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

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.

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.

Count the crosses on the magnesium atom in the diagram: there are two, and each cross represents one outer-shell electron. Magnesium is in Group II with the arrangement 2,8,2, so those two electrons are its entire outer shell. Losing both leaves the stable full arrangement 2,8, so the magnesium atom loses 2 electrons and becomes Mg²⁺.

The arrow in the diagram shows those two electrons moving across to the oxygen atom, which has six outer electrons drawn and needs exactly two more to reach eight. That perfect match is why magnesium oxide has the formula MgO.

Answering 3 would suit aluminium, not magnesium. The number of outer-shell dots or crosses drawn is the direct clue, so count them before deciding.

Adding aqueous sodium hydroxide to a solution of a metal ion precipitates the metal hydroxide, and the colour of that precipitate identifies the cation. Transition-metal hydroxides are coloured because these ions have a partly filled d sub-shell; main-group metal hydroxides are white.

In the table the light blue, green and red-brown precipitates belong to copper(II), iron(II) and iron(III), all transition-element ions. The white precipitate belongs to calcium, and calcium is a Group II metal, so it is the ion that is not from a transition element.

Note the question asks which is NOT a transition-element ion, so the odd one out is wanted. White precipitate means a main-group cation such as calcium, zinc or aluminium rather than a transition metal.

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.

Gas particles move rapidly and randomly in all directions. Each time a particle strikes the container wall it exerts a tiny force on it, and the enormous number of such collisions every second adds up to a steady force spread over the wall area, which is what we call pressure.

The particles do not stick to the wall or dissolve into it; they bounce off and travel on. Nor do they pull the wall inwards, because the forces of attraction between gas particles are negligible. Heating the gas makes the particles move faster, so collisions become both harder and more frequent and the pressure rises.

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.

The graph and the table answer two different questions. The rising curve shows that the amount of methane in the atmosphere has increased over time, but it says nothing about where the gas comes from. The source is read from the table instead.

The shares given are 40% from cattle, 25% from landfill sites, 20% from rice fields and 15% from other sources. Cattle therefore release the most, and the figures add to 100%, confirming that the whole supply has been accounted for.

Methane is produced by microbes acting on plant material without oxygen, in the digestive systems of cattle, in flooded rice paddies and in buried waste. When a question gives both a graph and a table, take each piece of information from the one that actually shows it.

Trace the flow chart from left to right. Nitrogen from the air is converted to ammonia, and the arrow into the final box is labelled +HNO₃, nitric acid. An acid neutralised by ammonia always gives an ammonium salt named after that acid's anion, and nitric acid gives the nitrate ion, so the product is ammonium nitrate:

\[ \mathrm{NH_3 + HNO_3 \rightarrow NH_4NO_3} \]

Ammonium chloride would need hydrochloric acid and ammonium sulfate would need sulfuric acid, neither of which appears on the chart. Potassium nitrate is impossible because no potassium enters the process at any stage; both halves of a salt's name must come from something actually supplied. Read the label on the arrow before naming the product.

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.

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.

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.

Acidified potassium manganate(VII) is purple because of the MnO₄^- ion, and it is a strong oxidising agent. It turns colourless only when something reduces the manganese to the colourless Mn²⁺ ion, so the test is looking for a reducing gas. Sulfur dioxide is that gas: it is oxidised to sulfate, SO₂ + 2H₂O → SO₄^2- + 4H⁺ + 2e^-, decolourising the solution.

Carbon dioxide is acidic but not a reducing agent, so it leaves the purple colour unchanged; it is identified with limewater instead. Nitrogen and argon are both unreactive and give no result at all. Learn the pairing: sulfur dioxide decolourises acidified manganate(VII), carbon dioxide turns limewater milky.

The difference lies in the physical nature of the two oxide layers. Aluminium oxide forms as a thin, continuous film that grips the metal tightly, so it seals the surface and blocks further attack. Rust, hydrated iron(III) oxide, is porous and flaky, and it lifts off the surface as it builds up.

Once the rust falls away, fresh iron is exposed to water and oxygen again, so corrosion continues layer by layer until the object is eaten through.

Rust is a solid, not a gas, and it does not change back into iron on its own; extracting iron from its oxide requires reduction in a blast furnace. Describing rust as impenetrable contradicts the observation being explained.

The correct pairing is diesel oil with fuel for lorries. Diesel oil is a middle fraction of moderately long chains, so it is a liquid that vaporises enough to burn in a compression engine while still being dense in energy per litre, which suits heavy road transport.

