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KS3 · Chemistry

K7 Atoms, Elements and Compounds

AQA AQA-KS3-CHEM-C1 · Calculator allowed · about 70 minutes

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Quickfire: circle the correct letter for each question.

(a)Which subatomic particle has no overall electric charge?(1)

A) Proton
B) Neutron
C) Electron
D) Nucleus

(b)Which of these substances is a compound?(1)

A) Oxygen gas (O2)
B) Iron (Fe)
C) Carbon dioxide (CO2)
D) Air

(c)The atomic number of an atom tells you the number of...(1)

A) neutrons
B) protons
C) protons and neutrons added together
D) electron shells

(d)Which chemical symbol represents the element sodium?(1)

A) S
B) So
C) Na
D) N

All atoms are made up of smaller subatomic particles.

(a)Name the three subatomic particles found inside an atom.(3)

(b)State where the protons and neutrons are located within an atom.(1)

Each subatomic particle has a relative charge and a relative mass.

(a)State the relative electric charge of a proton, a neutron and an electron.(3)

(b)State the relative mass of a proton and of a neutron, compared with the relative mass of an electron.(2)

The atomic number of an element equals its number of protons. The mass number equals the number of protons plus neutrons. Use this to complete the table below for three elements.

(a)Sodium has atomic number 11 and mass number 23. State the number of protons and electrons, and calculate the number of neutrons.(2)

(b)Aluminium has atomic number 13 and mass number 27. State the number of protons and electrons, and calculate the number of neutrons.(2)

(c)Chlorine has atomic number 17 and mass number 35. State the number of protons and electrons, and calculate the number of neutrons.(2)

Classify each of the following substances as an element, a compound or a mixture.

(a)(i) oxygen gas, O2 (ii) carbon dioxide, CO2 (iii) air (iv) sodium chloride (table salt) (v) bronze (vi) sea water. Classify each as an element, a compound or a mixture.(6)

The periodic table arranges all known elements into groups (vertical columns) and periods (horizontal rows).

(a)State what elements in the same group of the periodic table have in common, and state what the period number of an element tells you about its atoms.(2)

(b)Using the periodic table, identify the group number and the period number of chlorine.(2)

A chemical formula shows how many atoms of each element are present in a substance.

(a)State the number of atoms of each element present in one molecule of water, H2O.(2)

(b)State the number of atoms of each element present in one molecule of carbon dioxide, CO2.(2)

(c)State the number of atoms of each element present in one formula unit of calcium carbonate, CaCO3.(3)

The relative formula mass (Mr) of a substance is found by adding together the relative atomic masses (Ar) of all the atoms shown in its formula. Use these Ar values: H = 1, C = 12, O = 16, Ca = 40.

(a)Calculate the relative formula mass (Mr) of water, H2O.(2)

(b)Calculate the relative formula mass (Mr) of carbon dioxide, CO2.(2)

(c)Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3. Show your working.(3)

Magnesium metal burns in oxygen to form magnesium oxide.

(a)Write a word equation for this reaction.(2)

(b)The unbalanced symbol equation for this reaction is: Mg + O2 -> MgO. Balance this equation.(2)

Balance the following symbol equations by writing the correct numbers in front of each formula where needed.

(a)Balance: H2 + O2 -> H2O(2)

(b)Balance: N2 + H2 -> NH3(2)

A pupil opens a bottle of perfume at the front of a still classroom. After a few minutes, pupils sitting at the back of the room can smell it, even though no one has moved and there are no draughts.

(a)Name the process by which the perfume particles spread through the room.(1)

(b)Explain, in terms of particles, why the smell spreads through the room.(2)

Three particle diagrams show small circles representing atoms. Diagram A shows many identical single circles scattered randomly. Diagram B shows pairs of two different-coloured circles joined together, repeated throughout, with no single circles present. Diagram C shows a mixture of single identical circles and joined pairs of two different-coloured circles, all scattered together.

