Chemistry key terms

Topic 1 · Principles of chemistry

The Three States of Matter

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Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random movement.

Exam-exact meanings.

Melting

solid → liquid (heat)

Freezing

liquid → solid (cool)

Boiling / evaporating

liquid → gas (heat)

Condensing

gas → liquid (cool)

Subliming

solid → gas directly (heat)

Deposition

gas → solid directly (cool)

Pure Substances, Mixtures and How to Separate Them

Exam-exact meanings.

Solvent

the liquid a substance dissolves in

Solute

the substance that dissolves

Solution

the mixture formed when a solute dissolves in a solvent

Saturated solution

a solution holding as much dissolved solute as it can at that temperature

Element

a substance containing only one kind of atom, which cannot be broken down into anything simpler by chemical means

Compound

two or more different elements chemically bonded together in fixed proportions

Mixture

two or more substances that are not chemically bonded, present in any proportions, and separable by physical means

Pure substance

a single element or a single compound, with nothing else in it — so it melts and boils at fixed, sharp temperatures

'Pure' in everyday use

means natural or unadulterated, as in 'pure orange juice'. Chemically that juice is a mixture, and it will not melt sharply

Inside the Atom

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Atom

The smallest particle of an element that can exist.

Molecule

Two or more atoms chemically bonded together by covalent bonds. A molecule may be an element (O₂, N₂, Cl₂) or a compound (H₂O, CO₂) — 'molecule' says how the atoms are joined, not whether they are the same.

Atomic number

The number of protons in the nucleus of an atom. It defines which element the atom is, and it never changes in a chemical reaction.

Mass number

The total number of protons and neutrons in the nucleus of an atom. Always a whole number, because it is a count of particles.

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons.

Relative atomic mass (Ar)

The weighted mean mass of the isotopes of an element, taking their abundances into account, compared with 1/12 the mass of a carbon-12 atom.

Exam-exact meanings.

Mass number (top)

protons + neutrons

Atomic number (bottom)

protons — this defines the element

Neutrons

mass number − atomic number

Electrons

equal to protons in a neutral atom; adjusted by the charge in an ion

Ion

an atom (or group of atoms) that has lost or gained electrons and therefore carries a charge

Why an atom is neutral

equal numbers of protons (+1 each) and electrons (−1 each), so the charges cancel exactly

Electronic Configurations and the Periodic Table

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Why a group reacts alike

Elements in the same group have the same number of electrons in their outer shell, so they react in the same way and form ions with the same charge.

Why the noble gases are unreactive

Group 0 elements have a full outer shell of electrons, so they have no tendency to lose, gain or share electrons.

Solubility and Solubility Curves

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Solubility

The mass of a solute that dissolves in 100 g of a solvent to form a saturated solution at a stated temperature. Quoted in g per 100 g of solvent, and the temperature must always be stated.

Saturated solution

A solution that contains as much dissolved solute as it can at that temperature, in the presence of undissolved solute, with no further net dissolving taking place.

Formulae and Balanced Equations

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Relative formula mass (Mr)

The sum of the relative atomic masses of all the atoms shown in the formula of a substance. It has no units, because it is a comparison rather than a measurement.

Exam-exact meanings.

(s)

solid

(l)

liquid — a pure substance in its liquid state, such as water or molten lead

(g)

gas

(aq)

aqueous — dissolved in water. This is a solution, not a pure liquid

(l) against (aq)

H₂O(l) is pure water; NaCl(aq) is salt dissolved in water; NaCl(l) is molten salt above 800 °C

The Mole

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Mole

The unit for amount of substance. One mole contains 6.02 × 10²³ particles, and has a mass in grams equal to the relative atomic or formula mass of that substance.

Reacting Masses and Percentage Yield

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Theoretical yield

The mass of product calculated from the balanced equation, assuming the reaction is complete and nothing is lost.

Actual yield

The mass of product actually obtained and weighed in the experiment.

Percentage yield

The actual yield as a percentage of the theoretical yield.

