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NSSCO Chemistry (6117) practice questions

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Question 1Matter, separation techniques and practical chemistry

A lump of solid carbon dioxide (dry ice) left in a warm room changes directly into carbon dioxide gas without first forming a liquid. Which term describes this change of state?

  1. Condensation
  2. Evaporation
  3. Melting
  4. Sublimation Correct answer.

Explanation

Sublimation is the change of state in which a solid turns directly into a gas without passing through the liquid state, as dry ice does. Evaporation and melting both involve a liquid stage, and condensation is the reverse change from gas to liquid.

Question 2Matter, separation techniques and practical chemistry

A student must deliver exactly 25.0 cm³ of dilute hydrochloric acid into a conical flask for a titration. Which piece of apparatus gives this fixed volume with the highest accuracy?

  1. A 50 cm³ beaker
  2. A 25 cm³ measuring cylinder
  3. A 100 cm³ conical flask
  4. A 25.0 cm³ pipette Correct answer.

Explanation

A pipette is a fixed-volume graduated instrument made to deliver its stated volume to within about 0.05 cm³, so a 25.0 cm³ pipette measures 25.0 cm³ accurately. Beakers and conical flasks carry only coarse or no graduations, and a measuring cylinder is less precise than a pipette.

Question 3Matter, separation techniques and practical chemistry

Which of these mixtures is best separated by filtration through filter paper in a funnel?

  1. Ethanol and water
  2. Iron filings and sulfur
  3. Sand and water Correct answer.
  4. Iodine and ammonium chloride

Explanation

Sand is an insoluble solid suspended in water, so it is trapped by the filter paper while water passes through as the filtrate. Iodine and ammonium chloride are separated by sublimation, iron from sulfur with a magnet, and ethanol from water by fractional distillation.

Question 4Matter, separation techniques and practical chemistry

A mixture of two miscible liquids has boiling points of 78 °C and 79 °C. Which method is most suitable for separating them?

  1. Filtration through filter paper
  2. Simple distillation without a fractionating column
  3. Separation using a separating funnel
  4. Fractional distillation Correct answer.

Explanation

Fractional distillation uses a fractionating column in which repeated condensation and evaporation separate liquids whose boiling points differ by only a few degrees. Simple distillation works well only when the boiling points differ greatly, and the other two methods cannot separate miscible liquids.

Question 5Matter, separation techniques and practical chemistry

In a paper chromatography experiment, a coloured spot moved 3.0 cm from the origin line while the solvent front moved 8.0 cm from the same line. What is the Rf value of the pigment?

  1. 0.625
  2. 0.267
  3. 0.375 Correct answer.
  4. 2.67

Explanation

The Rf value is the distance travelled by the spot divided by the distance travelled by the solvent front, both measured from the origin line. Here 3.0 cm ÷ 8.0 cm = 0.375, and a valid Rf value always lies between 0 and 1.

Question 6Matter, separation techniques and practical chemistry

Kerosene (density 0.80 g/cm³) and water are shaken together, then left to stand in a separating funnel. Which statement about the separation is correct?

  1. The liquids are separated because they have different boiling points.
  2. Water forms the lower layer and runs off first, because it has the greater density. Correct answer.
  3. Kerosene forms the lower layer and runs off first, because it is the denser liquid.
  4. The mixture must be heated until one of the liquids boils away.

Explanation

Kerosene and water are immiscible, so they form two layers, and the denser liquid settles at the bottom and is run off through the tap first. Kerosene, density 0.80 g/cm³, is less dense than water, so water is the lower layer and kerosene the upper one.

Question 7Matter, separation techniques and practical chemistry

A student wants to obtain large, pure crystals of copper(II) sulfate from copper(II) sulfate solution. Which procedure is correct?

  1. Boil the solution for a long time, then pour off the liquid from the solid.
  2. Add excess water to the solution, then filter the resulting mixture.
  3. Warm the solution to concentrate it, cool it slowly, then filter off the crystals. Correct answer.
  4. Heat the solution to dryness in an evaporating basin and then grind the solid into a powder.

Explanation

Warming the solution concentrates it without removing all the water, and slow cooling allows large crystals to grow which can then be filtered off and dried. Heating to dryness drives off the water of crystallisation and leaves a powder instead of blue crystals.

