Chemistry 0620 | Paper 2 Mock 01 | Multiple Choice (Extended)

Hydrated copper(II) sulfate is blue because of the water of crystallisation locked into its lattice. Heating drives that water off as steam:

\[\mathrm{CuSO_4\cdot 5H_2O\ (blue)} \rightleftharpoons \mathrm{CuSO_4\ (white)} + 5\mathrm{H_2O}\]

So during heating the solid changes from blue to white, and the vapour labelled in the diagram condenses to colourless water at the cooler upper part of the tube. The row recording a blue to white colour change with water as the liquid formed is the one that matches.

White to blue describes the reverse change, seen when water is dripped onto the cooled anhydrous solid, which also releases heat. No oil is produced, since nothing organic is present, and there is certainly a visible change, so the row reporting nothing happening is ruled out by the observation.

Both strips sit in the same acid at the same temperature, and they are not touching, so each behaves independently. The only variable is the metal, which makes this a fair comparison of reactivity.

Magnesium is well above hydrogen in the reactivity series, so it readily gives electrons to \( \text{H}^{+} \) ions and hydrogen streams off its surface: \( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \). Copper is below hydrogen, so it cannot displace hydrogen at all and stays unchanged. Far more bubbles therefore form on the magnesium because it is the more reactive metal.

Density has nothing to do with it, and sharing one beaker of acid does not equalise the rates, since each metal reacts at its own speed with the acid it touches.

Methanol contains the \( \mathrm{-OH} \) group shown at the right of the structure, and sodium is reactive enough to displace the hydrogen atom bonded to that oxygen. The metal fizzes as hydrogen is released and dissolves as sodium methoxide forms:

\[ \mathrm{2CH_3OH + 2Na \rightarrow 2CH_3ONa + H_2} \]

This is the same type of reaction as sodium with water, but gentler, because an alcohol gives up its \( \mathrm{-OH} \) hydrogen less readily. You would confirm the gas with a lighted splint, which produces a squeaky pop. No chlorine is present anywhere in the reactants, and the oxygen stays bonded within the methoxide, so neither can be given off. Carbon dioxide would require the carbon to be oxidised, which needs burning, not a metal.

The wax melts on the copper rod, because copper is a metal and metals are good conductors of heat. The delocalised electrons in a metal are free to move, so they gain kinetic energy at the hot end and carry it rapidly along the rod, and the vibrating lattice of ions passes energy on as well. Energy therefore reaches the wax blob quickly and melts it.

Wood has no free electrons. Its energy can only be passed slowly from one vibrating molecule to the next, so it is an insulator and its wax stays solid. That is exactly why saucepans have metal bases and wooden or plastic handles.

Saying both rods behave the same ignores the whole point of the fair test set up here.

The volume of gas collected per minute follows the metal's position in the reactivity series: the higher the metal, the faster hydrogen bubbles off into the measuring cylinder.

Ranking the four given metals from the series gives calcium highest, then magnesium, then zinc, with iron lowest. Iron therefore reacts most sluggishly with dilute acid and gives the slowest rate of gas collection: \( \text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2 \).

Calcium is a tempting wrong choice for "slow" because it feels less familiar than magnesium, but calcium is a Group II metal that reacts even with cold water, so with acid it is violently fast. Rank by series position, not by how often the metal appears in lessons.

The student is correct. In an exothermic reaction the chemicals give out energy to the surroundings, so they must end up with less energy than they started with. On the diagram that shows as the product level lying below the reactant level, which is exactly what this profile shows.

Expressed as a calculation, \(\Delta H = E_{\text{products}} - E_{\text{reactants}}\), and a lower product energy makes \(\Delta H\) negative, which is the sign convention for exothermic.

The claim that the products are always higher describes an endothermic reaction instead, and equal levels would mean no net energy change at all, which is not a real reaction outcome. Temperature does not decide the direction either; it affects the rate. Fix the rule now: exothermic means products below reactants, endothermic means products above.

In a simple cell the voltage depends on how strongly one electrode gives up electrons compared with the other. The bigger the difference in reactivity between the two metals, the further apart their tendencies to lose electrons and the larger the reading on the voltmeter shown in the diagram.