Empirical and Molecular Formulae

Exam-exact meanings.

Empirical formula

The simplest whole-number ratio of the atoms of each element in a compound. Ethane's is CH₃.

Molecular formula

The actual number of atoms of each element in one molecule. Ethane's is C₂H₆.

Water of crystallisation

Water held within the structure of a crystal, written after a dot: CuSO₄·5H₂O. Driven off by heating.

Anhydrous

Containing no water of crystallisation — what remains after the hydrated salt has been heated.

Concentration and Gas Volumes

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Molar volume

The volume occupied by one mole of any gas at a stated temperature and pressure — 24 dm³ at room temperature and pressure. It is the same for every gas, because the particles occupy almost none of the volume.

Exam-exact meanings.

mol/dm³

Concentration — the moles dissolved in each cubic decimetre of solution.

dm³

A cubic decimetre, the same as a litre and the same as 1000 cm³.

cm³ → dm³

DIVIDE BY 1000. Burettes and pipettes are marked in cm³, so this is the commonest step in the topic.

rtp

Room temperature and pressure — the conditions at which the molar volume is 24 dm³.

Ionic Bonding

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Ion

An atom, or group of atoms, that has lost or gained electrons and so carries an overall charge. Losing electrons gives a positive ion; gaining them gives a negative ion. The number of protons never changes.

Exam-exact meanings.

Hydroxide

OH⁻ — charge 1−.

Nitrate

NO₃⁻ — charge 1−.

Carbonate

CO₃²⁻ — charge 2−.

Sulfate

SO₄²⁻ — charge 2−.

Ammonium

NH₄⁺ — charge 1+, and the only common positive one.

Ionic Structures and Their Properties

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Ionic bond

The strong electrostatic force of attraction between oppositely charged ions. It acts in all directions, which is why ions form a giant lattice rather than molecules.

Giant ionic lattice

A regular repeating three-dimensional arrangement in which each ion is surrounded by ions of the opposite charge. The formula gives the ratio of ions, not the contents of a molecule.

Covalent Bonding

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Covalent bond

A shared pair of electrons between two atoms, counting towards the outer shell of both. It is held together by the electrostatic attraction of both positive nuclei to the shared negative pair.

Exam-exact meanings.

Shared pair

The two electrons making up one covalent bond, counted by both atoms.

Lone pair

A pair of outer electrons on an atom that is not shared. Must be drawn on the diagram.

Double bond

Two shared pairs between the same two atoms, as in O₂.

Triple bond

Three shared pairs between the same two atoms, as in N₂.

Simple Molecular Substances

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Simple molecular substance

A substance consisting of small separate molecules, with strong covalent bonds inside each molecule and weak intermolecular forces between them.

Intermolecular forces

The forces of attraction between one molecule and another. They are much weaker than covalent bonds, and they are what is overcome when a molecular substance melts or boils.

Giant Covalent Structures

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Giant covalent structure

A structure in which covalent bonds continue throughout the whole piece of material, so there are no separate molecules. Melting requires breaking covalent bonds themselves.

Metallic Bonding

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Metallic bonding

The strong electrostatic attraction between a lattice of positive metal ions and the delocalised electrons that move freely throughout the structure.

Delocalised electrons

Electrons that are not attached to any particular atom and are free to move throughout the whole structure. In a metal they come from the outer shell of every atom.

Why Substances Conduct — the Four Structures Compared

Exam-exact meanings.

Cation

A POSITIVE ion. Attracted to the cathode.

Anion

A NEGATIVE ion. Attracted to the anode.

Cathode

The NEGATIVE electrode. Cations go there.

Anode

The POSITIVE electrode. Anions go there.

Electrolysis

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Electrolysis

The decomposition of an ionic compound by passing an electric current through it when molten or in aqueous solution, so that its ions are free to move to the electrodes.

Exam-exact meanings.

Electrolyte

The molten or dissolved ionic compound being decomposed.

