ESAT Chemistry: Bonding, structure and the groups, set 1

• Answer all questions. No calculator.
• Each question has exactly one correct answer.

1. 11 mark

   Sea water contains dissolved sodium chloride, magnesium sulfate and other salts in water, with the salts not chemically bonded to the water. Which term correctly describes sea water?

   1. A An element, because sea water occurs naturally in the environment.
   2. B A mixture, because the dissolved salts are not chemically combined with the water and could be separated from it by a physical method such as evaporation.
   3. C A compound, because the salts and the water always occur together in the sea in fixed proportions.
   4. D An isotope, because sea water contains atoms with different numbers of neutrons.
2. 21 mark

   Nitrogen atoms (electron configuration 2,5) and hydrogen atoms (electron configuration 1) both obtain the electron configuration of the nearest noble gas by reacting together to form ammonia, NH3. Nitrogen and hydrogen are both non-metals. What type of bonding holds the atoms together in ammonia?

   1. A Covalent bonding, because the atoms share pairs of electrons rather than transferring them.
   2. B Metallic bonding, because delocalised electrons are shared between all the atoms in the molecule.
   3. C Ionic bonding, because electrons transfer completely from the hydrogen atoms to the nitrogen atom.
   4. D No bonding is needed, because both atoms already have full outer shells before reacting.
3. 31 mark

   Aluminium forms Al3+ ions and oxygen forms O2- ions. What is the correct formula for aluminium oxide?

   1. A AlO
   2. B Al3O2
   3. C Al2O3
   4. D AlO3
4. 41 mark

   Silicon dioxide (SiO2) is a giant covalent structure in which every silicon atom is bonded to four oxygen atoms in a continuous lattice. Which property would you expect silicon dioxide to have?

   1. A Low melting point, because only weak intermolecular forces between separate molecules need to be overcome.
   2. B Good electrical conductivity, because delocalised electrons move freely through the structure.
   3. C Solubility in water, because the covalent bonds break apart in water to release ions.
   4. D High melting point, because many strong covalent bonds throughout the lattice must be broken.
5. 51 mark

   Which statement correctly explains why metals are good conductors of electricity?

   1. A The positive metal ions themselves move freely through the lattice, carrying charge as they go.
   2. B Delocalised electrons are free to move throughout the giant metallic structure and carry charge.
   3. C Covalent bonds between neighbouring metal atoms allow electrons to hop from bond to bond.
   4. D Metals contain two different types of mobile ion, of opposite charge, that balance each other as they move.
6. 61 mark

   Bromine exists as simple molecules, Br2, held together by weak intermolecular forces. When liquid bromine boils, which bonds or forces are broken?

   1. A Only the weak intermolecular forces between separate Br2 molecules.
   2. B Both the covalent bond within each Br2 molecule and the intermolecular forces between molecules.
   3. C Only the covalent bond within each Br2 molecule, releasing free bromine atoms.
   4. D No bonds or forces are broken; the molecules simply gain kinetic energy and move apart.
7. 71 mark

   Sodium chloride (an ionic giant lattice) has a much higher melting point than hydrogen chloride (simple covalent molecules). What is the best explanation for this difference?

   1. A Sodium chloride has covalent bonds, while hydrogen chloride has ionic bonds.
   2. B Sodium chloride molecules are larger and heavier than hydrogen chloride molecules, so more energy is needed to move them.
   3. C Both compounds are held together by the same type of bond, but sodium chloride simply has more atoms in each unit.
   4. D Sodium chloride's giant ionic lattice needs strong electrostatic forces between many ions to be overcome, while hydrogen chloride only needs weak intermolecular forces between molecules to be overcome.
8. 81 mark

   Which statement correctly describes the trend in reactivity of the alkali metals (Group 1) going down the group, from lithium to potassium?

   1. A Reactivity decreases going down the group, because the atoms become smaller and hold their outer electron more tightly.
   2. B Reactivity stays the same down the group, because all Group 1 elements have the same number of outer electrons.
   3. C Reactivity increases going down the group, because the outer electron is further from the nucleus and more easily lost.
   4. D Reactivity decreases going down the group, because the increasing number of protons attracts all the electrons more strongly.
9. 91 mark

   Chlorine water is added to a colourless solution of potassium bromide. Which observation, with the correct explanation, describes what happens?

