Elements, Compounds and Mixtures | OCR GCSE Combined Science Chemistry, Foundation tier (J250)

Elements, Compounds and Mixtures

221 questions
15 subtopics
Paper 3 (Chemistry)
Paper 3 (Chemistry)

Elements, Compounds and Mixtures is examined in Paper 3 (Chemistry).

It covers purity, and melting point data, relative formula mass and empirical formulae, formulations, and separating mixtures, paper and thin layer chromatography, interpreting chromatograms and Rf values, choosing a purification method, metals and non-metals in the Periodic Table, electron arrangement and position in the Periodic Table, ionic, covalent and metallic bonding compared, dot and cross diagrams, and the limits of models, reactions, electron arrangement and Mendeleev's table, carbon's four bonds, and the variety of organic compounds, diamond, graphite, fullerenes and graphene, bond strength, intermolecular forces and predicting state and bulk properties of ionic, molecular and giant structures.

Sample questions from Elements, Compounds and Mixtures

Answer each one closed book first, then open the answer.

Purity, and melting point data

A sample contains nothing but copper atoms. Is it an element, a compound or a mixture, and is it pure?

Show the answer
It is an element, and it is pure because only one kind of atom is present.
Relative formula mass and empirical formulae

A compound has the empirical formula CH₂O and a relative formula mass of 180. How many CH₂O units make up one molecule?

Show the answer
Six, because one CH₂O unit has a relative mass of 30 and 180 ÷ 30 = 6.
Paper and thin layer chromatography

When would a non-aqueous solvent such as ethanol be chosen instead of water?

Show the answer
When the substances being separated do not dissolve in water, so they need an organic solvent to carry them.
Interpreting chromatograms and Rf values

To how many significant figures should the answer to a calculation be given?

Show the answer
To the same number as the value with the fewest significant figures among the data used in the calculation.
Metals and non-metals in the Periodic Table

How does a non-metal oxide behave when it dissolves in water?

Show the answer
It is acidic and gives a solution with a pH below 7.
Electron arrangement and position in the Periodic Table

An atom has 19 electrons. Give its electron arrangement and its position in the Periodic Table.

Show the answer
Its arrangement is 2,8,8,1, so it is in Group 1 and period 4.
Dot and cross diagrams, and the limits of models

What would a dot and cross diagram of sodium chloride show?

Show the answer
It would show a sodium ion with an empty outer shell and a single positive charge, and a chloride ion with eight outer electrons and a single negative charge, each drawn in square brackets.
Reactions, electron arrangement and Mendeleev's table

Why did leaving gaps make Mendeleev's table more convincing?

Show the answer
Because he used the gaps to predict the properties of undiscovered elements, and when those elements were found their properties matched his predictions.

The 15 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Purity, and melting point data	What pure means to a chemist and in everyday speech, orange juice and spring water as mixtures, why chemists need the strict definition, and how a sharp melting point shows purity.	17
Relative formula mass and empirical formulae	What relative atomic, molecular and formula mass mean, calculating relative formula masses such as CO₂ and Ca(OH)₂, why the totals balance on both sides of an equation, what an empirical formula is, and working out empirical and molecular formulae from molecules, models, diagrams and relative formula mass.	16
Formulations, and separating mixtures	What a formulation is and why it is never pure, examples such as paints, medicines and brass, the jobs of the pigment and solvent in a paint, why proportions matter and alloys are harder than pure metals, and how filtration, crystallisation, simple distillation and fractional distillation separate mixtures.	15
Paper and thin layer chromatography	How a paper chromatogram is set up with a pencil baseline above the solvent, what a thin layer plate is made of and its advantage over paper, choosing a solvent, revealing colourless spots with a locating agent or ultraviolet light, the stationary and mobile phases in paper and thin layer chromatography, and why substances separate.	16
Interpreting chromatograms and Rf values	What an Rf value means, the formula and where the distances are measured from, worked Rf calculations, why Rf never exceeds 1, why the solvent must match, and reading purity and identity from a chromatogram.	15
Choosing a purification method	Matching filtration, simple distillation, crystallisation and fractional distillation to mixtures, the property each relies on, purifying rock salt, choosing paper or thin layer chromatography, and how gas chromatography works.	16
Metals and non-metals in the Periodic Table	The typical physical properties of metals and non-metals, the ions each forms and why, metal oxides as basic and non-metal oxides as acidic, graphite as a non-metal that conducts, where metals and non-metals sit in the Periodic Table, why metallic character increases down a group, the elements beside the stepped line, and predicting ion charges from outer electrons.	16
Electron arrangement and position in the Periodic Table	What the group and period numbers say about electron shells, reading a position from an arrangement such as 2,8,1, atomic number and electrons, ordering by atomic number, and why Group 0 sits at the right.	11
Ionic, covalent and metallic bonding compared	The bonding and arrangement of particles in ionic compounds, simple molecules, giant covalent structures, polymers and metals, which kinds of atom each type of bonding joins, electron transfer and sharing, the electrostatic attractions that hold ionic, covalent and metallic bonds together, the ions in sodium chloride and magnesium oxide, and why atoms bond at all.	16
Dot and cross diagrams, and the limits of models	Why dots and crosses are used, the diagrams for hydrogen chloride, oxygen, methane, sodium chloride and magnesium oxide, and what dot and cross, ball and stick and flat drawings each fail to show.	16
Reactions, electron arrangement and Mendeleev's table	Why a group reacts alike, the reactivity trends in Groups 1, 7 and 0, predicting how an atom with 2,8,7 reacts, how Mendeleev ordered the elements and left gaps, and ordering by atomic number.	15
Carbon's four bonds, and the variety of organic compounds	Why a carbon atom forms four covalent bonds and never five, how many bonds remain after a double bond, why carbon forms so many compounds in chains, branches and rings, homologous series such as the alkanes, ring structures, compounds with the same formula but different properties, and natural and synthetic organic compounds.	14
Diamond, graphite, fullerenes and graphene	The structures of diamond and graphite, why diamond is hard and does not conduct while graphite is soft, slippery and conducting, what graphene is and why it is strong, light and conducting, what a fullerene is, buckminsterfullerene C₆₀, and why fullerenes melt at far lower temperatures than diamond.	11
Bond strength, intermolecular forces and predicting state	Why simple molecular substances melt and boil at low temperatures without breaking covalent bonds, why ionic compounds, giant covalent structures and metals have high melting points, why boiling points rise as molecules get larger, and predicting whether a substance is a solid, liquid or gas at a given temperature from its melting and boiling points.	16
Bulk properties of ionic, molecular and giant structures	Why bulk properties belong to many atoms rather than one, why ionic compounds conduct only when molten or dissolved and are brittle, why simple molecular substances are often gases or liquids and do not conduct, why metals are malleable and conduct, why giant covalent structures are insoluble, why polymers soften on heating, and identifying a structure from its properties.	11

Elements, Compounds and Mixtures is 221 of the 797 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.