Cathode

The NEGATIVE electrode. Cations go there, GAIN electrons, and are REDUCED.

Anode

The POSITIVE electrode. Anions go there, LOSE electrons, and are OXIDISED.

Inert electrode

Graphite or platinum — takes no part in the reaction, only delivering and removing electrons.

Topic 2 · Inorganic chemistry

Group 1 — the Alkali Metals

Exam-exact meanings.

Alkali metal

A Group 1 metal. Named because it reacts with water to give a metal hydroxide, which forms an alkaline solution.

Group

A vertical column of the periodic table. All its elements have the same number of outer electrons, so they react in the same way.

Tarnish

To lose its shine as the freshly cut surface reacts with oxygen in the air to form the oxide.

Shielding

The reduction in attraction felt by an outer electron because full inner shells lie between it and the nucleus.

Group 7 — the Halogens

Exam-exact meanings.

Halogen

A Group 7 non-metal. Each exists as diatomic molecules and reacts by gaining one electron.

Halide

The ion formed when a halogen gains an electron — chloride, bromide, iodide — or a compound containing one.

Displacement

A reaction in which a more reactive halogen takes the place of a less reactive one in a solution of its salt.

Diatomic

Existing as molecules of two atoms. All the halogens are diatomic: Cl₂, Br₂, I₂.

The Gases in the Air

Exam-exact meanings.

Nitrogen

About 78%. Very unreactive, because the triple bond in N₂ is hard to break.

Oxygen

About 21%. The reactive component — everything that burns, burns in this.

Argon

About 0.9%. A noble gas, so it takes part in no reactions at all.

Carbon dioxide

About 0.04%. A very small fraction, but the one whose amount is changing.

Combustion and Carbonates

Exam-exact meanings.

Combustion

Reaction with oxygen, releasing energy. Two reactants are needed: the substance and oxygen.

Thermal decomposition

The breaking down of ONE substance into two or more by heating. Nothing reacts with it.

Greenhouse effect

The warming of the Earth caused by gases absorbing infrared radiation emitted by the surface and re-emitting some of it back.

Basic oxide

A metal oxide, which reacts with acids and gives an alkaline solution if it dissolves.

Acidic oxide

A non-metal oxide, which gives an acidic solution when it dissolves in water.

The Reactivity Series

Exam-exact meanings.

Reactivity series

A list of metals in order of how readily they react, established by reactions with water, acid and by displacement.

Displacement

A reaction in which a more reactive metal takes the place of a less reactive one in a solution of its salt.

Native metal

A metal found uncombined in the ground because it is too unreactive to have formed compounds — gold and silver.

Rusting and its Prevention

Exam-exact meanings.

Rusting

The corrosion of iron to form hydrated iron(III) oxide. Both water AND oxygen are required.

Barrier method

Preventing rusting by keeping water and oxygen away — painting, oiling, plastic coating, tin plating.

Sacrificial protection

Attaching a MORE reactive metal, which corrodes in preference and so protects the iron.

Galvanising

Coating iron with zinc, which acts as a barrier AND as sacrificial protection if scratched.

Redox

Exam-exact meanings.

Oxidation

Gain of oxygen, OR loss of electrons. Remember OIL — Oxidation Is Loss.

Reduction

Loss of oxygen, OR gain of electrons. Remember RIG — Reduction Is Gain.

Redox

A reaction in which oxidation and reduction both occur. They always do, since electrons lost by one substance are gained by another.

Oxidising agent

The substance that causes oxidation by TAKING electrons — so it is itself REDUCED.

Reducing agent

The substance that causes reduction by GIVING electrons — so it is itself OXIDISED.

Extracting Metals from their Ores

Exam-exact meanings.

Ore

A rock containing enough of a metal compound to make extraction worthwhile.

Native metal

A metal found uncombined in the ground, because it is too unreactive to have formed compounds — gold and silver.

Reduction

Removal of oxygen from a compound, or gain of electrons. Extracting a metal from its oxide is always a reduction.