Question 8Matter, separation techniques and practical chemistry

Cotton wool soaked in concentrated ammonia is placed at one end of a long glass tube and cotton wool soaked in concentrated hydrochloric acid at the other end. A white ring of ammonium chloride forms inside the tube. Where does the ring form, and why?

  1. Nearer the ammonia end, because ammonia gas is denser than hydrogen chloride gas.
  2. At the ammonia end only, because hydrogen chloride gas is too heavy to move along the tube.
  3. Nearer the hydrochloric acid end, because ammonia molecules are lighter and diffuse faster. Correct answer.
  4. Exactly at the middle of the tube, because both gases are released at the same moment.

Explanation

Ammonia has a smaller relative molecular mass (17) than hydrogen chloride (36.5), so at the same temperature its molecules have higher average speeds and travel further in the same time. The gases therefore meet nearer the hydrochloric acid end, where the white ring of ammonium chloride forms.

Question 9Matter, separation techniques and practical chemistry

A pure crystalline solid is heated steadily from room temperature to well above its boiling point, and its temperature is plotted against time. The curve shows two horizontal sections, one at 80 °C and one at 100 °C. Which statement about the curve is correct?

  1. The two horizontal sections show that the sample is a mixture, because a pure substance melts over a range of temperatures.
  2. The temperature rises quickly at each horizontal section because the particles gain kinetic energy.
  3. Energy supplied at each horizontal section increases the average kinetic energy of the particles.
  4. Energy at each horizontal section overcomes forces between particles, so the temperature stays constant. Correct answer.

Explanation

During a change of state the temperature stays constant because the energy supplied is used to overcome the attractive forces between particles instead of raising their kinetic energy. The two horizontal sections therefore mark the melting point (80 °C) and boiling point (100 °C) of a pure substance.

Question 10Matter, separation techniques and practical chemistry

A mixture contains iron filings, sand and sodium chloride. Which sequence of steps separates all three components?

  1. Add water and stir, filter, hold a magnet over the filtrate, then evaporate the solution.
  2. Add water and stir, then run off the salt solution using a separating funnel.
  3. Filter the mixture, then distil the residue and condense the vapour produced.
  4. Use a magnet, add water and stir, filter, then evaporate the filtrate to dryness. Correct answer.

Explanation

Iron is removed first with a magnet because it is the only magnetic component. Adding water then dissolves the sodium chloride while the sand stays solid; filtration traps the sand, and evaporating the filtrate recovers the salt.

Question 11Matter, separation techniques and practical chemistry

A gas is sealed in a rigid container at room temperature. Which description best matches the arrangement and movement of its particles?

  1. Particles are close together but are able to slide past one another freely.
  2. Particles are far apart and are held in a regular lattice by strong forces.
  3. Particles are far apart and move rapidly in random directions, colliding with the container walls. Correct answer.
  4. Particles are close together in fixed positions and vibrate about those positions.

Explanation

In a gas the particles are widely spaced and move randomly at high speed. They collide with one another and with the walls of the container, producing pressure. The particles are not held in fixed positions or in a lattice, so the descriptions of solids and liquids do not apply.

Question 12Matter, separation techniques and practical chemistry

Which of these samples is a mixture rather than a pure substance?

  1. Air taken from a compressed cylinder. Correct answer.
  2. Distilled water collected from a laboratory still.
  3. Copper wire used to connect a circuit.
  4. Carbon dioxide gas released from a cylinder.

Explanation

Air is a mixture because it contains nitrogen, oxygen, argon, carbon dioxide and other substances that are not chemically combined. These gases can be separated by fractional distillation of liquid air. Carbon dioxide, copper and distilled water each consist of only one substance.

Question 13Matter, separation techniques and practical chemistry

A student reads the volume of water in a measuring cylinder. The water meniscus is curved downwards. Which reading procedure gives the correct volume?

  1. View at eye level with the top of the meniscus on the graduation line.
  2. View from above with the top of the meniscus on the graduation line.
  3. View at eye level with the bottom of the meniscus on the graduation line. Correct answer.
  4. View from below with the bottom of the meniscus on the graduation line.

Explanation

For water the meniscus is concave, so the volume is read at the bottom of the curve. The eye must be level with the meniscus to avoid parallax error. Viewing from above or below makes the recorded volume too high or too low.

Question 14Matter, separation techniques and practical chemistry

Four solid samples of the same compound are heated separately. Which result gives the strongest evidence that the sample is pure?

