Chemistry 0620 | Paper 1 Mock 01 | Multiple Choice (Core)

With a limited air supply the carbon in the natural gas cannot be fully oxidised, so incomplete combustion produces carbon monoxide together with unburnt carbon as soot, and this is the pair that makes the heater dangerous. Carbon monoxide is colourless and has no smell, and it binds to haemoglobin in the blood more strongly than oxygen does, so it stops the blood carrying oxygen and can kill without warning. Soot blocks the burner and blackens surfaces, and it also signals that the flame is burning inefficiently.

Carbon dioxide and water are the products of complete combustion in plenty of air, which is the safe outcome, not the hazard. Nitrogen and argon are unreactive components of air and are not products. Hydrogen and oxygen are not formed when a hydrocarbon burns.

A yellow, sooty flame is the visible warning sign.

The plunger moves out fastest at the beginning because that is when the reaction is producing hydrogen most quickly. As time passes, the reactants are steadily consumed.

The reaction slows because the acid is being used up, so its concentration falls. With fewer acid particles in each cubic centimetre, collisions with the zinc surface become less frequent, so the gradient of the volume against time curve decreases.

No extra acid is being added, and the temperature does not keep rising; even the slight warming from this exothermic reaction would speed it up rather than slow it. Zinc is a reactant here, not a catalyst, and it is consumed too, so the exposed surface shrinks as well.

Judge the alloy on both rows of the table together. The correct reason is its greater strength combined with a similarly low density. Relative strength rises from 1 to 4, a fourfold gain, while density rises only from \(2.70\) to \(2.80\ \mathrm{g/cm^3}\), an increase of under 4%. The aircraft therefore gains a great deal of load-bearing ability for almost no extra mass.

Saying the alloy has the lower density contradicts the table, because the alloy is the denser of the two. No conductivity or melting point data appear in the table at all, so neither of those claims can be supported; inventing a property that was not measured is the trap here.

In data questions, quote the numbers that actually appear before choosing.

Rusting needs both oxygen and water, and the moist iron supplies the water while the trapped air supplies the oxygen. As the iron rusts, oxygen is taken out of the air and locked into hydrated iron(III) oxide, a solid. The volume of gas in the jar therefore falls, and water rises to fill the space left behind.

Oxygen is about 21% of air, close to one fifth, so that is roughly how far the level moves. No gas is produced by rusting, so the volume cannot rise, and it cannot stay the same because a reaction is clearly happening. Nitrogen is far too unreactive to be used up, so a fall of four fifths is impossible. This is the standard way of measuring the oxygen content of air.

Two conditions have to be met at once, so work through the table using both columns. Turning cobalt(II) chloride paper pink shows that water is present, which rules out both liquids that leave the paper blue. That leaves the two liquids turning the paper pink.

Of those, one boils at 108 °C, which is above the fixed boiling point of water and shows a dissolved solute raising the boiling point, so it is a solution rather than pure water. The other boils at exactly 100 °C and turns the paper pink, so the pure water is liquid B.

The trap is the liquid that boils at 100 °C but leaves the paper blue: a correct boiling point alone is not enough when the water test fails.

Read the scale on the chart before comparing the bars. The vertical axis is percentage by volume, marked at 0, 40 and 80. Bar P rises almost to the 80 line, bar Q stands at roughly 20, and bar R is barely visible above the axis.

Those readings match the real composition of air: nitrogen about 78%, oxygen about 21%, and the remaining trace gases such as argon at about 0.9% and carbon dioxide at about 0.04%. So the tallest bar P is nitrogen. Choosing oxygen means confusing the tallest bar with bar Q beside it, and argon or carbon dioxide are far too small to reach anywhere near 78%. Always read values off the axis rather than judging heights by eye.

Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the ground, warming it. The warmed Earth then radiates energy back upwards, but at a much longer wavelength, as infra-red or heat radiation. That upward arrow is Y. Greenhouse gas molecules such as carbon dioxide and methane absorb infra-red and re-emit some of it downwards, so the energy is trapped and the lower atmosphere warms.