So find the pair that is furthest apart in the reactivity series. Magnesium is high up and copper is low down, a very wide gap, so magnesium paired with copper gives the largest voltage, with magnesium as the negative electrode losing electrons.

\[\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}\]

Iron with tin, lead with copper and copper with silver are all close neighbours in the series, so each gives only a small voltage. Judge these questions by the size of the gap, not by how reactive either single metal is.

Each nail sits in the same dilute acid, so the only variable is the coating metal. Bubbles of hydrogen come from the metal displacing hydrogen from the acid, and the further above hydrogen the metal is, the faster that happens.

Place the three coatings in the reactivity series: zinc is the highest, tin comes below it but is still above hydrogen, and copper lies below hydrogen. So the nail coated with zinc reacts fastest and bubbles most vigorously.

\[\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2}\]

The tin coated nail bubbles slowly, and the copper coated nail gives no bubbles at all because copper cannot displace hydrogen from a dilute acid. The results cannot be equal, since the whole point of the comparison is that reactivity differs. This is also why tin plating protects steel only while the coating stays unbroken.

Choose the method whose separating principle matches the difference between the components. An insoluble solid keeps its particles in large clusters that cannot pass through the pores of filter paper, while the liquid flows through freely, so filtration is the correct method for this task, exactly as the first row of the table states.

Crystallisation is for recovering a solid that has dissolved, so it is the wrong tool when the solid never dissolved in the first place. Distillation is for obtaining the solvent from a solution using boiling point, which wastes energy and is unnecessary here. Chromatography separates dissolved substances by how far they travel on paper. The deciding question is always whether the solid is soluble or insoluble.

A neutral oxide reacts with neither acids nor alkalis and gives a solution of pH 7. Of the substances listed, water is itself the oxide of hydrogen, H₂O, and fits exactly, since pure water has pH 7 and is neither acid nor base in this classification.

Sodium oxide is basic and dissolves to give sodium hydroxide at about pH 13. Sulfur dioxide is acidic and forms sulfurous acid. Aluminium oxide is amphoteric, reacting with both acids and alkalis; its near-neutral pH in water reflects only its very low solubility, not neutrality. Carbon monoxide and nitrogen monoxide are the other neutral oxides worth remembering.

A homologous series is a family of compounds with the same general formula, whose members differ by CH₂ and share the same functional group. Reactions occur where a bond is polar or can break easily, and that is exactly what the functional group provides: -OH in alcohols, C=C in alkenes, -COOH in carboxylic acids. Because every member carries the same reactive site, all members undergo the same reactions.

The hydrocarbon chain is comparatively unreactive, so counting hydrogen atoms or comparing relative molecular masses predicts physical trends such as boiling point and viscosity, not chemical behaviour. Colour is irrelevant, since most of these compounds are colourless.

Addition polymerisation is the one type of polymerisation with a single product. Each C=C opens and the units bond directly to one another, so every atom of every ethene molecule ends up in the chain and nothing is left over. The equation is n CH₂=CH₂ → -(CH₂-CH₂)_n-, giving poly(ethene) only.

Water is lost in condensation polymerisation, as in making nylon or a polyester, which needs monomers carrying functional groups at each end rather than a double bond; expecting water here is the usual confusion between the two types. No hydrogen is released, and ethane cannot form because no extra hydrogen is supplied.

Non-metals form covalent oxides that dissolve in water to release hydrogen ions, making the solution acidic. The question states that P₄O₁₀ dissolves to give phosphoric acid, H₃PO₄, so its solution has a pH below 7 and the oxide is acidic. It also reacts with alkalis to form salts, in this case phosphates.

The evidence given, a non-metal element and an acid in water, rules out the alternatives directly: basic oxides come from metals and give alkaline solutions, neutral oxides such as carbon monoxide give no reaction and pH 7, and amphoteric oxides such as aluminium oxide react with both acids and alkalis. Use the element type as the first clue and the water test to confirm.

The strip is displacing silver, so metal X must be above silver in the reactivity series and must give a coloured solution when its own ions form.