(a)Identify which diagram represents an element.(1)

(b)Identify which diagram represents a compound.(1)

(c)Identify which diagram represents a mixture, and explain your answer.(2)

A student is given a mixture of 20.0 g of salt (sodium chloride) and 10.0 g of sand. She wants to separate the mixture and recover the salt as dry, pure crystals.

(a)Below are four steps from the separation method, listed in the wrong order. 1: Evaporate the filtrate gently until dry salt crystals remain. 2: Add water to the mixture and stir until the salt dissolves. 3: Collect and dry the sand left on the filter paper. 4: Filter the mixture using a filter funnel and filter paper. Write down the four steps in the correct order.(4)

(b)Explain why the sand remains on the filter paper during filtration, but the salt passes through.(2)

(c)Name a piece of apparatus that could be used to gently heat the salt solution so that the water evaporates safely.(1)

Continuing from Question 13 (20.0 g salt + 10.0 g sand = 30.0 g total mixture), after filtration and evaporation the student recovers 17.5 g of dry salt crystals and 9.2 g of dry sand.

(a)Calculate the percentage of the original mass of salt that was recovered as dry crystals.(3)

(b)Calculate the total mass of solid recovered from the whole experiment, and suggest one reason why it is less than the original total mixture mass of 30.0 g.(2)

A mixture contains salt, sand and iron filings, all mixed together as dry solids. Describe, step by step, how you could separate this mixture into three pure, dry substances. In your answer, explain which physical property is used to separate the substances at each stage.

(6)

Metals and non-metals have different physical properties.

(a)State two physical properties of metals.(2)

(b)State two physical properties of non-metals.(2)

(c)Classify the following elements as a metal or a non-metal: sodium, sulfur, iron, oxygen, copper, chlorine.(3)

Atoms of the same element can have different numbers of neutrons. These are called isotopes.

(a)Explain what is meant by the term isotope, using chlorine-35 and chlorine-37 as examples.(3)

(b)Calculate the number of neutrons in an atom of chlorine-37.(1)

Bronze is an alloy made from 88% copper and 12% tin by mass.

(a)State why bronze is a mixture and not a compound.(2)

(b)Calculate the mass of tin in a bronze statue with a total mass of 250 kg.(2)

Mark scheme · K7 Atoms, Elements and Compounds

Question 1

(a) B1 B (neutron) cao
(a) Answer: B) Neutron
(b) B1 C (carbon dioxide) cao
(b) Answer: C) Carbon dioxide (CO2)
(c) B1 B (protons) cao
(c) Answer: B) protons
(d) B1 C (Na) cao
(d) Answer: C) Na

Question 2

(a) B1 proton
(a) B1 neutron
(a) B1 electron (any order)
(a) Answer: Proton, neutron, electron (any order).
(b) B1 in the nucleus (centre of the atom) oe
(b) Answer: In the nucleus, at the centre of the atom.

Question 3

(a) B1 proton = +1
(a) B1 neutron = 0
(a) B1 electron = -1
(a) Answer: Proton = +1, neutron = 0, electron = -1.
(b) B1 proton and neutron each have a relative mass of 1
(b) B1 electron has a very small (negligible) relative mass, about 1/2000 of a proton oe
(b) Answer: Proton = 1, neutron = 1, electron = very small/negligible (about 1/2000 of a proton).

Question 4

(a) B1 protons = 11, electrons = 11 (equal to atomic number)
(a) B1 neutrons = mass number - atomic number = 23 - 11 = 12 cao (ft own atomic number if misread but used consistently)
(a) Answer: Protons = 11, electrons = 11, neutrons = 12.
(b) B1 protons = 13, electrons = 13
(b) B1 neutrons = 27 - 13 = 14 cao
(b) Answer: Protons = 13, electrons = 13, neutrons = 14.
(c) B1 protons = 17, electrons = 17
(c) B1 neutrons = 35 - 17 = 18 cao
(c) Answer: Protons = 17, electrons = 17, neutrons = 18.