Ultraviolet has a shorter wavelength than visible light and is the radiation absorbed by the ozone layer, a completely separate issue. Sound waves are vibrations of matter and cannot cross space. Visible light mostly passes straight back out. Remember: sunlight in, infra-red out and trapped.

Groups are the vertical columns of the periodic table, and the group number is read from the heading directly above the column an element occupies. The shaded box holding element E lies in the column headed VI, so element E is in Group VI. Elements there have 6 electrons in their outer shell and typically form 2- ions, as oxygen and sulfur do.

Group VII is the next column to the right and Group IV lies further to the left, so both come from counting the columns from the wrong end or by one place. Group 0 is the final column of noble gases at the far right, which is not where E is shaded.

The temperature scale on the diagram runs from 40 °C beside the top of the column to 350 °C beside the base, so the column is coolest at the top and hottest at the bottom. The outlet drawn level with the 40 °C label, at the very top, is therefore at the lowest temperature.

This gradient is what makes fractional distillation work. Hot vapour enters at the base and cools as it rises; each fraction condenses at the height where the temperature has dropped to its own boiling point. The smallest, most volatile molecules travel furthest before condensing.

The usual error is to link "top" with "hot" because heat rises. In this column the heat source is at the base and the vapour loses energy as it climbs, so the lower outlets are always the hotter ones.

The outer strands are aluminium because aluminium has a lower density, making the cable lighter. At about 2.7 g/cm³ it is roughly one third as dense as copper at 8.9 g/cm³, so a long span between pylons weighs far less and sags much less. Its oxide layer also stops it corroding in the open air.

Copper is in fact the better conductor, so conductivity is not the reason; aluminium conducts well enough and the weight saving decides the matter. Aluminium is not magnetic, and saying it does not conduct contradicts its use as the current-carrying strands in the diagram.

Carbon can only remove oxygen from the oxide of a metal that is less reactive than carbon itself. Metals above carbon in the reactivity series hold their oxygen too strongly, so they have to be extracted by electrolysis of the molten ore, where an electric current supplies the electrons directly.

Aluminium sits well above carbon, so aluminium oxide is electrolysed after being dissolved in molten cryolite, and Al³⁺ ions gain electrons at the cathode: Al³⁺ + 3e^- → Al.

Iron and lead both lie below carbon, so their oxides are reduced by carbon or carbon monoxide in a furnace, which is much cheaper. Copper is lower still and is easily obtained this way. Remember the rule: above carbon means electrolysis.

The table is the only evidence needed: silver chloride is the single entry marked insoluble. That fits the general solubility rules for chlorides, which are soluble except for silver chloride and lead(II) chloride. Silver chloride appears as a white precipitate the instant silver nitrate solution meets a chloride solution:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

which is the standard test for chloride ions. Sodium sulfate, potassium nitrate and ammonium chloride are all soluble, following a rule worth memorising: every sodium, potassium and ammonium salt, and every nitrate, dissolves in water without exception.

Positive ions travel to the negative electrode, and in acidified water the ion available there is H⁺, which gains electrons to give hydrogen: 2H⁺ + 2e^- → H₂. So the gas at electrode A is hydrogen.

The volumes confirm it. At each time the reading at A is exactly double the reading at B: 4 and 2, then 8 and 4, then 12 and 6, so the ratio is a constant 2 : 1. That is precisely what 2H₂O → 2H₂ + O₂ predicts, since equal volumes of gases contain equal numbers of molecules under the same conditions.

Quoting 1 : 2 reverses the order asked for, and calling the cathode gas oxygen swaps the electrodes. Check the direction of the ratio against the wording before writing it down.

On a bar chart the height of each bar shows the size of the quantity, so the shortest bar means the smallest release of carbon dioxide.

Reading the chart against the same axis, the first bar is the tallest, the third bar is next, then the second, and bar S is clearly the shortest of the four, standing at roughly a third of the height of the tallest. The fuel represented by bar S therefore releases the least carbon dioxide for the same mass burned, which makes it the cleanest of the four in this respect. The mistake to avoid is picking the tallest bar, which answers the opposite question by giving the fuel that releases the most.