  1. It softens at 110 °C and becomes completely liquid only at 121 °C.
  2. It melts sharply at 121 °C. Correct answer.
  3. It melts at 105 °C and remains partly solid at 120 °C.
  4. It melts gradually over the range 115 °C to 125 °C.

Explanation

A pure crystalline solid melts sharply at a fixed temperature. Impurities lower the melting point and make the solid melt over a range of temperatures. A sharp melting point close to the accepted value is therefore the best evidence of purity.

Question 15Matter, separation techniques and practical chemistry

In paper chromatography, the start line is drawn with a pencil rather than a pen, and the solvent level is kept below this line. Why is this done?

  1. A pencil line conducts the solvent evenly, and the solvent must completely cover the samples.
  2. A pencil line does not dissolve in the solvent, and the solvent must not wash the samples directly into the tank. Correct answer.
  3. A pencil line is easier to see, and the solvent must be heated before it is used.
  4. A pencil line stops the paper from tearing, and the solvent must be alkaline.

Explanation

Pencil graphite is insoluble in the solvents normally used, so it stays on the paper and does not create extra spots. Pen ink would dissolve and travel with the solvent. Keeping the solvent below the start line stops the samples from being washed straight into the solvent instead of separating on the paper.

Question 16Matter, separation techniques and practical chemistry

In a simple distillation set-up, cold water is passed through the condenser. Which arrangement and reason are correct?

  1. Water enters at the bottom and leaves at the bottom, so the vapour is cooled quickly.
  2. Water enters at the bottom and leaves at the top, so the condenser stays full and vapour is cooled efficiently. Correct answer.
  3. Water enters at the top and leaves at the top, so the condenser stays full.
  4. Water enters at the top and leaves at the bottom, so the condenser fills completely.

Explanation

Cooling water enters at the lower end of the condenser and leaves at the upper end. This keeps the water jacket full and gives efficient counter-current cooling. Vapour condensing on the inner tube is therefore cooled along its whole length.

Question 17Matter, separation techniques and practical chemistry

A beaker contains cloudy water with very fine clay particles that settle only very slowly. A student needs clear water quickly. Which method is most suitable?

  1. Evaporate the mixture to dryness and add water again.
  2. Filter the mixture through ordinary filter paper in a funnel.
  3. Use a separating funnel and run off the lower layer.
  4. Centrifuge the mixture, then decant the clear liquid. Correct answer.

Explanation

Very fine clay particles can pass through ordinary filter paper or block its pores slowly. Centrifuging spins the mixture so that the denser particles are forced to the bottom quickly, leaving a clear liquid that can be poured off. This gives clear water in a short time.

Question 18Matter, separation techniques and practical chemistry

At 20 °C, 50 g of water dissolves a maximum of 18 g of potassium nitrate. A student adds 22 g of potassium nitrate to 50 g of water at 20 °C and stirs thoroughly. What is observed?

  1. All 22 g dissolves, giving a saturated solution.
  2. 22 g dissolves only after the mixture is cooled.
  3. All 22 g dissolves, giving an unsaturated solution.
  4. 18 g dissolves and 4 g remains as solid at the bottom. Correct answer.

Explanation

At 20 °C, 50 g of water can dissolve only 18 g of potassium nitrate, so 18 g dissolves and 4 g remains undissolved. The solution is saturated with potassium nitrate. The temperature is unchanged, so the extra solid does not dissolve.

Question 19Matter, separation techniques and practical chemistry

Iodine is much more soluble in hexane than in water. A student shakes an iodine solution with hexane in a separating funnel and lets the mixture stand. Hexane is less dense than water and does not mix with it. Which layer contains most of the iodine, and how is that layer collected?

  1. The lower hexane layer, which is run off first through the tap.
  2. The upper hexane layer, which is run off first through the tap.
  3. The upper hexane layer, which is poured out from the top of the funnel. Correct answer.
  4. The lower water layer, which is poured out from the top of the funnel.

Explanation

Hexane is less dense than water and immiscible with it, so hexane forms the upper layer. Iodine is much more soluble in hexane, so most of it moves into that layer. The lower aqueous layer is run off through the tap first, and the upper hexane layer is then poured out from the top of the funnel.

Question 20Matter, separation techniques and practical chemistry

A student heats a crucible containing hydrated copper(II) sulfate, cools it in a desiccator and weighs it. The heating, cooling and weighing are repeated until two successive masses agree. Why is this done?