   1. A No visible change occurs, because chlorine and bromine are both halogens and do not react with each other's compounds.
   2. B The solution turns orange/brown, because chlorine is more reactive than bromine and displaces it from potassium bromide.
   3. C The solution turns orange/brown, because bromine is more reactive than chlorine and displaces it from the solution.
   4. D A white precipitate forms, because potassium chloride is insoluble in water.
10. 101 mark

    Which statement correctly explains why the noble gases (Group 18) are so unreactive?

    1. A They already have a full outer shell of electrons, so they have little tendency to gain, lose or share electrons.
    2. B They have no electrons in their outer shell at all, so they cannot form any bonds.
    3. C They are all monatomic gases at room temperature, and gases never react with other substances.
    4. D They have very high melting points, so their atoms are too tightly bonded to each other to react with anything else.
11. 111 mark

    Copper cannot usually be found as an element in its ores; instead it exists as compounds, such as copper oxide. Which type of process is needed to obtain the metal element from its ore?

    1. A A physical process, such as filtration, because copper ore is a mixture of small solid particles.
    2. B A physical process, such as evaporation, because heating the ore is enough to release the copper.
    3. C A chemical process, such as reduction, because a chemical reaction is needed to break the bonds holding copper and oxygen together.
    4. D No process is needed, because copper compounds naturally break down into pure copper over time.
12. 121 mark

    A student has a mixture of sand (insoluble in water) and salt (soluble in water), stirred into water. Which physical process should be used FIRST to remove the insoluble sand from the mixture?

    1. A Chromatography, because it separates substances in a mixture using how far they travel through a material.
    2. B Fractional distillation, because it separates liquids with different boiling points.
    3. C Crystallisation, because it separates a dissolved solid by evaporating off the solvent.
    4. D Filtration, because it separates an insoluble solid from a liquid.
13. 131 mark

    A mixture of two miscible liquids, ethanol (boiling point 78 degrees C) and water (boiling point 100 degrees C), needs to be separated into its two pure components. Which separation technique should be used?

    1. A Simple distillation, because it can separate any two liquids regardless of how close their boiling points are.
    2. B Fractional distillation, because it separates miscible liquids with different boiling points.
    3. C A separating funnel, because it separates any two liquids by density.
    4. D Filtration, because it separates a mixture into its component substances using a filter paper.
14. 141 mark

    A student runs a paper chromatography experiment on a food dye and calculates an Rf value of 0.60 for one of the coloured spots. What does this Rf value represent, and what would confirm the dye is a pure substance?

    1. A Rf = distance travelled by the spot divided by distance travelled by the solvent; a pure substance produces just one spot on the chromatogram, with one Rf value.
    2. B Rf = distance travelled by the solvent divided by distance travelled by the spot; a pure substance shows the largest possible number of spots.
    3. C Rf = distance travelled by the spot divided by distance travelled by the solvent; a higher Rf value always means a purer substance.
    4. D Rf values cannot be used to assess purity, only to identify unknown coloured substances.
15. 151 mark

    Which statement correctly compares the arrangement and movement of particles in a liquid with those in a gas?

    1. A In a liquid, particles are fixed in a regular pattern and vibrate on the spot; in a gas, particles are close together and slide past each other.
    2. B Particles in a liquid and a gas are arranged and move in exactly the same way; only the particles' size differs between the two states.
    3. C In a liquid, particles are close together and can move and slide past each other; in a gas, particles are far apart and move quickly in random directions, with almost no forces between them.
    4. D In a liquid, particles are far apart and move quickly in random directions; in a gas, particles are close together and slide past each other.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: B

   1. An element contains only one type of atom (one proton number); sea water contains many different substances, so it is not an element.
   2. A compound forms when different elements are chemically bonded together in fixed proportions; the dissolved salts in sea water are not chemically bonded to the water.
   3. A mixture contains two or more substances that are not chemically combined and that can be separated by physical means, such as evaporation, which is exactly how the salts could be recovered from sea water.
   4. Sea water therefore fits the definition of a mixture, so the answer is B.
   • Why not A: Sea water contains several different substances, not one type of atom, so it cannot be classed as an element; an element is a substance made of only one type of atom.
   • Why not C: A compound requires its elements to be chemically bonded together in fixed proportions; the salts dissolved in sea water are not chemically bonded to the water molecules, which is why they can be separated out again by evaporation.
   • Why not D: Isotope is a term from atomic structure, describing atoms of the same element with different numbers of neutrons; it is not one of the terms used to classify a substance as an element, compound or mixture, so this option confuses two separate ideas.