The diagram labels the gases inside the cylinder as nitrogen and oxygen from the air, at high temperature. That heat provides enough energy to break the very strong triple bond in N₂, so the two gases combine: N₂ + O₂ → 2NO. Nitrogen monoxide is the pollutant formed first, and it is oxidised later in the exhaust to nitrogen dioxide, which dissolves in rain water to give nitric acid.

Making ammonia requires hydrogen, which is not among the gases shown. The equations giving N₂O or NO₂ may balance, but neither is the first product of nitrogen and oxygen combining. Check which reactants the diagram actually supplies before choosing an equation.

Read the direction of each arrow. Arrow G points from the air into the plants, which is photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, so carbon dioxide is taken out of the atmosphere. Arrow R points from the plants back to the air, which is respiration, and that adds carbon dioxide.

The other major removal route is carbon dioxide dissolving in the oceans, where it forms hydrogencarbonate ions and is used by organisms to build shells. So photosynthesis together with dissolving in the oceans gives two genuine removal processes. Respiration, combustion and volcanic activity all release carbon dioxide instead. Decide for each named process whether it adds carbon or removes it.

Burning in plenty of air means complete combustion, and a hydrocarbon contains only carbon and hydrogen. Every carbon atom is fully oxidised to carbon dioxide and every hydrogen atom ends up in water, as in CH₄ + 2O₂ → CO₂ + 2H₂O. That is why the label beneath the bar names carbon dioxide and water as the collected products, with M the larger share.

Carbon monoxide only forms when oxygen is limited, which the phrase plenty of air rules out. Sulfur dioxide would need sulfur, and a hydrocarbon contains none. Hydrogen is not released as a gas, because it is bound up in the water formed. Check the oxygen supply before deciding which carbon oxide forms.

Boiling point depends on the strength of the forces of attraction between separate atoms, not on any bond inside them, since noble gases exist as single atoms. Going down Group 0 from helium to xenon, each atom has more shells and more electrons, so the atoms become larger and heavier and their electron clouds are more easily distorted. The weak attractions between neighbouring atoms therefore become stronger, and more energy is needed to pull them apart into a gas, so the boiling point rises steadily. Atoms do not become smaller down a group; they become bigger. The number of outer electrons stays at eight throughout, and the noble gases remain non-metals rather than turning into metals.

In this route the base is added in excess so that every last bit of acid is used up. That deliberately leaves some solid base sitting in the mixture, and since it is insoluble it can be trapped by filter paper. The filter step therefore removes the unreacted excess base, and the filtrate that passes through is a solution of the salt in water.

The water and the dissolved salt both pass straight through the paper, which is why the next steps are evaporate and crystallise. The acid cannot be removed here because none is left; it was all neutralised, which is the whole reason for adding the base in excess in the first place.

Acid rain comes from sulfur dioxide, which is formed when the sulfur impurity in a fuel burns: S + O₂ → SO₂. So the fuel to avoid is the one releasing most sulfur dioxide.

Both sets of data agree. In the table the sulfur contents are 2.1%, 0.1%, 1.3% and 0.5%, and fuel P is highest at 2.1%. On the bar chart the bar for P is also by far the tallest, while the bar for Q is the shortest, matching its 0.1% sulfur. Fuel P is therefore the one to avoid.

Fuel Q would be the best choice, not the worst, so picking the smallest bar is the usual error. Check that the chart and the table tell the same story before answering.

Hydrogen contains no carbon at all, so burning it cannot produce carbon dioxide. The only product is water: 2H₂ + O₂ → 2H₂O. Diesel is a hydrocarbon mixture, so its carbon is oxidised to carbon dioxide, a greenhouse gas that contributes to climate change. Removing that output at the point of use is the main environmental advantage of a hydrogen bus.