Cryolite

The molten compound in which aluminium oxide is dissolved, lowering the working temperature from over 2000 °C to about 950 °C.

Uses of Metals, and Alloys

Exam-exact meanings.

Alloy

A MIXTURE of a metal with one or more other elements. Not a compound: no fixed formula, and no reaction has occurred.

Steel

An alloy of iron with a small amount of carbon, usually under 1%. Far harder than pure iron.

Stainless steel

Steel alloyed with chromium and nickel, which resists corrosion — used for cutlery and surgical instruments.

Acids, Alkalis and the pH Scale

Exam-exact meanings.

Indicator

A substance that changes colour depending on whether it is in an acid or an alkali.

pH scale

A scale from 0 to 14 measuring how acidic or alkaline a solution is. Below 7 acidic, 7 neutral, above 7 alkaline.

Universal indicator

A mixture of indicators that changes colour gradually across the whole pH range, giving an approximate pH.

Neutral

At pH exactly 7 — neither acidic nor alkaline. Pure water is neutral.

Hydrogen Ions and Neutralisation

Exam-exact meanings.

Acid

A substance that produces hydrogen ions, H⁺, when dissolved in water.

Alkali

A substance that produces hydroxide ions, OH⁻, when dissolved in water.

Base

Any substance that neutralises an acid — metal oxides, metal hydroxides and ammonia.

Spectator ion

An ion present during a reaction but unchanged by it, taking no part in the chemistry.

Titrations

Exam-exact meanings.

Titre

The volume delivered from the burette — the final reading minus the initial reading.

End point

The moment at which the indicator changes colour permanently, showing neutralisation is complete.

Concordant results

Repeat titres agreeing within about 0.1 cm³. Only these are averaged.

Rough titration

A deliberately quick first run to find roughly where the end point is. It is discarded, not averaged.

Solubility Rules and Proton Transfer

Exam-exact meanings.

Proton

A hydrogen ion, H⁺. A hydrogen atom that has lost its single electron is just a proton.

Acid

A PROTON DONOR — a substance that gives away a hydrogen ion.

Base

A PROTON ACCEPTOR — a substance that takes a hydrogen ion.

Proton transfer

The movement of a hydrogen ion from the acid to the base. Every acid–base reaction is one.

The Reactions of Acids

Exam-exact meanings.

Metal oxide

A base. Copper oxide and magnesium oxide both neutralise acids.

Metal hydroxide

A base. Sodium hydroxide and copper hydroxide both neutralise acids.

Ammonia

A base, despite containing no hydroxide, because it accepts a proton to become NH₄⁺.

Alkali

A base that DISSOLVES in water. Sodium hydroxide is one; copper oxide is not.

Preparing a Soluble Salt

Exam-exact meanings.

Filtrate

The liquid that passes through the filter paper. Here it is the solution of the salt.

Residue

The solid left on the filter paper. Here it is the excess insoluble reactant.

Hydrated salt

A salt whose crystals contain water of crystallisation, such as CuSO₄·5H₂O — blue rather than white.

Point of crystallisation

The stage at which crystals first appear at the edges of the evaporating basin. Stop heating here.

Preparing an Insoluble Salt

Exam-exact meanings.

Precipitate

An insoluble solid formed when two solutions are mixed.

Precipitation reaction

A reaction in which two soluble compounds react in solution to form an insoluble product.

Residue

The solid caught on the filter paper — here the precipitate itself, which is the product wanted.

Spectator ion

An ion present but unchanged — here the sodium and nitrate ions, which stay dissolved and are washed away.

Tests for Gases, and for Water

Exam-exact meanings.

Anhydrous

Containing no water of crystallisation. Anhydrous copper(II) sulfate is WHITE.

Hydrated

Containing water of crystallisation. Hydrated copper(II) sulfate is BLUE.

Limewater

Calcium hydroxide solution. It turns milky with carbon dioxide, forming insoluble calcium carbonate.