The colour change is the decisive clue. The solution starts colourless and turns pale blue, and blue in aqueous solution means copper(II) ions. So copper atoms are dissolving as \( \text{Cu}^{2+} \) while silver ions are being deposited as shiny crystals:

\[ \text{Cu} + 2\text{AgNO}_3 \rightarrow \text{Cu(NO}_3)_2 + 2\text{Ag} \]

Silver itself cannot be metal X, since a metal cannot displace its own ions and nothing would change. Gold and platinum both sit below silver, so no reaction would occur and the solution would stay colourless. Use the solution colour to name the metal that has dissolved.

Read the displayed formula atom by atom. Two carbon atoms are joined by the double bond; the left-hand carbon carries two hydrogen atoms, and the right-hand carbon carries one hydrogen atom and a CH₃ group. Adding these gives 2 + 1 = 3 carbon atoms and 2 + 1 + 3 = 6 hydrogen atoms, so propene is C₃H₆.

C₃H₈ is propane, the saturated alkane, and choosing it usually means the double bond was ignored. Alkenes follow the general formula C_nH_2n, so three carbon atoms must carry six hydrogen atoms. C₂H₄ leaves out the methyl carbon altogether.

Haematite is the common iron ore and it contains iron(III) oxide. Iron in the +3 oxidation state pairs with oxide ions, O²⁻, so balancing the charges gives two Fe³⁺ for every three O²⁻, that is Fe₂O₃. This is the compound reduced in the blast furnace by carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂.

FeO is iron(II) oxide and Fe₃O₄ is magnetite, a different ore containing iron in two oxidation states. FeCO₃ is siderite, an iron carbonate, not an oxide at all.

Build the formula from the charges rather than recalling it blindly: name the ion charges, then balance them to zero.

Neutralisation removes the acidic hydrogen from the carboxyl group and leaves a negatively charged ion: \[ \mathrm{CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O} \] The ethanoate ion is therefore \( \mathrm{CH_3COO^-} \), carrying a single negative charge on the site where the hydrogen used to be.

The hydroxide ion comes from the alkali, not the acid, and is used up in forming water. The carbonate ion belongs to a different family and carries a 2- charge. \( \mathrm{CH_3OH} \) is methanol, a neutral molecule with no charge and no carboxyl group. To write any carboxylate ion, delete the acidic hydrogen and add one negative charge.

Copper is above silver in the reactivity series, so copper atoms give up electrons more readily than silver atoms do. Placed in silver nitrate solution, copper is oxidised and silver ions are reduced:

\[ \text{Cu} + 2\text{AgNO}_3 \rightarrow \text{Cu(NO}_3)_2 + 2\text{Ag} \]

The wire therefore dissolves while needle-like grey crystals of silver grow on it, and the solution slowly turns blue as \( \text{Cu}^{2+} \) ions build up.

The reverse process, silver dissolving and copper depositing, would need silver to be the more reactive metal, which it is not. "Nothing changes" would only be right if copper were below silver. No brown gas forms because nitrogen dioxide requires concentrated nitric acid, not a nitrate solution.

An ion is an atom that has gained or lost electrons. Losing electrons removes negative charge while the number of protons stays the same, so the particle is left with more protons than electrons and is positively charged. A positive ion is called a cation, and during electrolysis it travels to the cathode, the negative electrode, which is a useful way to remember the name.

An anion is the opposite, formed when an atom gains electrons and so becomes negative.

An isotope is an atom of the same element with a different number of neutrons, and since neutrons are uncharged this does not make an ion. A molecule is a neutral group of atoms joined by covalent bonds, so it carries no charge either.

Check the three rows of the table against what you know about ionic compounds. They are built from oppositely charged ions rather than molecules or free electrons, they have high melting points because the attractions in the giant lattice are strong, and they do not conduct as solids because the ions are fixed.

Column Y shows exactly that combination: ions, a high melting point and no conduction as a solid.

Column Z contains atoms and electrons and conducts as a solid, which describes a metal, where delocalised electrons move through a lattice of positive ions. Column X contains molecules with a low melting point, which describes a simple molecular substance held together by weak intermolecular forces.