  1. To make sure that all the water of crystallisation has been removed. Correct answer.
  2. To make sure that the salt has decomposed into copper and sulfate.
  3. To make sure that the crucible has absorbed water from the air.
  4. To make sure that the balance reading increases with each heating.

Explanation

Repeating the heating, cooling and weighing until the mass is constant shows that no further water is being lost. This ensures that all the water of crystallisation has been driven off, so the mass loss represents only that water. Cooling in a desiccator prevents the anhydrous salt from absorbing water from the air before weighing.

Question 21Atomic structure

The atomic number of an element is equal to the number of

  1. protons in the nucleus Correct answer.
  2. nucleons in the nucleus
  3. neutrons in the nucleus
  4. occupied electron shells

Explanation

The atomic number, also called the proton number, is the number of protons in the nucleus of an atom of the element. In a neutral atom this is also equal to the number of electrons orbiting the nucleus. The mass number is a different quantity because it counts protons plus neutrons.

Question 22Atomic structure

Two atoms are isotopes of the same element. Which statement about the two atoms is always true?

  1. They contain the same number of protons. Correct answer.
  2. They contain the same number of neutrons.
  3. They have the same mass number.
  4. They have the same number of nucleons.

Explanation

Isotopes are atoms of the same element, so by definition they have identical proton numbers and therefore identical numbers of electrons. They differ in the number of neutrons in the nucleus. Because neutrons differ, the mass number (nucleon number) also differs between the isotopes.

Question 23Atomic structure

Which statement about the filling of electron shells in the first 20 elements is correct?

  1. The first shell holds a maximum of eight electrons.
  2. Electrons are added to shells in no fixed order.
  3. The third shell fills before the second shell.
  4. The first shell holds a maximum of two electrons. Correct answer.

Explanation

Electrons occupy shells of increasing energy, and each shell has a maximum capacity. The first shell can hold only two electrons and is always filled before the second shell begins to fill. The second shell then takes up to eight electrons, and the third shell takes up to eight before the fourth is occupied in this part of the periodic table.

Question 24Atomic structure

Magnesium has three stable isotopes with mass numbers 24, 25 and 26 and percentage abundances of 79.0%, 10.0% and 11.0%. What is the relative atomic mass of magnesium, to two decimal places?

  1. 25.00
  2. 24.32 Correct answer.
  3. 24.00
  4. 25.32

Explanation

Relative atomic mass is the abundance-weighted average of the isotope masses, calculated as (24 × 0.79) + (25 × 0.10) + (26 × 0.11). This gives 18.96 + 2.50 + 2.86 = 24.32. The value lies between the lightest and heaviest isotope masses, closer to 24 because that isotope is most abundant.

Question 25Atomic structure

A magnesium atom has 12 protons and 12 neutrons. It forms an ion with a 2+ charge. Which description of this ion is correct?

  1. 10 protons, 12 neutrons and 10 electrons
  2. 12 protons, 12 neutrons and 12 electrons
  3. 12 protons, 14 neutrons and 10 electrons
  4. 12 protons, 12 neutrons and 10 electrons Correct answer.

Explanation

A 2+ charge means the atom has lost two electrons, so 12 − 2 = 10 electrons remain. Protons and neutrons are unchanged when ions form, so the nucleus still contains 12 protons and 12 neutrons. The ion is therefore written as 24Mg2+.

Question 26Atomic structure

An element has the electron arrangement 2,8,6. In which group and period of the periodic table does it belong?

  1. Group 6, Period 3
  2. Group 14, Period 3
  3. Group 6, Period 2
  4. Group 16, Period 3 Correct answer.

Explanation

The number of occupied shells gives the period, and there are three shells here, so the element is in period 3. For main-group elements, the number of outer-shell electrons gives the group, and six outer electrons place the element in group 16 (group VI). The element is sulfur.

Question 27Atomic structure

The relative atomic mass of copper is 63.5, yet all copper atoms have a mass number of 63 or 65. Which statement best explains the value 63.5?

  1. Half of all copper atoms have a mass number of 63 and the other half have a mass number of 65.
  2. Electrons in a copper atom add about half a unit of mass.
  3. Some copper atoms have 63.5 protons in the nucleus.
  4. Copper is a mixture of isotopes, averaged by abundance. Correct answer.