The other pairings swap the ends of the column. Bitumen is the involatile residue used for road surfacing, so it cannot fuel a car; that job belongs to gasoline. Refinery gas is the most volatile fraction and is sold as bottled gas, not spread on roads. Kerosene is a fuel for aircraft, while lubricating engines is the job of the lubricating oil fraction, which is thicker and does not burn.

Check each row against chain length before selecting: uses follow volatility and viscosity.

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.

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.

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.

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.

Hot copper reacts with oxygen only: 2Cu + O₂ → 2CuO, and the black copper(II) oxide stays behind as a solid, so that gas is removed from the air being pushed to and fro between the syringes.

The volume falls from 100 cm³ to 79 cm³, a drop of 21 cm³. That is 21% of the original sample, which matches the proportion of oxygen in air exactly. Nitrogen is the wrong choice because 79 cm³ is what remains rather than what was lost, and nitrogen is far too unreactive to combine with copper. Carbon dioxide and argon are present at only about 0.04% and 0.9%, far too little to explain a 21 cm³ change. Subtract to find the gas used.

Read the displayed formula first. One carbon atom is joined by four single bonds to four hydrogen atoms, so the fuel is methane, CH₄. In complete combustion every carbon atom is fully oxidised to carbon dioxide and every hydrogen atom ends up in water: CH₄ + 2O₂ → CO₂ + 2H₂O. The products are therefore carbon dioxide and water.

Carbon and carbon monoxide are products of incomplete combustion, which happens only when oxygen is limited, and the question states a plentiful supply. Hydrogen is never a combustion product, because the hydrogen atoms are what react with oxygen to make the water. Any hydrocarbon burned in excess oxygen gives the same two products.

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.

Compare both columns for each row. Fermentation is run at only 30 degrees Celsius, and its catalyst column shows none, because the enzymes come from yeast rather than from a chemical catalyst. Adding steam to ethene is run at 300 degrees Celsius, ten times hotter, and uses phosphoric acid, so that is the method needing both a high temperature and a catalyst. Choosing both methods ignores that 30 degrees Celsius is barely above room temperature and that no chemical catalyst is listed against fermentation. The high temperature is needed because adding water across the double bond of ethene in the gas phase is otherwise slow, and the acid catalyst speeds it further without being used up.

Read the plotted line from left to right. Every point sits higher than the one before it, and the steps between the points are similar in size, so the concentration has climbed steadily across the whole 60 year period with no reversal at any stage.

On these axes the vertical scale is carbon dioxide in parts per million and the horizontal scale is the year, so a line rising to the right means the concentration is increasing. A rise then a fall would need the line to turn downwards somewhere, which it never does. A steady fall would need a line sloping down to the right, and no change would give a flat horizontal line. Describe a graph by the direction of its line.

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.

The arrow points down Group I through lithium, sodium, potassium and rubidium, and along that direction reactivity increases. Rubidium reacts violently with cold water while lithium fizzes gently.

Each step down adds an occupied electron shell, so the single outer electron lies further from the nucleus and is screened from it by more inner shells. The electrostatic attraction on that electron is weaker, so it is removed more easily and the 1+ ion forms more readily. Since losing that electron is what these metals do when they react, easier loss means greater reactivity:

\[ \mathrm{2K + 2H_2O \rightarrow 2KOH + H_2} \]

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.

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.

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 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.

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.

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.

The named test in the table is the reliable one. Acidified potassium manganate(VII) is purple, and it fades to colourless only when it takes electrons from something else and is itself reduced to manganese(II). Taking electrons is what an oxidising agent does, so substance 3, the one showing purple to colourless, is acting as the oxidising agent.

Substances that show no change have not taken part in any electron transfer, so neither of those rows can be an agent of any kind. The brown to colourless row does not name its reagent, so no safe redox conclusion can be drawn from it; only the labelled manganate(VII) result is decisive.

Learn the two key colour tests: manganate(VII) fading, and colourless iodide turning brown.

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.

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.

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.

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.

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.

Compare the two figures for the same power station before and after the change. The sulfur dioxide released falls from 80 units to 12 units, and the bar on the chart drops to about a sixth of its original height. The removal is therefore \( \frac{80 - 12}{80} \times 100 = 85\% \), so the best conclusion is that the scrubbers greatly reduce the sulfur dioxide released.

The data directly contradict the claims that the release increases or stays the same, since the second bar is clearly much shorter. Nothing in the table or chart refers to carbon dioxide, so no conclusion about it can be drawn from this evidence. Scrubbers work by passing the waste gases through an alkaline slurry such as calcium oxide or calcium hydroxide, which neutralises the acidic sulfur dioxide.