S1.1 Introduction to the particulate nature of matter

1. Oxygen, since each of its molecules contains two atoms bonded together — A student who equates 'molecule' with 'compound' picks this. O₂ molecules contain two atoms of the SAME element, so oxygen is an element. A compound needs atoms of different elements bonded together.
2. Air, since it contains several elements in the same proportions everywhere — A student who counts elements, or who reads air's percentages as a fixed formula, picks this. The nitrogen and oxygen in air are separate molecules, not bonded to each other, so air is a mixture; its proportions also vary (water vapour especially).
3. Carbon dioxide, since it contains atoms of two elements bonded in a fixed ratio — Carbon dioxide, CO₂, contains carbon and oxygen atoms chemically bonded in a fixed 1:2 ratio, which is what makes a substance a compound.
4. Salt water, since the salt and the water have combined to form one new substance — A student who thinks dissolving is a chemical change picks this. Salt water is a mixture: the salt-to-water ratio can vary and evaporating the water gives back sodium chloride unchanged.

Syllabus statement S1.1.1 · Read this in Learn

1. Evaporation can take place only once the water has reached its boiling point — A student who confuses evaporation with boiling picks this. Evaporation happens from the surface at any temperature; only boiling requires the boiling point.
2. Evaporation takes place at the surface, even well below the boiling point — Water particles have a range of kinetic energies. At any temperature some particles at the surface have enough energy to overcome the attractions and escape, which is why puddles dry at 20 °C. Boiling, with bubbles of vapour throughout the liquid, happens at the boiling point.
3. The large bubbles in boiling water contain hydrogen and oxygen — A student who thinks boiling splits water into its elements picks this. The bubbles contain water vapour, H₂O(g). Boiling overcomes the attractions between water molecules; the covalent bonds within them are unchanged.
4. The large bubbles in boiling water are air from the gaps between the particles — A student who thinks there is air between particles picks this. The space between particles is empty. The large bubbles that form throughout boiling water contain water vapour, H₂O(g).

Syllabus statement S1.1.2 · Read this in Learn

1. A temperature of 0 K is the same as a temperature of 273 °C — A student who always adds 273 when converting picks this. To convert kelvin to Celsius, subtract 273: 0 K = −273 °C, absolute zero.
2. Doubling a temperature in °C doubles the average kinetic energy — A student who takes average kinetic energy to be proportional to the Celsius temperature picks this. It is proportional to the temperature in kelvin: 20 °C (293 K) to 40 °C (313 K) raises it by only about 7%.
3. A rise of 1 K is exactly the same size as a rise of 1 °C — The kelvin has the same incremental value as the Celsius degree; the two scales differ only in their zero (0 K = −273 °C). So a temperature change of 25 °C is a change of 25 K.
4. Temperature in K measures how much heat a sample holds — A student who treats temperature and heat as the same picks this. Temperature is a measure of the average kinetic energy of the particles and does not depend on the size of the sample; heat is energy transferred because of a temperature difference.

Syllabus statement S1.1.3 · Read this in Learn

1. Its hydrogen and oxygen can be separated again simply by boiling it — A student who thinks compounds can be separated by physical methods picks this. Boiling turns water into water vapour, H₂O(g); the molecules stay intact. Separating water into hydrogen and oxygen needs a chemical change, such as electrolysis.
2. Its hydrogen and oxygen are bonded together in a fixed ratio — Water is a compound: every H₂O molecule contains hydrogen and oxygen atoms chemically bonded in a 2:1 ratio, so pure water always has the same composition by mass, whatever proportions of the gases were mixed.
3. Its hydrogen and oxygen each keep their own properties — A student who thinks the elements keep their properties in a compound picks this. Hydrogen burns and oxygen supports burning; water does neither. A compound has new properties of its own.
4. Its hydrogen and oxygen are in whatever ratio of gases was mixed — A student who thinks a compound's composition depends on the amounts used to make it picks this. Water always contains hydrogen and oxygen atoms in a 2:1 ratio; if more of one gas is mixed, the excess is left over unreacted, not built into the water.

Syllabus statement S1.1.1 · Read this in Learn

1. They are close together but not in fixed positions — This is a liquid. Its particles are close together, as in a solid (liquids are barely compressible), but not held in fixed positions, so they move past one another and the liquid takes the shape of its container.
2. They are spaced about halfway between those of a solid and a gas — A student who pictures a liquid as halfway between a solid and a gas in spacing picks this. Liquid particles are almost as close as in a solid, which is why a liquid, like a solid, is hardly compressible.
3. The spaces between the particles are filled with air — A student who thinks there is air between particles picks this. The spaces between particles are empty. Air is itself made of particles, so it cannot fill the gaps between particles.
4. Each one is soft, so it changes shape as the substance flows — A student who gives particles the properties of the bulk substance picks this. A liquid flows because its particles move past one another; the particles themselves do not soften or change shape.

