared with their own size and move rapidly in random directions, colliding with each other and the container walls. This explains why gases fill any container and are easily compressed.

Question 22States of matter, particle theory and separation techniques

A sealed syringe contains air. The plunger is pushed in while the temperature stays constant. Why does the air pressure inside rise?

  1. The mass of each air particle increases.
  2. The number of air particles inside the syringe increases.
  3. The particles hit the walls more frequently because they are confined to a smaller volume. Correct answer.
  4. The air particles become larger as they are squeezed.

Explanation

Pressure arises from particle collisions with the walls. Reducing the volume at constant temperature keeps particle speed the same but forces more collisions per unit area per second, so pressure increases.

Question 23States of matter, particle theory and separation techniques

Which change of state involves particles gaining energy and moving from a fixed, regular arrangement into a random, closely packed arrangement?

  1. Melting Correct answer.
  2. Condensation
  3. Freezing
  4. Deposition

Explanation

Melting converts a solid, whose particles vibrate about fixed lattice positions, into a liquid where particles are still close but randomly arranged and able to slide. Energy is absorbed to overcome the forces holding the lattice together.

Question 24States of matter, particle theory and separation techniques

During boiling of pure water at normal atmospheric pressure, the temperature stays at 100 °C even though heating continues. What is the added energy used for?

  1. Increasing the pressure of the surrounding air
  2. Increasing the average kinetic energy of the water molecules
  3. Overcoming the forces of attraction between water molecules so they escape as vapour Correct answer.
  4. Breaking the covalent bonds inside each water molecule

Explanation

At the boiling point the energy supplied is used as latent heat to separate molecules from the liquid rather than to raise their average kinetic energy. Since temperature measures average kinetic energy, it remains constant during the change of state.

Question 25States of matter, particle theory and separation techniques

A sample of muddy water is left to stand and a layer of solid settles at the bottom. Which separation technique has occurred, and what is the clear liquid above the solid called?

  1. Filtration; the clear liquid is the filtrate.
  2. Chromatography; the clear liquid is the mobile phase.
  3. Distillation; the clear liquid is the distillate.
  4. Sedimentation; the clear liquid is the supernatant. Correct answer.

Explanation

When suspended solids settle under gravity the process is sedimentation, and the clear liquid remaining above the settled solid is the supernatant. Filtration would require a filter medium to trap the solid particles.

Question 26States of matter, particle theory and separation techniques

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

  1. Fractional distillation Correct answer.
  2. Simple filtration
  3. Evaporation to dryness
  4. Magnetic separation

Explanation

Ethanol and water are miscible liquids with different boiling points, so fractional distillation separates them by repeated vaporisation and condensation. Filtration and magnetic separation work only on undissolved solids.

Question 27States of matter, particle theory and separation techniques

A student uses paper chromatography to test a green food dye. The dye separates into a yellow spot and a blue spot. What does this show about the dye?

  1. The dye contains water as its only solvent.
  2. The dye has reacted with the paper to form new colours.
  3. The dye is a pure substance with a single component.
  4. The dye is a mixture of at least two coloured substances with different solubilities. Correct answer.

Explanation

Separation on the paper occurs because components travel at different rates depending on how strongly they are attracted to the stationary paper and how soluble they are in the moving solvent. Two spots therefore indicate at least two components, so the dye is a mixture.

Question 28States of matter, particle theory and separation techniques

Which statement correctly distinguishes a compound from a mixture?

  1. A compound has a variable melting point, while a mixture has a sharp melting point.
  2. A mixture always contains only one element, while a compound contains several.
  3. A compound has components in fixed proportions joined chemically, while a mixture has components in variable proportions not chemically joined. Correct answer.
  4. A compound can be separated by filtration, while a mixture cannot.

Explanation

In a compound the elements are chemically combined in fixed ratios and can only be separated by chemical means, giving sharp melting and boiling points. A mixture's components are not chemically bonded, can be present in any proportion, and are separable by physical techniques.