Soot is a product of incomplete combustion of a carbon-containing fuel, so hydrogen produces none; suggesting large amounts of soot reverses the chemistry. Sulfur dioxide comes from sulfur impurities in fossil fuels, which pure hydrogen does not contain. Hydrogen is not a fossil fuel, and no fuel never runs out.

Note the honest limit: the benefit depends on how the hydrogen itself was manufactured.

Calcium hydroxide must be neutral overall. Calcium forms Ca²⁺ with a 2+ charge, while each hydroxide ion OH^- carries only 1-, so two hydroxide ions are needed to cancel one calcium ion. Because hydroxide is a group of two atoms, it is enclosed in a bracket before the subscript is added: Ca(OH)₂.

CaOH leaves an overall charge of 1+ and Ca(OH)₃ leaves 1-, so neither is a neutral compound. Ca₂OH suggests two calcium ions to one hydroxide, which is even further out of balance.

Bracket any polyatomic ion you need more than one of, otherwise the subscript wrongly changes the ion itself.

In every example in the table the coating metal dissolves in the electrolyte as positive ions, such as \(\mathrm{Ag^+}\), \(\mathrm{Zn^{2+}}\), \(\mathrm{Cr^{3+}}\) or \(\mathrm{Au^{3+}}\). Positive ions are attracted to the negative electrode, so the object being plated is always connected at the cathode, the negative electrode. There the ions gain electrons and stick as a thin metal layer, for instance \(\mathrm{Zn^{2+}} + 2e^- \rightarrow \mathrm{Zn}\) on the iron nail.

Connecting the object at the positive anode would oxidise and dissolve it instead of coating it; the anode is where the pure coating metal is placed. An object sitting loose in the electrolyte carries no charge and attracts no ions, and nothing outside the cell is plated at all.

Object to be coated equals cathode, every time.

The table shows no conduction in the solid but conduction both when molten and when dissolved, so sodium chloride conducts when molten or in aqueous solution.

Conduction requires mobile charge carriers. Sodium chloride is built from Na⁺ and Cl⁻ ions, but in the solid the strong attractions hold every ion in a fixed lattice position. Melting or dissolving breaks the lattice apart, freeing the ions to move towards the electrodes and carry the current.

The common mistake is thinking the ions are created when the solid melts. They were always there; only their freedom to move changes. Cooling does the reverse, locking them more firmly in place.

Carbon monoxide, CO, is formed when a carbon-containing fuel burns in a limited supply of air, because there is not enough oxygen to convert every carbon atom fully to carbon dioxide: 2CH₄ + 3O₂ → 2CO + 4H₂O.

Carbon monoxide is toxic because it binds to haemoglobin in red blood cells more strongly than oxygen does, so the blood can no longer carry enough oxygen. It is colourless and odourless, which is what makes a faulty gas appliance so dangerous.

Carbon dioxide is the product of complete combustion and is not poisonous in the same way. Sulfur dioxide comes from sulfur impurities in fuels and causes acid rain, not from a shortage of air. Nitrogen from the air passes through the flame largely unchanged.

Potassium is a Group I metal. Group I metals are soft, have low densities (potassium floats on water) and react vigorously with water and acids. Transition metals such as iron, copper and nickel are the opposite: dense, hard, high melting, and much less reactive.

So the property that separates potassium from a typical transition metal is its low density combined with high reactivity.

The other statements describe transition metals, not potassium. A very high melting point, coloured compounds and catalytic behaviour are all transition-metal features. Potassium melts at only about 63 °C, its compounds such as potassium nitrate are white, and it is not used as a catalyst. When a question asks what distinguishes a metal, check that the property named genuinely belongs to that metal.

Use the state and the conductivity together to identify each structure. A simple molecular covalent compound has weak forces between its molecules, so it melts and boils at low temperatures and is often a gas at room temperature, and it contains no ions or free electrons, so it never conducts electricity.

The substance labelled B is the only one that is a gas and does not conduct, so it is the simple molecular covalent compound.

