Chemistry

What is the smallest particle of an element that can exist?

• AA molecule
• BAn electron
• CAn atom✓
• DA compound

Answer: An atom is the smallest particle of an element that retains its chemical identity.

Which subatomic particle has a negative charge?

• AProton
• BNeutron
• CElectron✓
• DNucleus

Answer: Electrons have a charge of −1; protons are +1 and neutrons are neutral.

What is the mass number of an atom equal to?

• ANumber of electrons
• BNumber of protons only
• CProtons minus neutrons
• DProtons plus neutrons✓

Answer: Almost all of an atom's mass sits in the nucleus, so the mass number counts the particles there: protons + neutrons. (The number of protons on its own is the atomic number.)

Elements in the same group of the periodic table have the same number of:

• AOuter-shell (valence) electrons✓
• BNeutrons
• CProtons
• DEnergy levels

Answer: Elements in the same group have the same number of outer-shell electrons, so they react in similar ways. For groups 1-7 this matches the group number; group 0 atoms have a full outer shell.

What type of bonding occurs between a metal and a non-metal?

• ACovalent
• BHydrogen
• CIonic✓
• DMetallic

Answer: Metals lose electrons and non-metals gain them, forming oppositely charged ions held by ionic bonds.

What is the pH of a neutral solution at 25°C?

• A0
• B7✓
• C10
• D14

Answer: A neutral solution (such as pure water) has a pH of 7. Below 7 is acidic, above 7 is alkaline.

Which gas is produced when an acid reacts with a metal?

• ACarbon dioxide
• BOxygen
• CChlorine
• DHydrogen✓

Answer: Acid + metal → salt + hydrogen. The hydrogen gas gives a squeaky pop with a lit splint.

What is the test for carbon dioxide gas?

• AIt relights a glowing splint
• BIt gives a squeaky pop
• CIt bleaches damp litmus paper
• DIt turns limewater cloudy/milky✓

Answer: Carbon dioxide turns limewater (calcium hydroxide solution) cloudy white.

A reaction that releases energy to the surroundings is described as:

• AEndothermic
• BReversible
• CCatalytic
• DExothermic✓

Answer: Exothermic reactions transfer energy to the surroundings, usually raising the temperature (e.g. combustion).

What does a catalyst do in a chemical reaction?

• AIs used up during the reaction
• BLowers the temperature of the products
• CSpeeds up the reaction without being used up✓
• DIncreases the yield

Answer: A catalyst increases the rate of reaction by lowering the activation energy and is not used up.

Which of these is a pure substance?

• ABrass
• BSea water
• CDistilled water✓
• DAir

Answer: A pure substance contains only one element or compound. Distilled water is pure H₂O; the others are mixtures.

During electrolysis, positive ions move towards which electrode?

• ANeither electrode
• BBoth equally
• CThe cathode (negative electrode)✓
• DThe anode (positive electrode)

Answer: Positive ions (cations) are attracted to the negative cathode, where they gain electrons (reduction).

What is the chemical formula of water?

• AH₂O₂
• BH₂O✓
• CH₃O
• DHO

Answer: Water is H₂O: two hydrogen atoms covalently bonded to one oxygen atom.

The Earth's early atmosphere is thought to have been mainly which gas?

• ACarbon dioxide✓
• BHelium
• COxygen
• DHydrogen

Answer: The early atmosphere is believed to have been rich in carbon dioxide, with little or no oxygen.

Which separation technique would best separate a soluble salt from its solution?

• ACrystallisation/evaporation✓
• BFiltration
• CChromatography
• DDecanting

Answer: Evaporating the water (crystallisation) leaves the dissolved salt behind as crystals.

What is the relative formula mass (Mr) of calcium hydroxide, Ca(OH)₂? (Relative atomic masses: Ca = 40, O = 16, H = 1)

• A57
• B73
• C74✓
• D114

Answer: The bracket means there are two OH groups, so Mr = 40 + 2 × (16 + 1) = 40 + 34 = 74. Ignoring the subscript 2 altogether gives 57, and doubling only the oxygen gives 73.

How many moles are there in 8.0 g of magnesium oxide, MgO? (Relative atomic masses: Mg = 24, O = 16)

• A0.2 mol✓
• B0.5 mol
• C5 mol
• D320 mol

Answer: Mr of MgO = 24 + 16 = 40. Moles = mass ÷ Mr = 8.0 ÷ 40 = 0.2 mol. Turning that division upside down gives 5 mol, and multiplying the mass by the Mr gives 320 mol.

What is the mass of 0.25 mol of carbon dioxide, CO₂? (Relative atomic masses: C = 12, O = 16)

• A0.006 g
• B7 g
• C11 g✓
• D176 g

Answer: Mr of CO₂ = 12 + (2 × 16) = 44. Mass = moles × Mr = 0.25 × 44 = 11 g. Dividing 44 by 0.25 instead of multiplying gives 176 g, and counting only one oxygen atom gives an Mr of 28 and a mass of 7 g.

6.0 g of magnesium is heated in air and 10.0 g of magnesium oxide is formed. What mass of oxygen has reacted with the magnesium?

• A1.7 g
• B4.0 g✓
• C10.0 g
• D16.0 g

Answer: Mass is conserved, so the total mass of the reactants equals the mass of the product: 6.0 g of magnesium + the mass of oxygen = 10.0 g, which gives 4.0 g of oxygen. Adding the two figures in the question instead gives 16.0 g, and dividing them gives 1.7 g.