Explanation

Copper occurs naturally as a mixture of isotopes, mainly 63Cu and 65Cu, present in unequal amounts. Relative atomic mass is the weighted average of these isotope masses, so it can fall between two whole numbers. Equal amounts of the two isotopes would give exactly 64, which is not observed.

Question 28Atomic structure

In the alpha-particle scattering experiment, most alpha particles passed straight through the gold foil. Which conclusion is supported by this observation?

  1. Alpha particles carry a positive charge.
  2. Electrons orbit the nucleus in fixed shells.
  3. Most of the volume of an atom is empty space. Correct answer.
  4. The nucleus contains both protons and neutrons.

Explanation

Alpha particles are only deflected when they pass close to a concentrated positive charge, so particles passing straight through the foil suggest that they met very little matter. This supports the model of an atom as mostly empty space with a tiny, dense, positive nucleus. The small number of large deflections showed that this nucleus is very small and very dense.

Question 29Atomic structure

Boron consists of two isotopes, 10B and 11B, and its relative atomic mass is 10.8. What is the percentage abundance of 10B?

  1. 10%
  2. 20% Correct answer.
  3. 45%
  4. 80%

Explanation

Let x be the percentage of 10B, so the percentage of 11B is (100 − x). Using the weighted average: [10x + 11(100 − x)] ÷ 100 = 10.8, which simplifies to 1100 − x = 1080, giving x = 20. The heavier isotope therefore makes up 80% of natural boron, which is why the relative atomic mass is closer to 11.

Question 30Atomic structure

A cation X2+ has the electron arrangement 2,8,8. What is the proton number of element X?

  1. 16
  2. 20 Correct answer.
  3. 18
  4. 22

Explanation

The arrangement 2,8,8 contains 18 electrons, and this is the number left in the ion after the atom has lost two electrons. The neutral atom therefore had 18 + 2 = 20 electrons and hence 20 protons, so X is calcium. The ion has the same electron arrangement as argon because it has lost its two outer electrons.

Question 31Atomic structure

Which statement about an electron in an atom is correct?

  1. It has a negative charge and is located in a shell around the nucleus. Correct answer.
  2. It has a positive charge and is located in the nucleus.
  3. It has a negative charge and is located in the nucleus.
  4. It has no charge and is located in the nucleus.

Explanation

Electrons are the negatively charged subatomic particles in an atom. They occupy regions called shells, or energy levels, outside the nucleus, while protons and neutrons make up the nucleus.

Question 32Atomic structure

Which statement about the nucleus of an atom is correct?

  1. It contains protons and neutrons and most of the atom's mass. Correct answer.
  2. It contains protons and electrons and most of the atom's mass.
  3. It contains only neutrons and most of the atom's mass.
  4. It contains protons and neutrons and very little of the atom's mass.

Explanation

The nucleus is the central part of an atom and contains protons and neutrons. These particles account for nearly all of the mass of the atom, because electrons have negligible mass by comparison.

Question 33Atomic structure

An iron atom has a mass number of 56 and an atomic number of 26. How many neutrons does this atom contain?

  1. 82
  2. 56
  3. 30 Correct answer.
  4. 26

Explanation

The mass number is the total number of protons and neutrons in the nucleus. Subtracting the atomic number, which gives the number of protons, from the mass number gives 56 − 26 = 30 neutrons.

Question 34Atomic structure

Chlorine has proton number 17. A chlorine atom gains one electron to form a chloride ion, Cl⁻. Which electron arrangement represents this ion?

  1. 2,7,8
  2. 2,8,8,1
  3. 2,8,7
  4. 2,8,8 Correct answer.

Explanation

A neutral chlorine atom has 17 electrons, arranged 2,8,7. Gaining one electron gives the chloride ion a total of 18 electrons, so its electron arrangement becomes 2,8,8.

Question 35Atomic structure

Which statement correctly distinguishes mass number from relative atomic mass?

  1. Mass number is the number of protons in one atom, while relative atomic mass is the total number of neutrons in one atom.
  2. Mass number is the weighted average mass of the isotopes of an element, while relative atomic mass is the total number of protons and neutrons in one atom.
  3. Mass number and relative atomic mass are always the same whole number for every element, regardless of its isotopes.
  4. Mass number counts protons plus neutrons in one atom; relative atomic mass is the weighted average mass of an element's isotopes. Correct answer.

Explanation

Mass number applies to a single atom and equals the number of protons plus neutrons. Relative atomic mass is an average for the naturally occurring isotopes, weighted by their abundances, so it is often not a whole number.