Pure substance

A single substance with nothing else in it, melting and boiling at fixed temperatures rather than over a range.

Flame Tests

Exam-exact meanings.

Nichrome or platinum wire

These give no flame colour of their own, so they add nothing to the result.

Cleaning until colourless

Any residue from a previous test would give its own colour, especially sodium.

Hydrochloric acid

Converts the compound to a more volatile chloride, so more metal reaches the flame and the colour is stronger.

Blue flame

A yellow safety flame has its own colour and would mask a faint result.

Tests for Ions

Exam-exact meanings.

Purpose of the acid

To remove any carbonate present, which would give its own white precipitate and a false positive.

Nitric acid for halides

Hydrochloric acid would add chloride ions and guarantee a white precipitate whatever the sample was.

Hydrochloric acid for sulfates

Sulfuric acid would add sulfate ions and guarantee a white precipitate whatever the sample was.

The rule

Use the acid that cannot supply the ion you are testing for.

Topic 3 · Physical chemistry

Exothermic and Endothermic Reactions

Exam-exact meanings.

Exothermic

A reaction that GIVES OUT heat energy to the surroundings. The temperature of the surroundings RISES.

Endothermic

A reaction that TAKES IN heat energy from the surroundings. The temperature of the surroundings FALLS.

Surroundings

Everything other than the reacting chemicals — the water, the container and the air. This is what the thermometer measures.

Calorimetry

Measuring the heat energy transferred in a reaction, usually by recording a temperature change in a known mass of water.

Calorimetry Calculations

Exam-exact meanings.

Q

The heat energy transferred, in joules. Calculated from the temperature change of the solution.

Specific heat capacity, c

The energy needed to raise 1 g of a substance by 1 °C. Take it as 4.18 J/g/°C for dilute aqueous solutions.

ΔT

The temperature change — the final temperature minus the starting temperature.

Molar enthalpy change, ΔH

The energy change per mole of reactant, in kJ/mol. Negative for exothermic, positive for endothermic.

Energy Diagrams and Bond Energies

Exam-exact meanings.

Bond breaking

ENDOTHERMIC — energy must be supplied. Always, for every bond.

Bond making

EXOTHERMIC — energy is released. Always, for every bond.

Bond energy

The energy needed to break one mole of a particular bond, in kJ/mol. Always positive, and the same amount is released when the bond forms.

Activation energy

The minimum energy colliding particles must have for a reaction to occur — the height of the peak on an energy level diagram.

Rates of Reaction

Exam-exact meanings.

Rate of reaction

How quickly reactants are used up or products formed. Measured as an amount per unit time.

Surface area

The area of solid exposed to the other reactant. Powdering a solid greatly increases it.

Catalyst

A substance that speeds up a reaction without being used up in it.

Limiting reactant

The reactant that runs out first, which is what decides the total amount of product and where the graph levels off.

Collision Theory

Exam-exact meanings.

Collision theory

Particles must collide, with sufficient energy and in the correct orientation, for a reaction to occur.

Successful collision

A collision that results in a reaction — one with at least the activation energy and the correct orientation.

Activation energy

The minimum energy colliding particles must have for a reaction to occur.

Reaction profile

A diagram showing the energy through a reaction, with the activation energy as the peak height and ΔH as the gap between reactants and products.

Catalysts

Exam-exact meanings.

Catalyst

A substance that increases the rate of a reaction and is chemically unchanged at the end of it.

Alternative pathway

The different route from reactants to products that a catalyst provides, having a lower activation energy.

Chemically unchanged

The same amount of the same substance is present at the end. The catalyst takes part but is regenerated.

Reversible Reactions

Exam-exact meanings.

Reversible reaction

One in which the products can react together to re-form the reactants. Written with the ⇌ symbol.

⇌

The reversible reaction symbol, with a head at each end, meaning both directions are possible.

Hydrated

Containing water of crystallisation. Hydrated copper(II) sulfate is BLUE.