Question 5

(a) B1 oxygen gas = element
(a) B1 carbon dioxide = compound
(a) B1 air = mixture
(a) B1 sodium chloride = compound
(a) B1 bronze = mixture
(a) B1 sea water = mixture
(a) Answer: Oxygen gas: element. Carbon dioxide: compound. Air: mixture. Sodium chloride: compound. Bronze: mixture. Sea water: mixture.

Question 6

(a) B1 elements in the same group have the same number of electrons in their outer shell, giving similar chemical properties oe
(a) B1 the period number equals the number of electron shells (energy levels) occupied by electrons oe
(a) Answer: Same group = same number of outer-shell electrons (similar chemical properties). Period number = number of electron shells occupied.
(b) B1 group 7
(b) B1 period 3
(b) Answer: Group 7, Period 3.

Question 7

(a) B1 2 hydrogen atoms
(a) B1 1 oxygen atom
(a) Answer: 2 hydrogen atoms and 1 oxygen atom.
(b) B1 1 carbon atom
(b) B1 2 oxygen atoms
(b) Answer: 1 carbon atom and 2 oxygen atoms.
(c) B1 1 calcium atom
(c) B1 1 carbon atom
(c) B1 3 oxygen atoms
(c) Answer: 1 calcium atom, 1 carbon atom and 3 oxygen atoms.

Question 8

(a) M1 (1 x 2) + 16
(a) A1 = 18 cao
(a) Answer: Mr(H2O) = 18
(b) M1 12 + (16 x 2)
(b) A1 = 44 cao
(b) Answer: Mr(CO2) = 44
(c) M1 identifies 1 Ca, 1 C and 3 O atoms from the formula
(c) M1 40 + 12 + (16 x 3)
(c) A1 = 100 cao
(c) Answer: Mr(CaCO3) = 100

Question 9

(a) B1 magnesium + oxygen ->
(a) B1 -> magnesium oxide
(a) Answer: magnesium + oxygen -> magnesium oxide
(b) M1 2Mg on the left and MgO adjusted to 2MgO oe working shown
(b) A1 2Mg + O2 -> 2MgO fully balanced cao
(b) Answer: 2Mg + O2 -> 2MgO

Question 10

(a) M1 2H2 and O2 correctly placed on the left
(a) A1 2H2 + O2 -> 2H2O fully balanced cao
(a) Answer: 2H2 + O2 -> 2H2O
(b) M1 3H2 correctly placed on the left
(b) A1 N2 + 3H2 -> 2NH3 fully balanced cao
(b) Answer: N2 + 3H2 -> 2NH3

Question 11

(a) B1 diffusion
(a) Answer: Diffusion.
(b) M1 gas particles move constantly and randomly in all directions (have kinetic energy)
(b) A1 particles move from an area of higher concentration (near the bottle) to an area of lower concentration until spread evenly, without needing the air itself to move oe
(b) Answer: Perfume particles move randomly and spread out from where they are concentrated to where they are less concentrated, until they are evenly spread through the room.

Question 12

(a) B1 Diagram A
(a) Answer: Diagram A.
(b) B1 Diagram B
(b) Answer: Diagram B.
(c) B1 Diagram C
(c) B1 it contains both single particles of one substance (an element) and joined pairs (a compound), not chemically bonded to each other, so it is a mixture oe
(c) Answer: Diagram C, because it contains an element and a compound mixed together but not chemically joined to each other.

Question 13

(a) B1 step 2 first (add water and stir)
(a) B1 step 4 second (filter the mixture)
(a) B1 step 3 third (collect and dry sand)
(a) B1 step 1 fourth (evaporate the filtrate) ft consistent logical order
(a) Answer: 2, 4, 3, 1
(b) B1 sand is insoluble in water and its particles are too large to pass through the tiny holes (pores) in the filter paper, so it is retained oe (ft consistent with candidate's order in (a))
(b) B1 salt dissolves in water to form a solution; the dissolved particles are small enough to pass through the filter paper with the water, forming the filtrate oe
(b) Answer: Sand is insoluble and its particles are too large to pass through the filter paper, so it stays behind. Salt dissolves to form a solution whose particles are small enough to pass through with the water.
(c) B1 evaporating basin (dish) oe, accept water bath
(c) Answer: Evaporating basin (evaporating dish).