When a reactive metal meets water it forms a soluble hydroxide and hydrogen, which is why the universal indicator turns alkaline: \[2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\]

The more reactive the metal, the more hydroxide is produced and the higher the pH. Reading the colours, metal 4 gives purple, the strongly alkaline end of the scale, so it produces the most hydroxide and is the most reactive.

Green means the solution stayed neutral, so that metal did not react with water at all. Blue and dark blue show weaker alkalinity, that is, slower or partial reaction. Learn the indicator sequence green (neutral), blue, dark blue, purple (strongly alkaline), and read pH as a measure of how much hydroxide has formed.

Ethanol and water are miscible with boiling points of 78℃ and 100℃, which are close enough that simple distillation lets some water vapour through with the ethanol. Fractional distillation adds a tall packed column in which the vapour condenses and re-evaporates many times on the way up. Each of these cycles enriches the rising vapour in the lower boiling component, so fractional distillation gives the best separation.

A separating funnel only works for immiscible liquids that form two layers, and these two mix completely. Filtration separates by particle size and cannot act on two liquids, and crystallisation needs a dissolved solid. Reach for a fractionating column whenever two miscible liquids have similar boiling points.

A correct row needs a property that is both true of ethanol and the actual reason for the use.

Ethanol is a flammable liquid that burns in air to release energy, \( \mathrm{C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O} \), so the row linking use as a fuel with burning to release energy is the correct pairing. The other fuel row contradicts itself by claiming ethanol does not burn. The solvent row fails on the property rather than the use, because ethanol mixes with water in all proportions, so calling it insoluble is false; that miscibility is part of why it is such a useful solvent. Ethanol is not a strong acid either, and it is sulfuric acid that car batteries contain.

Read straight down the percentage column and compare the values: 90 per cent, 5 per cent, 3 per cent and 2 per cent. The largest share by a very wide margin is 90 per cent, so the alkane present in the greatest proportion is methane, \(\text{CH}_4\), the simplest member of the series.

Ethane at 5 per cent and propane at 3 per cent are minor components, so choosing either misreads the ranking. The other gases entry at 2 per cent is the smallest figure and is not even a single named alkane, so it cannot be the answer to a question asking which alkane dominates.

Note the useful check that the four percentages sum to 100, confirming the table accounts for the whole sample.

The name says it: a hydrocarbon is a compound containing only hydrogen and carbon, with no other element present. Methane \(\text{CH}_4\), octane \(\text{C}_8\text{H}_{18}\) and the alkanes generally, formula \(\text{C}_n\text{H}_{2n+2}\), are all hydrocarbons, and crude oil is a mixture of thousands of them.

Something made only of carbon atoms is an element such as graphite or diamond, not a compound and not a hydrocarbon. Adding oxygen to carbon and hydrogen makes a different family altogether, for example ethanol \(\text{C}_2\text{H}_5\text{OH}\), which is a compound of three elements and therefore not a hydrocarbon. A mixture of water and carbon is not a compound at all, since its components are not chemically joined.

The word "only" is what earns the mark; do not omit it.

The dead algae are a huge food supply for decomposing bacteria, so the bacteria multiply rapidly. These bacteria are aerobic, meaning they respire using oxygen:

\[\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}\]

The oxygen they use is the oxygen dissolved in the lake water, so as decay proceeds the bacteria use up the dissolved oxygen as they respire and fish and other aerobic organisms suffocate.

The lake does not evaporate away, and oxygen atoms cannot turn into nitrogen atoms, since that would require a change in the nucleus rather than a chemical reaction. Decaying algae are dead, so they photosynthesise no longer and release no oxygen.

Each arrow means "this metal can displace the metal in that salt solution", which places the metal above that salt's metal in the reactivity series.

One arrow runs from zinc to iron(II) sulfate, so zinc is above iron: \( \text{Zn} + \text{FeSO}_4 \rightarrow \text{ZnSO}_4 + \text{Fe} \). A second arrow runs from iron to copper(II) sulfate, so iron is above copper. Chaining these gives zinc above iron above copper, making zinc the most reactive metal shown.

Iron is the trap, because it has an arrow leaving it and so looks active, but it also sits at the receiving end of the zinc arrow. A metal that no arrow points to is the top of the chain. Copper has no arrow leaving it at all, so it is the least reactive.