Anhydrous

Containing no water of crystallisation. Anhydrous copper(II) sulfate is WHITE.

Dynamic Equilibrium

Exam-exact meanings.

Dynamic equilibrium

The state in which the forward and reverse reactions occur at equal rates, so concentrations remain constant.

Closed system

A container from which nothing can enter or leave. Equilibrium can only be reached in one.

Position of equilibrium

How much product there is relative to reactant. To the RIGHT means mostly products; to the LEFT means mostly reactants.

Dynamic

Both reactions are STILL HAPPENING. The reaction has not stopped — the two rates are simply equal.

Le Chatelier's principle

If a condition of a system at equilibrium is changed, the position of equilibrium shifts to counteract the change.

Topic 4 · Organic chemistry

What Organic Chemistry Is

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Isomers

Compounds with the same molecular formula but different structural formulae. They are different substances with different physical properties.

Exam-exact meanings.

Hydrocarbon

A compound containing hydrogen and carbon ONLY.

Homologous series

A family of compounds with the same general formula and the same functional group, whose members differ by CH₂.

Functional group

The atom or group of atoms that gives a homologous series its characteristic chemical reactions.

General formula

The formula for a whole homologous series, written in terms of n, the number of carbon atoms.

Naming, Drawing And Classifying

Exam-exact meanings.

Substitution

A reaction in which an atom or group in a molecule is replaced by a different atom or group.

Addition

A reaction in which two molecules join to form a single product, with nothing else formed.

Combustion

A reaction of a substance with oxygen that releases heat energy.

Structural isomers

Compounds with the same molecular formula whose atoms are joined in a different order.

Crude Oil And Its Fractions

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Crude oil

A mixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient marine organisms.

Exam-exact meanings.

Fraction

A group of hydrocarbons with similar chain lengths and boiling points, collected together at one level of the column.

Fractional distillation

Separation of a mixture of liquids by their different boiling points.

Viscosity

How thick a liquid is and how poorly it flows. Long-chain hydrocarbons are more viscous.

Volatility

How readily a liquid turns to vapour. Short-chain hydrocarbons are the more volatile.

Burning Fuels

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Fuel

A substance that releases heat energy when it is burned.

Exam-exact meanings.

Complete combustion

Burning in plenty of oxygen, giving carbon dioxide and water only.

Incomplete combustion

Burning in a limited supply of oxygen, giving carbon monoxide and/or carbon as well as water.

Soot

Solid carbon released when combustion is very incomplete. It makes a flame yellow and smoky.

Haemoglobin

The red pigment in red blood cells that carries oxygen around the body.

Pollutants And Acid Rain

Exam-exact meanings.

Sulfur dioxide

Formed when sulfur impurities present in the fuel are burned. SO₂.

Oxides of nitrogen

Formed when nitrogen and oxygen from the AIR react in the high temperature of an engine.

Acid rain

Rain made significantly more acidic than normal by dissolved sulfur dioxide and oxides of nitrogen.

Impurity

A substance present in small amounts alongside the main component, here the sulfur compounds in crude oil.

Catalytic converter

A device fitted in a vehicle's exhaust that converts oxides of nitrogen back to nitrogen. It is unchanged by the reaction.

Cracking

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Cracking

The breaking down of long-chain alkanes into shorter, more useful alkanes and alkenes, using heat and a catalyst.

Exam-exact meanings.

Thermal decomposition

The breaking down of one substance into two or more others by heating.

Catalyst

A substance that speeds up a reaction and is unchanged at the end. It is not a reactant.

Alkene

A hydrocarbon containing a carbon-to-carbon double bond. Barely present in crude oil, and produced by cracking.

Feedstock

A raw material fed into an industrial process to make something else — here, alkenes for making plastics.

Alkanes

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Saturated hydrocarbon

A hydrocarbon containing only single carbon-to-carbon bonds, so that it holds the maximum possible number of hydrogen atoms and nothing further can be added.

Exam-exact meanings.