The substance that is solid and conducts only when molten is ionic: its ions become free to move once the lattice breaks down. The solid that conducts while still solid has delocalised electrons, so it is a metal. The solid that does not conduct but has a very high melting point is giant covalent, like silicon(IV) oxide.

An electrode must carry a current and survive the heat and the electrolyte without melting or reacting, so graphite is the choice. The table lists it as the only substance that both conducts electricity and has a high melting point. Each carbon in graphite bonds to only three others, leaving one delocalised electron per atom free to move through the layers and carry the charge.

Diamond is also giant covalent and heat resistant, but every outer electron is locked into a bond, so it cannot conduct. Poly(ethene) is soft and non-conducting and would melt, and carbon dioxide is a gas, not a solid electrode material.

Transition elements form coloured compounds because their ions have a partly filled d sub-shell that absorbs some wavelengths of visible light. Compounds of Group I and Group II metals are normally white solids that give colourless solutions.

In the diagram the only coloured solution is the blue one, labelled copper(II) sulfate, so that is the tube containing a transition-element compound. Copper is in the transition block and aqueous Cu²⁺ ions are blue.

Sodium chloride and potassium nitrate are drawn as colourless because sodium and potassium are Group I metals with no partly filled d sub-shell. Choosing all three ignores the evidence in the diagram. Use the colour written in each tube as the deciding data rather than the familiarity of the name.

Rusting needs both oxygen and water. The damp iron wool supplies the water and the trapped air supplies the oxygen, so as the iron rusts the oxygen is removed from the air and locked into solid hydrated iron(III) oxide. Since gas disappears, water rises up the tube to take its place.

The water rose by about one fifth of the tube, and oxygen is about 21% of air, so the two figures match. Hydrogen is not present in air in any measurable amount. Carbon dioxide is only about 0.04%, so losing it could never move the level by a fifth. Nitrogen at 78% is far too unreactive to rust iron. A one fifth rise is the fingerprint of oxygen.

The diagram shows larger atoms of a second metal sitting among the smaller atoms of the main metal, and that is why the alloy is harder: the differently sized atoms stop the layers sliding over each other. In the pure metal every atom is the same size, so the layers slip past one another easily under an applied force and the metal is soft. The larger atoms interrupt the rows, so a far greater force is needed to make the layers move.

The delocalised electrons are not removed by alloying, which is why the alloy still conducts. No electrons are shared in pairs between atoms, so no covalent compound is formed; alloying is mixing, not reacting. The layers in the diagram are clearly not perfectly regular, since the large atoms distort them.

Copper(II) chloride solution contains blue \(\mathrm{Cu^{2+}}\) ions and \(\mathrm{Cl^-}\) ions. The positive copper ions move to the negative electrode and gain electrons there, \(\mathrm{Cu^{2+}} + 2e^- \rightarrow \mathrm{Cu}\), so the pinkish-brown layer building up on the cathode is copper. Its colour matches the metal.

Chlorine is made at the positive anode by \(2\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2e^-\) and is a pale green gas, so it could not form a solid layer. Hydrogen is not discharged because copper is below hydrogen in the reactivity series, and oxygen would need hydroxide ions to be discharged in preference to the far more easily oxidised chloride.

Use the colour clue: a pinkish-brown deposit at a cathode is almost always copper.

Neutralisation of an acid by an alkali releases energy to the surroundings, so the mixture warms up. The reaction is the joining of hydrogen ions and hydroxide ions to make water:

H⁺(aq) + OH^-(aq) → H₂O(l)

Forming the strong O-H bonds in water gives out more energy than is needed to separate the ions, so \(\Delta H\) is negative and the temperature rises.

The other changes all absorb energy. Dissolving ammonium nitrate cools the water, which is why it is used in cold packs. Melting a solid needs energy to separate the particles, and thermal decomposition of a carbonate continues only while heat is supplied.

Count only the electrons on the outermost ring. The diagram shows 2 electrons on the inner shell and 6 on the outer shell, so the atom has 6 outer-shell electrons, which matches the oxygen row of the table, and that row gives 2 bonds.