2. Question 2Answer: A

   1. Nitrogen has the electron configuration 2,5 and hydrogen has 1, so neither atom has a full outer shell before reacting.
   2. Both nitrogen and hydrogen are non-metals, so when they react together the bonding formed is covalent rather than ionic, since ionic bonding needs a metal and a non-metal.
   3. In covalent bonding, atoms share pairs of electrons rather than transferring them completely; each nitrogen atom shares three pairs of electrons with three hydrogen atoms in NH3.
   4. This sharing gives nitrogen 8 outer electrons (the configuration of neon) and each hydrogen 2 outer electrons (the configuration of helium), so the bonding is covalent, and the answer is A.
   • Why not B: Metallic bonding describes a lattice of positive metal ions surrounded by a sea of delocalised electrons; ammonia contains no metal atoms at all, so this bonding type cannot apply.
   • Why not C: Ionic bonding involves the complete transfer of electrons between a metal and a non-metal; nitrogen and hydrogen are both non-metals, so their atoms share electrons rather than transferring them, which is covalent bonding, not ionic.
   • Why not D: Nitrogen has only 5 electrons in its outer shell and hydrogen has only 1, so neither atom already has the electron configuration of a noble gas before reacting; this is exactly why they react together.

3. Question 3Answer: C

   1. Aluminium ions carry a charge of 3+ and oxide ions carry a charge of 2-, so the compound's overall charge must balance to zero.
   2. Using the criss-cross method, the subscript for aluminium comes from the size of the oxide ion's charge (2), and the subscript for oxide comes from the size of the aluminium ion's charge (3).
   3. This gives Al2O3, which has a total positive charge of 2 x 3+ = 6+ and a total negative charge of 3 x 2- = 6-, so the charges balance exactly.
   4. The correct formula is therefore Al2O3, so the answer is C.
   • Why not A: This assumes a simple 1:1 ratio and ignores the different ionic charges; balancing a 3+ charge against a 2- charge actually requires two aluminium ions for every three oxide ions.
   • Why not B: This swaps the cross-multiplied subscripts, placing aluminium's own charge (3) as its own subscript instead of using oxygen's charge (2); each ion's subscript should come from the size of the OTHER ion's charge.
   • Why not D: This uses only the oxide subscript needed to balance aluminium's charge (3) but forgets to also adjust the aluminium subscript to 2, using oxygen's charge; the compound's overall charge is left unbalanced with only one aluminium ion.

4. Question 4Answer: D

   1. Silicon dioxide is described as a giant covalent structure, meaning every atom is joined to its neighbours by strong covalent bonds extending throughout the whole lattice, not just within small individual molecules.
   2. Melting a giant covalent structure means breaking a very large number of these strong covalent bonds, which requires a large amount of energy.
   3. This is different from a simple molecular substance, where the covalent bonds within each molecule stay intact on melting and only the weak forces between separate molecules are overcome.
   4. Because breaking covalent bonds throughout the lattice needs far more energy than overcoming intermolecular forces, silicon dioxide has a high melting point, so the answer is D.
   • Why not A: This describes the low melting point of a simple molecular covalent substance, where only weak intermolecular forces between separate molecules are overcome on melting; silicon dioxide is a giant covalent structure, where strong covalent bonds extend throughout the whole lattice, so its melting point is high, not low.
   • Why not B: Good electrical conductivity from delocalised electrons is a property of metallic bonding; in a giant covalent structure like silicon dioxide, every electron is held in a localised covalent bond, so there are no free charge carriers and it does not conduct electricity.
   • Why not C: Dissolving to release ions is a property of ionic compounds; silicon dioxide contains no ions at all, only covalent bonds, so it does not dissolve in water in this way.