Syllabus statement S1.1.2 · Read this in Learn

1. The energy supplied is lost to the surroundings, so X absorbs none of it as it melts — A student who thinks energy absorbed must always raise the temperature concludes that a flat section means the energy is not reaching X. X does absorb energy at a constant rate while it melts; the energy overcomes the attractions between molecules instead of increasing their average kinetic energy, so the temperature stays constant.
2. The energy absorbed overcomes attractions between molecules; their average Ek is constant — Temperature is a measure of the average kinetic energy of the particles. While X melts, the energy absorbed separates the molecules against their attractions, so the average kinetic energy, and the temperature, stay constant until all the solid has melted.
3. The energy absorbed breaks the covalent bonds within each of the molecules of X — A student who thinks melting breaks the bonds within molecules picks this. Melting overcomes the attractions between molecules; the covalent bonds are unchanged, so the liquid is still X.
4. The energy absorbed is used to make each molecule expand as the solid melts — A student who gives molecules the properties of the bulk substance picks this. Molecules do not expand or melt; melting is a change in how the molecules are arranged and move, as the attractions between them are overcome.

Syllabus statement S1.1.3 · Read this in Learn

1. It is a heterogeneous mixture, since the dissolved salt slowly sinks to the bottom — A student who thinks dissolved solids settle picks this. Dissolved ions are kept in constant random motion among the water molecules and stay evenly distributed; only undissolved solid settles, as in a heterogeneous mixture such as sand in water.
2. It is a compound, since the salt and water have bonded together in solution — A student who thinks dissolving forms a new substance picks this. The salt-to-water ratio is not fixed and the salt is recovered unchanged when the water is evaporated, so the solution is a mixture.
3. It is a pure substance, since the salt no longer exists once it has dissolved — A student who thinks a dissolved solid disappears picks this. The salt is still there: the solution tastes salty, its mass is the mass of water plus salt, and evaporating the water gives the salt back.
4. It is a homogeneous mixture, since its composition is the same throughout the solution — The Na⁺ and Cl⁻ ions spread evenly through the water, each surrounded by water molecules, in a single phase. Every sample has the same composition, which is what homogeneous means; the ratio of salt to water can vary, so it is a mixture.

Syllabus statement S1.1.1 · Read this in Learn

1. In the filtrate, as ions surrounded by water molecules — Sodium chloride dissolves: its Na⁺ and Cl⁻ ions separate and are surrounded by water molecules (solvation). These ions are far too small to be held back by filter paper, so they pass into the filtrate. The insoluble sand stays on the paper. Evaporating the filtrate recovers the salt.
2. On the filter paper, since solid cannot pass through it — A student who thinks filtration removes dissolved solids picks this. Only the undissolved sand is held back. The dissolved sodium chloride is present as separate ions surrounded by water molecules, which pass through the paper.
3. Nowhere, since the salt stopped existing when it dissolved — A student who thinks dissolved substances disappear picks this. The sodium chloride is still present as ions in the filtrate; evaporating the water gives it back.
4. In the filtrate, as salt that has melted in the hot water — A student who confuses dissolving with melting picks this. Sodium chloride melts at about 800 °C; in hot water it dissolves, forming a solution. The salt is in the filtrate, but as dissolved ions, not as molten salt.

Syllabus statement S1.1.1 · Read this in Learn

1. Evaporation, as the water is collected when it leaves the dish — A student who treats evaporation and distillation as the same process picks this. In evaporation the water vapour escapes into the air and is lost; the method recovers the copper(II) sulfate, not the water.
2. Filtration, as the dissolved solid is caught by the paper — A student who thinks filter paper can catch a dissolved solid picks this. Dissolved Cu²⁺ and SO₄²⁻ ions pass straight through filter paper, so the filtrate is still blue copper(II) sulfate solution.
3. Electrolysis, as the solute and water are chemically bonded — A student who thinks a solution is a compound, so needs a chemical method, picks this. The solution is a mixture, separable by a physical method. Electrolysis causes chemical changes (copper is deposited at the negative electrode); it does not collect pure water.
4. Distillation, as the water vapour is condensed and collected — Heating vaporizes the water; the vapour is cooled in a condenser and the pure liquid water collected. Copper(II) sulfate does not vaporize at 100 °C, so it stays in the flask.

Syllabus statement S1.1.1 · Read this in Learn

1. It is a mixture of several different dyes — More than one spot shows more than one component. Each dye is carried a different distance up the paper, according to how strongly it is attracted to the paper compared with the water. The dyes were already present and are unchanged.
2. It is a mixture of several different elements — A student who uses 'element' to mean any component picks this. The ink is a mixture, but its components are dyes, which are compounds; the word element means a substance that cannot be broken down chemically.
3. It is a compound the paper has separated out — A student who thinks compounds can be separated by physical methods picks this. Paper chromatography is a physical method: it separates the components of a mixture, not the elements of a compound, so several spots show a mixture.
4. It is a single dye that has reacted with water — A student who thinks separation makes new substances picks this. Chromatography is a physical process: the dyes in the spots were in the ink all along, and they are unchanged.