Question 29States of matter, particle theory and separation techniques

A crystal of potassium manganate(VII) is placed in a beaker of water and left undisturbed. After several hours the purple colour has spread throughout the water without stirring. Which explanation is correct?

  1. Water molecules chemically react with the crystal to produce a purple compound.
  2. The crystal dissolves and the heavy purple particles sink and push the water upwards.
  3. The crystal melts and forms a separate purple layer on the water.
  4. Particles of the crystal move randomly and spread through the water by diffusion. Correct answer.

Explanation

Particles in a liquid are in constant random motion, so dissolved particles gradually spread from a region of high concentration to low concentration until evenly distributed. This is diffusion and needs no stirring.

Question 30States of matter, particle theory and separation techniques

A balloon is taken from a warm room into a cold refrigerator. Assuming the pressure remains the same, what happens to the volume of the gas inside?

  1. It increases, because cold gas particles expand.
  2. It decreases, because the particles move more slowly and collide with the walls less forcefully. Correct answer.
  3. It stays the same, because the number of particles is unchanged.
  4. It decreases, because some gas particles are destroyed by the cold.

Explanation

Cooling reduces the average kinetic energy of the gas particles, so they hit the walls with less force and the volume contracts at constant pressure. This is Charles's law behaviour; the particles themselves do not shrink.

Question 31States of matter, particle theory and separation techniques

Which pair of processes both involve a gas changing into a liquid or solid?

  1. Condensation and deposition Correct answer.
  2. Freezing and sublimation
  3. Melting and boiling
  4. Sublimation and evaporation

Explanation

Condensation is gas to liquid and deposition is gas directly to solid; both release energy as particles come closer together. The other processes listed involve solids or liquids gaining energy to become less ordered.

Question 32States of matter, particle theory and separation techniques

A mixture of sand and salt is stirred with water and then filtered. What remains on the filter paper and what is in the filtrate?

  1. Salt remains on the paper; sand is dissolved in the filtrate.
  2. Neither remains on the paper; both pass through as a suspension.
  3. Both sand and salt remain on the paper; the filtrate is pure water.
  4. Sand remains on the paper; dissolved salt is in the filtrate. Correct answer.

Explanation

Sand is insoluble, so its particles are trapped by the filter paper, while salt dissolves and passes through with the water as the filtrate. Evaporating the filtrate would then recover the salt.

Question 33States of matter, particle theory and separation techniques

A student heats a pure solid and records the temperature every minute. The temperature rises, then stays constant for several minutes before rising again. What does the constant-temperature section represent?

  1. The thermometer has reached its maximum reading.
  2. The solid is cooling towards room temperature.
  3. The solid is melting and the latent heat is breaking down the lattice. Correct answer.
  4. The solid is subliming directly to a gas with no liquid stage.

Explanation

During melting the energy supplied is used to overcome the forces holding particles in the solid lattice rather than to raise kinetic energy, so the temperature stays constant at the melting point. Once all the solid has melted, the temperature rises again.

Question 34States of matter, particle theory and separation techniques

Two gas jars, one of hydrogen (very light) and one of carbon dioxide (denser than air), are placed mouth to mouth with the hydrogen on top. After some time hydrogen is found in the lower jar. What best explains this?

  1. The hydrogen dissolves in the carbon dioxide and sinks as a solution.
  2. The gas jars leak, allowing air to carry the hydrogen down.
  3. The carbon dioxide rises and pushes the hydrogen downwards.
  4. Gas particles move randomly in all directions, so diffusion occurs even against the density difference. Correct answer.

Explanation

Diffusion results from the random motion of particles, which move in all directions regardless of density, so light hydrogen particles travel into the lower jar over time. Density affects the rate but does not prevent mixing.

Question 35States of matter, particle theory and separation techniques

Which property is characteristic of a pure crystalline substance rather than a mixture?

  1. It can be separated into components by fractional distillation.
  2. Its composition can be varied by adding more of one component.
  3. It has a sharp, fixed melting point. Correct answer.
  4. It melts over a wide range of temperatures.