5. Question 5Answer: B

   1. A metal is a giant structure of positively charged ions held in fixed positions, surrounded by a sea of delocalised (free) electrons.
   2. When a voltage is applied, it is these delocalised electrons, not the fixed positive ions, that are free to move through the structure.
   3. The movement of these charged electrons through the lattice is what constitutes an electric current.
   4. Metals therefore conduct electricity because their delocalised electrons are free to move and carry charge, so the answer is B.
   • Why not A: In a metallic lattice, the positive metal ions stay in fixed positions; it is the delocalised electrons, not the ions themselves, that are free to move and so carry the charge.
   • Why not C: Metallic bonding is not covalent bonding; there are no discrete bonds between individual pairs of atoms for electrons to hop between, only a continuous sea of delocalised electrons surrounding fixed positive ions.
   • Why not D: This describes an ionic lattice, which contains two different types of charged ion; a metallic lattice contains only one type of positively charged metal ion, surrounded by delocalised electrons, not a second mobile ion.

6. Question 6Answer: A

   1. Boiling is a physical change in state, not a chemical reaction, so no covalent bonds within the Br2 molecules are broken.
   2. In liquid bromine, separate Br2 molecules are held close together by weak intermolecular forces.
   3. Boiling requires enough energy to overcome these intermolecular forces so that the molecules can separate and move freely as a gas.
   4. The covalent bond within each Br2 molecule is far stronger than the intermolecular forces and remains intact, so only the intermolecular forces are broken, giving answer A.
   • Why not B: Boiling is a physical change, so it only needs to overcome the weak intermolecular forces between molecules, not the strong covalent bond holding the two bromine atoms together within each molecule; breaking that covalent bond would be a chemical change.
   • Why not C: This assumes the Br2 molecules split apart into separate atoms on boiling, which would mean breaking the strong covalent bond; boiling is a physical change and the Br2 molecules remain intact in the gas phase, so only the forces between molecules are overcome.
   • Why not D: Overcoming the intermolecular forces holding the liquid together does require energy, which is why bromine has to be heated to reach its boiling point; boiling is not simply the molecules gaining energy with nothing being overcome.

7. Question 7Answer: D

   1. Sodium chloride is a giant ionic structure, in which every positive sodium ion is strongly attracted to negative chloride ions in every direction throughout the lattice.
   2. Melting sodium chloride means overcoming these strong electrostatic forces between huge numbers of ions, which needs a large amount of energy and gives it a high melting point.
   3. Hydrogen chloride, in contrast, exists as small covalent molecules; the covalent bond inside each molecule stays intact on melting, and only the weak intermolecular forces between separate molecules need to be overcome.
   4. Because weak intermolecular forces need far less energy to overcome than the strong electrostatic forces of a giant ionic lattice, hydrogen chloride has a much lower melting point than sodium chloride, so the answer is D.
   • Why not A: This swaps the bonding types around: sodium chloride is held together by ionic bonds between oppositely charged ions, while hydrogen chloride is held together by a covalent bond within each molecule, with only weak intermolecular forces between separate molecules.
   • Why not B: Ionic compounds such as sodium chloride do not exist as individual molecules, so comparing 'molecule size' does not apply here; the real reason for the melting point difference is the type and strength of the bonding involved, not the size of a particle.
   • Why not C: Sodium chloride and hydrogen chloride are not held together by the same type of bond: sodium chloride is a giant ionic lattice, while hydrogen chloride consists of covalently bonded molecules with only weak forces between them, and this difference in bonding is what causes the difference in melting point.

8. Question 8Answer: C

   1. Each Group 1 metal has one electron in its outer shell, and reacting means losing that single electron to form a 1+ ion with the electron configuration of a noble gas.
   2. Going down the group from lithium to sodium to potassium, each element has an extra electron shell, so the outer electron is further from the positive nucleus.
   3. Being further from the nucleus, and shielded by more inner shells, the outer electron is held less tightly and is easier to lose.
   4. Because the outer electron is more easily lost going down the group, reactivity increases from lithium to potassium, so the answer is C.
   • Why not A: This gets the atomic size trend backwards: atoms get larger going down a group, as extra electron shells are added, so the outer electron actually becomes easier, not harder, to lose.
   • Why not B: Having the same number of outer electrons explains why all Group 1 metals react in a similar way, by losing one electron to form a 1+ ion, but it does not mean they are equally reactive; the ease of losing that electron still changes down the group.
   • Why not D: Although the number of protons does increase down the group, the outer electron is also shielded by more inner shells and is further from the nucleus, so in practice the outer electron becomes easier to remove, not harder, and reactivity increases rather than decreases.