Syllabus statement S1.1.1 · Read this in Learn

1. X freezes, as the solution is cooled below the temperature at which X melts — A student who confuses dissolving with melting picks this. X was never molten: it was dissolved in the water, and the solution was never near the melting point of X. It crystallizes because its solubility falls.
2. X is less soluble in cold water than in hot water, so the excess comes out of solution — The hot solution is close to saturated. Cold water can dissolve less X, so on cooling the X that the water can no longer hold crystallizes. The small amounts of soluble impurity stay dissolved.
3. Dissolved X sinks and collects at the bottom of the flask as the water cools — A student who thinks dissolved solids settle picks this. A solution stays uniform; dissolved X does not sink. Crystals form only once the cold water cannot hold all the X in solution.
4. Water evaporates as it cools, since the solubility of X does not change — A student who thinks solubility does not depend on temperature picks this. Little water evaporates from a cooling solution. Crystals form because X is much less soluble in cold water than in hot.

Syllabus statement S1.1.1 · Read this in Learn

1. Filtration, as the dissolved solid is held back on the paper — A student who thinks filter paper can catch a dissolved solid picks this. Dissolved K⁺ and Cl⁻ ions pass through the paper with the water, so the filtrate is still potassium chloride solution.
2. Cooling, as the dissolved solid freezes when cold enough — A student who confuses dissolving with melting picks this. The potassium chloride is dissolved, not molten, so it cannot freeze. Cooling a dilute solution far enough freezes the water, not the dissolved salt.
3. Evaporation, as the water escapes and the solid is left behind — Heating the solution in an open dish vaporizes the water, which escapes into the air; potassium chloride does not vaporize, so the solid is left in the dish. Evaporation recovers the solute; distillation would be used to collect the water.
4. Decanting, as the dissolved solid settles to the bottom on standing — A student who thinks a dissolved solid settles picks this. A solution is a homogeneous mixture; the dissolved ions stay evenly spread and never settle, so pouring off the liquid leaves no solid behind.

Syllabus statement S1.1.1 · Read this in Learn

1. Melting: NaCl(s) → NaCl(aq) — A student who thinks dissolving is melting picks this. NaCl(s) → NaCl(aq) represents dissolving in water. Melting would be NaCl(s) → NaCl(l), which needs a temperature of about 800 °C.
2. Condensation: H₂O(g) → H₂O(s) — A student who calls any change from gas to a denser state condensation picks this. A direct change from gas to solid, as when frost forms, is deposition. Condensation is gas to liquid, H₂O(g) → H₂O(l).
3. Boiling: H₂O(aq) → H₂O(g) — A student who uses (aq) to mean any liquid picks this. (aq) means dissolved in water, so water itself is H₂O(l). Boiling is H₂O(l) → H₂O(g).
4. Sublimation: CO₂(s) → CO₂(g) — At atmospheric pressure solid carbon dioxide ('dry ice') changes directly into gas without becoming liquid. A direct change from solid to gas is sublimation, and the state symbols (s) and (g) show exactly that.

Syllabus statement S1.1.2 · Read this in Learn

1. Condensation — Condensation is gas to liquid, H₂O(g) → H₂O(l). Here no liquid forms: the vapour becomes solid directly, which is deposition.
2. Freezing — Freezing is liquid to solid, H₂O(l) → H₂O(s). The stem says no liquid water forms, so the change is gas to solid: deposition.
3. Deposition — A direct change from gas to solid, H₂O(g) → H₂O(s), is deposition. It is the reverse of sublimation.
4. Sublimation — Sublimation is the direct change from solid to gas, e.g. CO₂(s) → CO₂(g). The reverse, gas to solid, is deposition.

Syllabus statement S1.1.2 · Read this in Learn

1. 427 °C — Average kinetic energy is proportional to temperature in kelvin. 77 °C = 350 K; doubling gives 700 K; 700 − 273 = 427 °C.
2. 154 °C — A student who takes average kinetic energy to be proportional to the Celsius temperature doubles 77 °C to get 154 °C. The Celsius zero is not absolute zero, so the temperature must be doubled in kelvin: 350 K → 700 K = 427 °C.
3. 700 °C — A student who calculates the new temperature in kelvin, 700 K, but does not convert it back gives 700 °C. The question asks for °C: 700 K = 700 − 273 = 427 °C.
4. 973 °C — A student who always adds 273 to convert gives 700 + 273 = 973 °C. Converting from kelvin to Celsius means subtracting 273: 700 K = 427 °C.

Working Average Ek ∝ T (in K). T₁ = 77 + 273 = 350 K. Doubling the average kinetic energy doubles T: T₂ = 2 × 350 K = 700 K. In °C: 700 − 273 = 427 °C.

Syllabus statement S1.1.3 · Read this in Learn