What mass of magnesium oxide is formed when 4.8 g of magnesium burns completely in oxygen? 2Mg + O₂ → 2MgO (Relative atomic masses: Mg = 24, O = 16)

• A3.2 g
• B4.8 g
• C8.0 g✓
• D16.0 g

Answer: Moles of magnesium = mass ÷ Ar = 4.8 ÷ 24 = 0.2 mol. The equation shows magnesium and magnesium oxide react in a 2:2 ratio, so 0.2 mol of magnesium oxide forms. Mr of MgO = 24 + 16 = 40, so the mass is 0.2 × 40 = 8.0 g. Assuming the mass does not change gives 4.8 g, and 3.2 g is only the mass of the oxygen that was added.

Sodium reacts with chlorine: 2Na + Cl₂ → 2NaCl. How many moles of chlorine molecules react completely with 0.8 mol of sodium?

• A0.2 mol
• B0.4 mol✓
• C0.8 mol
• D1.6 mol

Answer: The large numbers in the equation give the reacting ratio. Sodium to chlorine is 2:1, so the number of moles of chlorine is half the number of moles of sodium: 0.8 ÷ 2 = 0.4 mol. Using the ratio the wrong way round gives 1.6 mol, and reading it as 1:1 gives 0.8 mol.

20 g of sodium chloride is dissolved in water to make 250 cm³ of solution. What is the concentration of the solution in g/dm³?

• A0.08 g/dm³
• B5 g/dm³
• C8 g/dm³
• D80 g/dm³✓

Answer: 1 dm³ = 1000 cm³, so 250 cm³ = 0.25 dm³. Concentration = mass ÷ volume = 20 g ÷ 0.25 dm³ = 80 g/dm³. Dividing by 250 without converting the volume first gives 0.08 g/dm³.

What is the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃? (Relative formula mass = 80; relative atomic mass of N = 14)

• A17.5%
• B35%✓
• C60%
• D286%

Answer: The formula contains two nitrogen atoms, so the mass of nitrogen is 2 × 14 = 28. Percentage by mass = 28 ÷ 80 × 100 = 35%. Counting only one nitrogen atom gives 17.5%, and 60% is the percentage by mass of the oxygen.

Which equation for the complete combustion of methane is correctly balanced?

• ACH₄ + O₂ → CO₂ + H₂O
• B2CH₄ + 2O₂ → 2CO₂ + H₂O
• CCH₄ + 4O₂ → CO₂ + 2H₂O
• DCH₄ + 2O₂ → CO₂ + 2H₂O✓

Answer: Balanced means the same number of atoms of each element on both sides. CH₄ + 2O₂ → CO₂ + 2H₂O has 1 carbon, 4 hydrogen and 4 oxygen atoms on each side. Putting 4O₂ on the left would give 8 oxygen atoms against 4 on the right. Only the large numbers in front of a formula may be changed when balancing.

Magnesium burns in oxygen: 2Mg + O₂ → 2MgO. How many moles of magnesium oxide are formed when 0.6 mol of magnesium burns completely in excess oxygen?

• A0.2 mol
• B0.3 mol
• C0.6 mol✓
• D1.2 mol

Answer: The large numbers in the equation give the reacting ratio. Magnesium to magnesium oxide is 2:2, which is the same as 1:1, so 0.6 mol of magnesium produces 0.6 mol of magnesium oxide. Doubling gives 1.2 mol and halving gives 0.3 mol.

In a paper chromatography experiment the solvent front moved 8.0 cm from the pencil line and a coloured spot moved 3.0 cm. What is the Rf value of the spot?

• A0.375✓
• B2.67
• C5.0
• D24

Answer: Rf = distance moved by the substance ÷ distance moved by the solvent = 3.0 ÷ 8.0 = 0.375. An Rf value has no units and is always less than 1, because the spot cannot travel further than the solvent front.

A reaction between magnesium and hydrochloric acid produced 60 cm³ of hydrogen in 40 seconds. What was the mean rate of reaction?

• A0.67 cm³/s
• B1.5 cm³/s✓
• C20 cm³/s
• D2400 cm³/s

Answer: Mean rate = quantity of product formed ÷ time taken = 60 cm³ ÷ 40 s = 1.5 cm³ per second. Dividing the time by the volume gives 0.67 and multiplying the two figures gives 2400.

In the required practical to prepare pure, dry copper sulfate crystals, copper oxide is added to warm dilute sulfuric acid until no more will dissolve. Why is the copper oxide added in excess?

• ATo act as a catalyst and speed the reaction up
• BTo lower the temperature so that crystals form faster
• CTo neutralise the copper sulfate solution after it has been formed
• DTo make sure that all of the acid is used up in the reaction✓

Answer: Adding the solid until no more will dissolve makes certain that none of the acid is left over, so the solution contains only copper sulfate and water. The copper oxide that has not reacted is then removed by filtration before the solution is crystallised.

Which piece of apparatus gives the most accurate measurement of the volume of gas given off during a reaction?

• AA gas syringe joined to the flask by a bung and delivery tube✓
• BA measuring cylinder held next to the flask to catch the gas as it escapes
• CA thermometer placed in the reaction mixture inside the flask
• DA stopwatch started the moment the two reactants are mixed

Answer: A gas syringe is sealed to the flask, so all of the gas produced is collected and its volume can be read straight from the scale. Gas rising into an open measuring cylinder would be lost, and a thermometer and a stopwatch do not measure volume at all.

A graph shows the total volume of gas collected against time for a reaction. The line is steep at first, gradually becomes less steep, and finally becomes horizontal at 60 cm³ after 90 seconds. What does the horizontal part of the line show?

• AThe reaction has finished because a reactant has been used up✓
• BThe reaction is speeding up as the mixture warms itself up
• CThe gas produced is being absorbed back into the solution
• DThe rate of the reaction reaches its highest value at this point

Answer: The gradient of the line gives the rate of reaction, so a horizontal line means no more gas is being produced and the reaction has stopped. The steepest part of the line is at the very start, when the reactants are at their most concentrated.