9. Question 9Answer: B

   1. Reactivity of the halogens (Group 17) decreases going down the group, so chlorine is more reactive than bromine, which is more reactive than iodine.
   2. In a displacement reaction, a more reactive halogen can displace a less reactive halide ion from solution, taking its place in the compound and releasing the less reactive halogen as the free element.
   3. Here chlorine is more reactive than bromine, so it displaces bromide ions from potassium bromide solution, forming potassium chloride and free bromine.
   4. The bromine released into solution gives the characteristic orange/brown colour, so the answer is B.
   • Why not A: This is exactly a displacement reaction: a more reactive halogen, chlorine, can displace a less reactive halogen, bromine, from a solution of its salt, which is why displacement reactions are used to compare halogen reactivity.
   • Why not C: This reverses the Group 17 reactivity trend: reactivity decreases going down the group, so chlorine, higher up the group, is more reactive than bromine, which is lower down; a less reactive halogen cannot displace a more reactive one.
   • Why not D: This confuses displacement with a precipitation reaction, such as the silver nitrate test for halide ions; potassium chloride is soluble in water, so no precipitate forms, and the colour change is due to bromine being released into solution, not a solid forming.

10. Question 10Answer: A

    1. Atoms react in order to obtain a full outer shell of electrons, generally matching the electron configuration of a noble gas.
    2. The noble gases themselves already have a full outer shell (eight electrons, except helium with two), so there is nothing to be gained by reacting.
    3. With no tendency to gain, lose or share electrons, the noble gases form very few compounds and exist as single, unreactive atoms.
    4. The noble gases are unreactive because their outer shell is already full, so the answer is A.
    • Why not B: The noble gases do have electrons in their outer shell (eight, or two for helium); it is that this shell is already full, not empty, which is why they have little tendency to react.
    • Why not C: Being a gas at room temperature is not itself a reason for chemical unreactivity; many other gases, such as chlorine and hydrogen, are very reactive. The real reason for the noble gases' lack of reactivity is their electron configuration, not their physical state.
    • Why not D: Noble gases actually have very low melting and boiling points, because only weak forces act between separate, unbonded atoms; a high melting point is not the reason for their lack of reactivity, and this option states the opposite of the real physical property.

11. Question 11Answer: C

    1. In copper oxide, copper and oxygen are chemically bonded together as a compound, not simply mixed as separate substances.
    2. Extracting the copper metal requires breaking this chemical bond between copper and oxygen, which needs a chemical reaction rather than a physical method.
    3. A reduction reaction, in which oxygen is removed from the copper oxide, is the type of chemical process used to displace the copper and leave the pure metal behind.
    4. Physical processes such as filtration or evaporation only separate substances that are already physically mixed, so they cannot break bonds within a compound; the answer is C.
    • Why not A: Filtration is a physical process used to separate an insoluble solid from a mixture; it cannot break the chemical bonds holding copper and oxygen together within a compound, so it cannot extract the metal element.
    • Why not B: Evaporation is a physical process for separating a dissolved solid from its solvent and involves no chemical reaction, so it cannot break the bonds within copper oxide to release copper metal.
    • Why not D: Copper compounds do not spontaneously break down into pure copper; a chemical reaction, providing the energy needed to break the existing bonds and form new ones, is required to displace copper from its compound.