Explanation

A pure substance has a definite composition and its particles all experience the same forces, so it melts sharply at one temperature. Impurities lower and broaden the melting range, which is why melting point is used as a test of purity.

Question 36States of matter, particle theory and separation techniques

A sample of air is cooled strongly until it liquefies, then warmed slowly and the fractions are collected. Which statement about this process is correct?

  1. Fractional distillation of liquid air separates components because they have different boiling points. Correct answer.
  2. Filtration of liquid air separates nitrogen from oxygen by particle size.
  3. The components of air cannot be separated because air is a compound.
  4. Chromatography is used because the gases have different colours.

Explanation

Air is a mixture of gases with different boiling points, so fractional distillation of liquefied air allows them to be separated as the temperature rises. Nitrogen, with the lowest boiling point, boils off first.

Question 37Atomic structure, the Periodic Table and chemical bonding

An atom of an element has the electron configuration 2,8,7. In which group and period of the Periodic Table does this element appear?

  1. Group 15, Period 3
  2. Group 17, Period 3 Correct answer.
  3. Group 7, Period 3
  4. Group 7, Period 2

Explanation

The number of occupied shells (three: 2, 8, 7) gives the period, so the element is in Period 3. The number of outer-shell electrons (seven) gives the group number for main-group elements, so the element is in Group 17, the halogens.

Question 38Atomic structure, the Periodic Table and chemical bonding

Which statement describes the structure of an atom of ³⁵Cl₁₇?

  1. 17 protons, 17 neutrons, 18 electrons
  2. 17 protons, 18 neutrons, 17 electrons Correct answer.
  3. 35 protons, 17 neutrons, 35 electrons
  4. 18 protons, 17 neutrons, 17 electrons

Explanation

The atomic number 17 fixes the number of protons at 17, and a neutral atom has the same number of electrons. The mass number 35 is the sum of protons and neutrons, so the neutron number is 35 − 17 = 18.

Question 39Atomic structure, the Periodic Table and chemical bonding

Which pair of elements would be expected to have the most similar chemical properties?

  1. F and Cl Correct answer.
  2. Na and Mg
  3. Li and Be
  4. Ne and Na

Explanation

Elements in the same group have the same number of outer-shell electrons and therefore similar chemical behaviour. Fluorine and chlorine are both Group 17 halogens with seven outer electrons, so they react in closely similar ways.

Question 40Atomic structure, the Periodic Table and chemical bonding

Which type of bonding is present in solid sodium chloride?

  1. Covalent bonding between sodium and chloride ions
  2. Ionic bonding between sodium ions and chloride ions Correct answer.
  3. Metallic bonding between sodium cations and delocalised electrons
  4. Intermolecular forces between sodium chloride molecules

Explanation

Sodium chloride is formed from a metal and a non-metal, so electrons are transferred to give Na⁺ and Cl⁻ ions. The oppositely charged ions are held in a giant lattice by strong electrostatic attraction, which is ionic bonding.

Question 41Atomic structure, the Periodic Table and chemical bonding

Which particle is responsible for the positive charge of the nucleus?

  1. Proton Correct answer.
  2. Neutron
  3. Electron
  4. Photon

Explanation

Protons carry a relative charge of +1 and are located in the nucleus, so they determine its positive charge. Neutrons are neutral and electrons are negative and orbit outside the nucleus.

Question 42Atomic structure, the Periodic Table and chemical bonding

How many electrons are needed to complete the outer shell of a nitrogen atom (atomic number 7)?

  1. 3 Correct answer.
  2. 7
  3. 5
  4. 2

Explanation

Nitrogen has the configuration 2,5, so its outer shell holds five electrons. A full outer shell for the second period holds eight electrons, so 8 − 5 = 3 electrons are needed.

Question 43Atomic structure, the Periodic Table and chemical bonding

Which statement about isotopes of the same element is correct?