The reason is the rule stated in the question: the outer shell is complete at eight electrons, so the number of covalent bonds needed is \(8 - 6 = 2\). Each covalent bond brings in one shared electron from the other atom, which is why oxygen forms two bonds, as in H-O-H.

Answering 4 uses the carbon row and answering 3 uses the nitrogen row; both come from misreading how many electrons sit on the outer ring. Count the outer shell first, then read across.

On a volume against time graph the gradient is the rate, so the steepest curve is the fastest run. Raising the temperature increases the rate, because particles collide more often and a greater proportion of collisions reach the activation energy.

The graph shows one curve rising almost vertically at first and flattening earliest, one rising less sharply, and one climbing most gently. The steepest curve is X, so X is the run at the highest temperature, which the table gives as 50 °C. The middle curve Y is 35 °C and the shallowest curve Z is 20 °C.

A common slip is to match curves by their final height instead of their steepness.

A pH meter gives a numerical reading that you then classify using the descriptive ranges in the table. Values below 7 are acidic, exactly 7 is neutral and values above 7 are alkaline, with the extremes at each end described as strongly rather than weakly.

The meter displays 12.5. That falls inside the band from 11 to 14, which the table labels strongly alkaline, so the solution is best described as strongly alkaline.

Calling it weakly alkaline would need a reading between 8 and 10, and 12.5 is well above that. Choosing either acidic description ignores that the reading is above 7 altogether. A reliable habit is to place the number on the scale first, then read off the description, rather than judging from the word strongly on its own.

Adding zinc to copper to make brass means the strength increases. Zinc atoms are a different size from copper atoms, so when they take up positions among them the neat layers are disrupted. The layers can no longer slide over one another when a force is applied, so a larger force is needed to deform the metal, which we measure as greater strength and hardness.

Alloying is only mixing at the atomic level, so the metal stays solid at room temperature and cannot turn into a gas. The strength rises rather than falling. The metallic bonding and its sea of delocalised electrons survive alloying, so brass still conducts electricity, although less well than pure copper because the irregular lattice scatters those electrons.

A pollutant is a substance released into the air that causes harm. Reading the classification column, nitrogen and water vapour are marked harmless and carbon dioxide is harmless in small amounts, while carbon monoxide is the one classed as a pollutant.

The chemistry behind that classification is worth knowing. Carbon monoxide forms in the engine when there is too little oxygen for complete combustion of the petrol, and it is dangerous because it is colourless and odourless and binds to haemoglobin much more strongly than oxygen does, so the blood can no longer carry enough oxygen.

Nitrogen simply passes through the engine unchanged, and water vapour is harmless. Carbon dioxide contributes to global warming in bulk, but it is not the acutely poisonous exhaust gas.

Sulfur dioxide from the chimney dissolves in cloud droplets and is oxidised by oxygen in the air, producing sulfuric acid, H₂SO₄, which then falls as acid rain. The overall change can be written 2SO₂ + O₂ + 2H₂O → 2H₂SO₄.

Nitric acid is also found in acid rain, but it comes from oxides of nitrogen made in hot engines and by lightning, not from the sulfur dioxide traced in this diagram. Carbonic acid forms when ordinary carbon dioxide dissolves in rain, which is why even unpolluted rain is slightly acidic at about pH 5.6; it is far too weak to be the main cause. Hydrochloric acid would need chlorine, which is not supplied here.

The air hole controls how much oxygen mixes with the gas before it burns. With the hole open there is plenty of oxygen, combustion is complete and the flame is blue and hot, giving carbon dioxide and water. With the hole closed the gas burns in a limited supply of oxygen, so combustion is incomplete and the flame is yellow, luminous and cooler.

Incomplete combustion is what produces carbon monoxide, because each carbon atom receives only one oxygen atom instead of two, and it also leaves unburnt carbon particles, which are the glowing soot that makes the flame yellow. The yellow flame with the air hole closed is therefore the one giving off the poisonous gas.

Link the colour to the oxygen supply: yellow and sooty always means too little oxygen.