12. Question 12Answer: D

    1. The mixture contains an insoluble solid (sand) suspended in a liquid that also contains a dissolved solid (salt).
    2. Filtration passes the mixture through a filter paper: the insoluble sand is too large to pass through and is retained, while the liquid and the dissolved salt pass through as the filtrate.
    3. This separates the insoluble sand from the rest of the mixture in one physical step, without needing to boil or evaporate anything.
    4. Chromatography and fractional distillation are the wrong techniques for this separation, and crystallisation is a later step used on the filtrate to recover the salt, so the first correct step is filtration, answer D.
    • Why not A: Chromatography separates soluble substances that travel at different rates through a stationary material, based on differences in solubility; it is not used to remove an insoluble solid, such as sand, from a liquid.
    • Why not B: Fractional distillation separates a mixture of miscible liquids with different boiling points; it is not appropriate here, because sand is an insoluble solid rather than a liquid, and the aim is not to separate two liquids.
    • Why not C: Crystallisation is used afterwards, to recover the dissolved salt by evaporating the solvent until crystals form; it does not remove the insoluble sand, which has to be filtered out first.

13. Question 13Answer: B

    1. Ethanol and water are miscible liquids, meaning they mix completely and do not form separate layers, so a separating funnel cannot be used.
    2. Because they have different boiling points (78 and 100 degrees C), the mixture can be separated by distillation.
    3. A fractionating column allows repeated evaporation and condensation as vapour rises, which separates liquids with boiling points that are reasonably close together far more effectively than simple distillation.
    4. Fractional distillation is therefore the correct technique for separating ethanol from water, so the answer is B.
    • Why not A: Simple distillation does not separate two liquids with reasonably close boiling points cleanly, because vapour from both liquids tends to rise together; a fractionating column, as used in fractional distillation, gives repeated condensation and evaporation that separates close boiling points far more effectively.
    • Why not C: A separating funnel works only for immiscible liquids that form two distinct layers, such as oil and water; ethanol and water are miscible and mix completely into a single layer, so a separating funnel cannot separate them.
    • Why not D: Filtration separates an insoluble solid from a liquid using a filter paper; it has no effect on two liquids that are fully mixed together, since both would simply pass straight through.

14. Question 14Answer: A

    1. The Rf value is defined as the distance travelled by a spot divided by the distance travelled by the solvent front, giving a number between 0 and 1 for that substance under those conditions.
    2. A pure single substance contains only one component, so it produces just one spot on the chromatogram, with one Rf value.
    3. If the sample were impure, the different substances in it would generally travel at different rates and separate into multiple spots with different Rf values.
    4. Since the food dye here shows just one spot with an Rf value of 0.60, this is consistent with the dye being a single, pure substance, so the answer is A.
    • Why not B: This inverts the Rf calculation, which is the distance travelled by the spot divided by the distance travelled by the solvent, not the other way round; it also reverses the purity rule, since a pure substance produces the fewest spots (just one), not the most.
    • Why not C: The size of a single Rf value reflects how strongly that particular substance interacts with the stationary and mobile phases, not how pure the sample is; a substance can have any Rf value and still be pure. Purity is judged by the number of spots and by comparing Rf values with known reference substances, not by how large one Rf value is.
    • Why not D: Chromatography is a standard method for checking purity: a pure substance gives a single spot with a single Rf value, while an impure sample separates into multiple spots with different Rf values, so it can be used to assess purity as well as to identify substances.

15. Question 15Answer: C

    1. In a liquid, particles are close together (though not held in a fixed regular pattern like a solid) and have enough energy to move around and slide past one another, which is why a liquid can flow and take the shape of its container.
    2. In a gas, particles have much more energy, are spaced far apart, and move quickly in random directions, with almost no forces holding them together.
    3. The particles themselves (their size) are identical in the liquid and gas states of the same substance; only their spacing, arrangement and speed of movement differ between the states.
    4. A liquid therefore has close, sliding particles while a gas has far-apart, fast-moving particles, so the answer is C.
    • Why not A: This describes a solid for the liquid (particles fixed in a regular pattern, vibrating on the spot) and describes a liquid for the gas (particles close together, sliding past each other); each description has been shifted one state out of place.
    • Why not B: The size of the particles themselves does not change between states of the same substance; what changes is how closely packed the particles are, how they are arranged, and how much they move, which is exactly what distinguishes a liquid from a gas.
    • Why not D: This swaps the liquid and gas descriptions with each other: it is gas particles that are far apart and move quickly in random directions, and liquid particles that stay close together and slide past one another.