Question 14

(a) M1 17.5 / 20.0
(a) M1 x 100
(a) A1 = 87.5% awrt
(a) Answer: 87.5%
(b) M1 17.5 + 9.2 = 26.7 g
(b) A1 valid reason, e.g. some salt solution was lost during filtering or transferring between containers, or solid was left on the filter paper/beaker/evaporating basin oe
(b) Answer: 26.7 g recovered. Reason: some mass was lost during the practical, for example solution left on the filter paper, funnel or beaker, or splashes during evaporation.

Question 15

Level 1 (1-2): Simple, isolated statements about separating the mixture, e.g. use a magnet and/or filter it, with little or no reference to the physical properties involved. Steps may be incomplete or in the wrong order.
Level 2 (3-4): A logically ordered description covering most of the separation stages, with some reference to physical properties (e.g. magnetism, solubility), but explanations may be incomplete or one stage may be missing or unclear.
Level 3 (5-6): A clear, logical, fully ordered description of all three separation stages (removing iron, separating sand from salt solution, recovering dry salt) with correct scientific explanation of the physical property used at each stage.
Indicative content:
- Use a magnet to attract and remove the iron filings from the dry mixture, because iron is magnetic and sand and salt are not (magnetism).
- Add water to the remaining sand and salt mixture and stir; the salt dissolves in the water because it is soluble, but the sand does not dissolve because it is insoluble.
- Filter the mixture; the insoluble sand is retained on the filter paper because its particles are too large to pass through the tiny pores, while the salt solution passes through as the filtrate.
- Allow or gently dry the sand collected on the filter paper to obtain pure, dry sand.
- Gently evaporate the filtrate (salt solution), for example using an evaporating basin, until all the water has evaporated and dry salt crystals remain.
- Correct overall order: magnet first, then dissolve and filter, then evaporate.
- May mention heating gently towards the end of evaporation to avoid spitting or decomposing the salt.

Question 16

(a) B1 any one from: good conductor of electricity/heat, shiny (lustrous), malleable, ductile, high melting/boiling point, high density, sonorous
(a) B1 any second correct property from the list above
(a) Answer: Any two of: good electrical/thermal conductor, shiny, malleable, ductile, high melting/boiling point, high density, sonorous.
(b) B1 any one from: poor conductor of electricity/heat (insulator), dull appearance, brittle when solid, low melting/boiling point, low density
(b) B1 any second correct property from the list above
(b) Answer: Any two of: poor electrical/thermal conductor, dull, brittle when solid, low melting/boiling point, low density.
(c) B1 sodium = metal AND iron = metal both correct
(c) B1 copper = metal AND oxygen = non-metal both correct
(c) B1 sulfur = non-metal AND chlorine = non-metal both correct
(c) Answer: Sodium: metal. Sulfur: non-metal. Iron: metal. Oxygen: non-metal. Copper: metal. Chlorine: non-metal.

Question 17

(a) B1 both are atoms of chlorine with the same number of protons (17), i.e. the same atomic number
(a) B1 they have different numbers of neutrons
(a) B1 this gives them different mass numbers (35 and 37) oe
(a) Answer: Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, and therefore different mass numbers. Chlorine-35 and chlorine-37 both have 17 protons but different neutron numbers.
(b) B1 = 20 cao (ft candidate's own atomic number of chlorine from (a) if consistent)
(b) Answer: 20 neutrons.

Question 18

(a) B1 the copper and tin are not chemically joined (bonded) together oe
(a) B1 the proportions of copper and tin can be varied, and each metal keeps its own individual properties oe
(a) Answer: Bronze is a mixture because the copper and tin atoms are not chemically bonded together; the ratio of the two metals can be varied and each metal keeps its own properties.
(b) M1 250 x 0.12 (or 250 x 12/100)
(b) A1 = 30 kg cao
(b) Answer: 30 kg

Atoms, Elements and Compounds - Worksheets, Questions and Revision