Rate of reaction with dilute acid is a direct measure of metal reactivity: the more reactive the metal, the faster it releases hydrogen and the sooner it disappears.

Read the bars as time, not as reactivity. The iron bar is the tallest, so iron takes the longest to dissolve; the bars then fall through zinc and aluminium to magnesium, whose bar is by far the shortest. Since a shorter time means a faster reaction, magnesium dissolves fastest and is therefore the most reactive metal shown.

The usual mistake is to pick the tallest bar because "bigger looks stronger". Here the tall bar belongs to the slowest, least reactive metal. Reaction: \( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \).

Always check what the axis actually measures before ranking bars.

Row 4 is correct: carbon monoxide comes from the incomplete combustion of fuels, when there is too little oxygen to oxidise all the carbon to CO₂.

The other rows have real sources attached to the wrong gas. Lightning in clouds provides enough energy to make oxides of nitrogen, not carbon monoxide. Burning sulfur impurities in coal gives sulfur dioxide, not methane; methane comes from decay, landfill sites and livestock. The reaction of N₂ with O₂ in hot engines makes oxides of nitrogen, not sulfur dioxide. Learning each pollutant with its own origin stops these scrambled pairings catching you out.

Chlorine is added to treated water as a disinfectant, so its main purpose is to kill bacteria and other microorganisms. It destroys the microbes that cause water-borne diseases such as cholera and typhoid, and because a small residual amount remains dissolved, the water is protected against recontamination as it travels through the pipes to homes.

Chlorine does not remove dissolved salts, since those stay in solution and would need distillation to be taken out. It does not remove large floating objects either, as those are caught by screening and settling long before this stage. Far from sweetening the water, too much chlorine gives an unpleasant taste and smell, which is why the dose is carefully controlled.

The steam test separates metals in the middle of the reactivity series. A metal that reacts with steam is reactive enough to strip oxygen from water and release hydrogen; one that does not is below that threshold.

Cleaned zinc glows and gives hydrogen that burns at the tube end: \( \text{Zn} + \text{H}_2\text{O} \rightarrow \text{ZnO} + \text{H}_2 \). Copper under identical conditions shows nothing, because copper cannot reduce water at all. The only valid conclusion is that zinc is more reactive than copper.

Note the fair test built into the apparatus: same steam supply, same heating, both surfaces cleaned of oxide. Because conditions are identical, the difference must come from the metals themselves, not from the setup.

The amount present is 0.05 mol. Convert the volume first: 100 cm³ ÷ 1000 = 0.100 dm³. Then n = c × V = 0.5 × 0.100 = 0.05 mol of CuSO₄.

Choosing 0.5 mol simply repeats the concentration, which is the amount in a full dm³, not in a tenth of one. Choosing 5 mol multiplies by the volume in cm³ without converting, or shifts the decimal the wrong way. Choosing 0.005 mol divides by 1000 twice. The rule is that concentration is moles per dm³, so the volume in the formula must always be in dm³.

Carbon can only remove oxygen from a metal oxide if the metal is below carbon in the reactivity series. If the metal is above carbon, it holds oxygen more strongly than carbon does and the reduction cannot happen.

The table shows metal recovered from lead oxide, copper oxide and iron oxide, all metals below carbon, for example \( 2\text{CuO} + \text{C} \rightarrow 2\text{Cu} + \text{CO}_2 \). Only aluminium oxide gives no metal, because aluminium is well above carbon; it is extracted instead by electrolysis of the molten oxide dissolved in cryolite.

The lesson is that heating harder will not help. The barrier is the reactivity order, not the temperature, so a different method is needed rather than a hotter crucible.

Most non-metals do not exist as lone atoms because their outer shells are incomplete, so they share electrons to form molecules. Oxygen exists as \(\mathrm{O_2}\), nitrogen as \(\mathrm{N_2}\) and chlorine as \(\mathrm{Cl_2}\), each pair of atoms joined by covalent bonds.

Argon is a noble gas in Group VIII. Its outer shell already holds 8 electrons, so it has no need to share any. With nothing to gain from bonding, argon exists as single unbonded atoms, described as monatomic. That is why argon is the substance made of separate atoms at room temperature.