Question 36Atomic structure

A particle has 16 protons, 18 neutrons and 18 electrons. Which description of this particle is correct?

  1. It is a chloride ion, Cl⁻, with mass number 34.
  2. It is a sulfide ion, S²⁻, with mass number 36.
  3. It is an argon atom, Ar, with mass number 34.
  4. It is a sulfide ion, S²⁻, with mass number 34. Correct answer.

Explanation

The element with 16 protons is sulfur, and its mass number is 16 + 18 = 34. The particle has two more electrons than protons, so it carries a 2− charge and is written S²⁻.

Question 37Chemical bonding and structure

Which statement describes the force that holds the ions together in an ionic compound?

  1. The sharing of one or more pairs of electrons between two atoms
  2. The weak attraction between molecules with permanent dipoles
  3. The electrostatic attraction between oppositely charged ions Correct answer.
  4. The attraction between metal nuclei and a sea of delocalised electrons

Explanation

Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom, producing positive and negative ions. The oppositely charged ions then attract each other electrostatically in all directions, forming a giant ionic lattice.

Question 38Chemical bonding and structure

Which substance is held together by ionic bonds?

  1. Methane
  2. Sodium chloride Correct answer.
  3. Iodine
  4. Carbon dioxide

Explanation

Sodium is a metal and chlorine is a non-metal, so sodium transfers an electron to chlorine to form Na⁺ and Cl⁻ ions. These ions are held in a giant ionic lattice by electrostatic attraction.

Question 39Chemical bonding and structure

Graphite conducts electricity along its layers. Which statement explains this?

  1. Carbon atoms carry a positive charge that can move between layers.
  2. One delocalised electron per carbon atom can move along each layer. Correct answer.
  3. The covalent bonds within each layer break to release ions.
  4. Weak forces between layers allow whole layers to slide and carry charge.

Explanation

In graphite, each carbon atom forms three covalent bonds, leaving one electron per atom delocalised. These electrons are free to move along the layer, so graphite conducts electricity; the weak forces between layers only explain why layers slide.

Question 40Chemical bonding and structure

An element X forms X²⁺ ions and an element Y forms Y³⁻ ions. Which formula is correct for the compound they form?

  1. XY
  2. X₂Y₃
  3. X₂Y
  4. X₃Y₂ Correct answer.

Explanation

The formula must give a total charge of zero. Three X²⁺ ions give +6 and two Y³⁻ ions give −6, so the simplest whole-number formula is X₃Y₂.

Question 41Chemical bonding and structure

Solid iodine melts at 114 °C even though the covalent I–I bond is very strong. Which statement explains the low melting point?

  1. Iodine contains ions that separate from each other when heated.
  2. The covalent bonds within iodine molecules break at a low temperature.
  3. Iodine is a giant covalent substance with weak bonds throughout.
  4. Only weak forces between iodine molecules have to be overcome. Correct answer.

Explanation

Iodine consists of small I₂ molecules, and the solid is held together by weak intermolecular forces. Melting overcomes only these weak forces, not the strong I–I covalent bonds, so the melting point is low.

Question 42Chemical bonding and structure

In a dot-and-cross diagram of a nitrogen molecule, N₂, how many electron pairs are shared between the two atoms?

  1. Six
  2. Two
  3. Three Correct answer.
  4. One

Explanation

Each nitrogen atom has five outer electrons and needs three more to complete an octet. The two atoms therefore share three pairs of electrons, forming a triple bond, with one lone pair left on each atom.

Question 43Chemical bonding and structure

Why can a piece of copper be hammered into a thin sheet without breaking?

  1. The copper atoms are joined by weak intermolecular forces that break easily.
  2. Copper ion layers slide while delocalised electrons keep them attracted. Correct answer.
  3. The copper ions are held in a rigid lattice that cannot move.
  4. Covalent bonds between copper atoms stretch when the metal is hit.

Explanation

In a metal the positive ions are arranged in layers surrounded by delocalised electrons. When the metal is hammered, the layers slide over one another but the electrons continue to attract the ions, so the metal bends rather than breaking.

Question 44Chemical bonding and structure

Solid sodium chloride does not conduct electricity, but molten sodium chloride does. Which statement explains this?