  1. They have different numbers of electrons in a neutral atom.
  2. They have the same number of protons but different numbers of neutrons. Correct answer.
  3. They have different numbers of protons.
  4. They have the same number of neutrons but different mass numbers.

Explanation

Isotopes are atoms of the same element, so they must have the same atomic number and therefore the same number of protons. They differ only in neutron number, which gives them different mass numbers.

Question 44Atomic structure, the Periodic Table and chemical bonding

A magnesium ion has the formula Mg²⁺. How many electrons does one Mg²⁺ ion contain? (Mg, atomic number 12)

  1. 12
  2. 10 Correct answer.
  3. 14
  4. 24

Explanation

A neutral magnesium atom has 12 electrons. The 2+ charge means two electrons have been lost, leaving 12 − 2 = 10 electrons.

Question 45Atomic structure, the Periodic Table and chemical bonding

Which description best explains why magnesium chloride has a high melting point?

  1. Strong electrostatic attractions between ions must be overcome in a giant lattice. Correct answer.
  2. Delocalised electrons must be separated from metal cations.
  3. Covalent bonds within each molecule must be broken.
  4. Weak intermolecular forces must be overcome between molecules.

Explanation

Magnesium chloride is an ionic compound with a giant lattice of Mg²⁺ and Cl⁻ ions. Melting requires breaking many strong electrostatic attractions between oppositely charged ions, so a large amount of energy is needed.

Question 46Atomic structure, the Periodic Table and chemical bonding

Which element in Period 3 has the highest first ionisation energy?

  1. Sodium
  2. Aluminium
  3. Sulfur
  4. Argon Correct answer.

Explanation

First ionisation energy generally increases across a period as nuclear charge rises while shielding stays similar. Argon, the noble gas at the end of Period 3, has a stable full outer shell and holds its outer electron most tightly.

Question 47Atomic structure, the Periodic Table and chemical bonding

Which statement correctly describes the bonding in a molecule of hydrogen chloride, HCl?

  1. Two electron pairs are shared equally between the atoms.
  2. The molecule is held together by electrostatic attraction between H⁺ and Cl⁻ ions.
  3. One electron pair is shared, with the pair pulled more strongly towards chlorine. Correct answer.
  4. One electron is transferred from hydrogen to chlorine.

Explanation

Hydrogen and chlorine form a single covalent bond by sharing one pair of electrons. Chlorine is more electronegative than hydrogen, so the shared pair lies closer to chlorine, giving a polar bond.

Question 48Atomic structure, the Periodic Table and chemical bonding

Why does the melting point of the Group 1 metals decrease down the group?

  1. The ionic charge on the metal cations increases down the group.
  2. The atoms lose more electrons down the group.
  3. The distance between the nucleus and the outer electron increases, weakening the metallic bond. Correct answer.
  4. The number of delocalised electrons per atom increases down the group.

Explanation

Metallic bonding depends on the attraction between metal cations and delocalised electrons. Down Group 1 the atoms get larger and the outer electron is further from the nucleus, so this attraction weakens and less energy is needed to melt the metal.

Question 49Atomic structure, the Periodic Table and chemical bonding

Which formula correctly represents the ionic compound formed between calcium and nitrogen?

  1. Ca₂N
  2. Ca₃N₂ Correct answer.
  3. CaN₂
  4. CaN

Explanation

Calcium forms Ca²⁺ ions and nitrogen forms N³⁻ ions. The total positive and negative charges must balance, so three Ca²⁺ ions (total +6) pair with two N³⁻ ions (total −6), giving Ca₃N₂.

Question 50Atomic structure, the Periodic Table and chemical bonding

Which property is characteristic of a simple molecular substance such as iodine?

  1. It conducts electricity when molten.
  2. It has a low melting point because only weak forces between molecules are overcome. Correct answer.
  3. It dissolves in water to give a conducting solution.
  4. It forms a giant lattice held together by ionic bonds.

Explanation

In a simple molecular solid, strong covalent bonds exist within each molecule, but the molecules are held together only by weak intermolecular forces. Melting breaks these weak forces, so the melting point is low.