The misconception behind choosing oxygen or nitrogen is treating the element symbol as the formula. The symbol \(\mathrm{O}\) names the element, but the substance in the air is \(\mathrm{O_2}\). Only the noble gases are monatomic, and that is a direct consequence of the full outer shell.

A metal displaces iron from iron(II) sulfate only if it is above iron in the series, because it must be able to give electrons to \( \text{Fe}^{2+} \) ions: \( \text{M} + \text{FeSO}_4 \rightarrow \text{MSO}_4 + \text{Fe} \).

Reading the ladder, calcium, magnesium and zinc all sit above the iron line, so each deposits a coating of iron. Copper is printed below iron, so it cannot reduce \( \text{Fe}^{2+} \) and no coating appears. That gives three metals producing a coating.

Counting four is the usual error, from assuming every metal added must do something. In practice calcium also attacks the water itself, so its coating is masked by fizzing, but by series position it still qualifies.

A metal can only make its salt by direct addition to an acid if it is above hydrogen in the reactivity series, because the reaction works by the metal displacing hydrogen from the acid. The table answers this for you: copper, silver and gold are all recorded as not reacting with dilute acid, while zinc is the one that does.

Zinc is therefore the suitable choice, and warming it with dilute sulfuric acid gives steady effervescence of hydrogen:

\[ \mathrm{Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2} \]

Excess zinc is filtered off and the solution is crystallised.

Copper, silver and gold sit below hydrogen, so no reaction occurs at all and no salt is formed. Copper(II) sulfate must instead be made from copper(II) oxide or copper(II) carbonate with the acid.

A metal strip becomes coated with copper only if it is more reactive than copper, so it can push copper out of solution: \[\text{Cu}^{2+}(aq) + 2\text{e}^{-} \rightarrow \text{Cu}(s)\]

The bigger the reactivity gap, the faster the electrons are handed over and the sooner the brown copper layer appears. Strip P was coated in only 5 s, against 40 s for Q and a full 3 minutes for R, so P is the most reactive of the three.

The misconception here is to read a long coating time as a "bigger" reaction because more time passed. Time is an inverse measure: a short time means a fast reaction. In displacement experiments, quickest change equals greatest reactivity.

Both gases set off at the same moment from opposite ends and stopped travelling at the same moment, when they met and formed the white ring. Since the times are equal, the distances directly compare their speeds.

The measurements show 36 cm travelled by ammonia against 24 cm by hydrogen chloride, so ammonia diffused faster, as it travelled the greater distance. The reason is its lower relative molecular mass, 17 against 36.5, which gives its molecules a higher average speed at the same temperature.

Claiming hydrogen chloride went further misreads the table, since 24 cm is the smaller value. Equal rates would have produced two equal distances of 30 cm each. And both gases plainly moved, otherwise no ring could have formed at all. Equal time is the hidden condition that makes distance a fair measure of rate.

The mass needed is 32 g. Mass = moles × A_r = 0.5 × 64 = 32 g, which makes sense because half a mole must weigh half of the molar mass.

Choosing 64 g weighs out a full mole and ignores the word half. Choosing 128 g multiplies by 2 instead of by 0.5, a very common slip when the fraction is written as a word rather than a decimal. Choosing 16 g quarters the molar mass, treating half a mole as half of a half. Writing the numbers into mass = n × A_r before calculating stops all three errors, and gives about 3.01 × 10²³ copper atoms.

Each fraction is used according to its boiling range and viscosity. Refinery gases are small, very volatile molecules, ideal as bottled gas; gasoline is the fraction used in car engines; kerosene is the aviation fuel. Those three uses are all correct.

The incorrect pairing is the row listing bitumen as fuel for lorries. Bitumen is the thick, almost solid residue left at the bottom of the column, with very large molecules and a very high boiling point. It is far too viscous to be pumped into an engine or vaporised for burning, so it is used to surface roads and roof buildings instead. Lorries run on diesel oil.

Match each fraction's use to its physical properties: light fractions burn easily, heavy fractions are used for surfacing and lubrication.