  1. In the solid, electrons are free to move; melting traps them.
  2. The ions are fixed in the solid lattice but mobile when molten. Correct answer.
  3. Melting breaks the covalent bonds to produce charged atoms.
  4. Melting converts sodium chloride into a metal.

Explanation

Electrical conduction in an ionic compound requires charged particles that can move. In solid sodium chloride the ions are held in fixed positions in the lattice, but melting releases them so they can carry current.

Question 45Chemical bonding and structure

Magnesium oxide melts at about 2852 °C, whereas sodium chloride melts at 801 °C. Both are ionic. Which statement best explains the higher melting point of magnesium oxide?

  1. Magnesium oxide is a simple molecular compound with strong covalent bonds.
  2. The Mg²⁺ and O²⁻ ions have higher charges, giving stronger electrostatic attraction. Correct answer.
  3. Magnesium oxide contains delocalised electrons that strengthen the lattice.
  4. The ions in magnesium oxide are larger, so they can pack more closely.

Explanation

The strength of an ionic lattice depends on the size and charge of the ions. Mg²⁺ and O²⁻ carry double the charge of Na⁺ and Cl⁻, and they are also smaller, so the electrostatic attraction is much stronger and more energy is needed to melt the solid.

Question 46Chemical bonding and structure

Silicon dioxide melts at about 1710 °C and does not conduct electricity when solid or when liquid. Which type of structure is consistent with these properties?

  1. A giant ionic lattice
  2. A giant covalent lattice Correct answer.
  3. A simple molecular lattice
  4. A metallic lattice

Explanation

A very high melting point shows that many strong bonds must be broken, which is typical of a giant covalent lattice such as silicon dioxide. It does not conduct when molten because it contains no free ions or electrons.

Question 47Chemical bonding and structure

Which statement describes the bonding in a solid metal such as copper?

  1. A lattice of positive ions in a sea of delocalised electrons Correct answer.
  2. Small molecules held together by weak intermolecular forces
  3. Atoms joined by shared pairs of electrons throughout a giant covalent network
  4. A lattice of positive and negative ions held by electrostatic attraction

Explanation

Metallic bonding consists of positive metal ions arranged in a lattice, with outer electrons delocalised and free to move through the structure. These delocalised electrons are attracted to the positive ions and hold the lattice together. The other options describe ionic, simple molecular and giant covalent structures respectively.

Question 48Chemical bonding and structure

When ammonia reacts with a hydrogen ion, the nitrogen atom donates a lone pair to form the ammonium ion, NH4+. Which type of bond is formed by this donation?

  1. A metallic bond
  2. A coordinate (dative covalent) bond Correct answer.
  3. A hydrogen bond
  4. An ionic bond

Explanation

A coordinate or dative covalent bond forms when one atom provides both electrons for a shared pair. In the ammonium ion, nitrogen donates its lone pair to the hydrogen ion, which has an empty orbital. Once formed, the coordinate bond is indistinguishable from the other N–H covalent bonds.

Question 49Chemical bonding and structure

An ionic crystal such as sodium chloride shatters when struck with a hammer. Which statement explains this behaviour?

  1. Covalent bonds between the ions break when the crystal is hit.
  2. The electrostatic attractions in the lattice are weak and are easily overcome.
  3. The ions are free to move, so they collide and break the lattice apart.
  4. When layers of ions slide, ions with the same charge line up and repel each other. Correct answer.

Explanation

In an ionic solid, ions are held in fixed positions by strong electrostatic attractions. When a crystal is struck, layers of ions can be displaced so that ions of the same charge become adjacent. The resulting repulsion causes the lattice to split along that plane, making the crystal brittle.

Question 50Chemical bonding and structure

Water boils at 100 °C, while hydrogen sulfide boils at −60 °C, although both are simple molecular substances. Which statement explains this difference?

  1. Hydrogen bonds between water molecules are stronger than the intermolecular forces between hydrogen sulfide molecules. Correct answer.
  2. The O–H covalent bonds in water are stronger than the S–H bonds in hydrogen sulfide.
  3. Hydrogen sulfide has a giant covalent structure, whereas water has a simple molecular structure.
  4. Water molecules are held together by ionic bonds, but hydrogen sulfide molecules are not.

Explanation

Boiling a simple molecular substance requires energy to overcome the forces between molecules, not the covalent bonds within molecules. Water molecules form hydrogen bonds, which are stronger than the dipole–dipole and dispersion forces between hydrogen sulfide molecules. Therefore water has the higher boiling point.