Alkane

A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂.

Saturated

Containing only single carbon-to-carbon bonds, so nothing further can be added.

Substitution

A reaction in which an atom or group is replaced by a different atom or group, giving two products.

Mono-substitution

Substitution of exactly one hydrogen atom, which is all the specification requires here.

Alkenes

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Unsaturated hydrocarbon

A hydrocarbon containing at least one carbon-to-carbon double bond, so that further atoms can be added across it.

Exam-exact meanings.

Alkene

An unsaturated hydrocarbon containing a carbon-to-carbon double bond, general formula CₙH₂ₙ.

C=C

The functional group of the alkenes. Every characteristic alkene reaction happens here.

Unsaturated

Containing a carbon-to-carbon double bond, so that more atoms can be added across it.

But-1-ene and but-2-ene

Isomers of C₄H₈, differing only in where along the chain the double bond sits.

Alkene Reactions And The Bromine Test

Exam-exact meanings.

Reagent

Bromine water, which is orange.

Positive result

The orange colour is decolourised — the mixture goes colourless. An alkene is present.

Negative result

No change; the mixture remains orange. An alkane is present.

What is being tested for

Unsaturation — the presence of a carbon-to-carbon double bond.

Alcohols

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Alcohol

A member of the homologous series whose functional group is –OH, with the general formula CₙH₂ₙ₊₁OH.

Exam-exact meanings.

–OH

The functional group of the alcohols, sometimes called the hydroxyl group.

Oxidation

Gain of oxygen. Ethanol gains oxygen in all three routes on this page.

Microbial oxidation

Oxidation carried out by bacteria present in the air, turning ethanol into ethanoic acid over time.

Potassium dichromate(VI)

The oxidising agent used with dilute sulfuric acid. Orange when it starts, green when it has been reduced.

Making Ethanol

Exam-exact meanings.

Fermentation

The conversion of sugar into ethanol and carbon dioxide by yeast, in the absence of air.

Hydration

Addition of water across a carbon-to-carbon double bond.

Batch process

One that is set up, run to completion and emptied, then started again.

Continuous process

One in which reactants are fed in and product drawn off without stopping.

Carboxylic Acids

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Carboxylic acid

A member of the homologous series whose functional group is –COOH, with the general formula CₙH₂ₙ₊₁COOH.

Exam-exact meanings.

–COOH

The carboxylic acid functional group: a carbon with a double-bonded oxygen and an –OH.

Ethanoate

The salt formed from ethanoic acid, for example sodium ethanoate, CH₃COONa.

Vinegar

An aqueous solution containing ethanoic acid, typically around five per cent.

Weak acid

One in which only a small proportion of the molecules release a hydrogen ion in solution.

Esters And How To Make One

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Ester

A compound formed when an alcohol reacts with a carboxylic acid, containing the –COO– functional group, with water also produced.

Exam-exact meanings.

–COO–

The ester functional group. It sits in the middle of the molecule, with a carbon chain on each side.

Esterification

The reaction of an alcohol with a carboxylic acid to form an ester and water.

Condensation reaction

One in which two molecules join and a small molecule, here water, is expelled.

Ethyl ethanoate

CH₃COOC₂H₅, made from ethanol and ethanoic acid. Smells sweet and fruity, like pear drops.

Working Out Esters, And What They Are For

Exam-exact meanings.

Volatile

Evaporating readily at ordinary temperatures. Esters are volatile, which is why their smell reaches the nose.

-yl

The ending taken from the alcohol, written FIRST in an ester's name. Ethanol gives ethyl.

-anoate

The ending taken from the acid, written SECOND in an ester's name. Ethanoic acid gives ethanoate.

Flavouring

A substance added to food to give it a particular taste or smell. Many are esters.

Addition Polymers

Exam-exact meanings.

Monomer

A small molecule that joins to many others to form a polymer.

Polymer

A very large molecule made by joining many monomers together.

Addition polymerisation

Many monomers containing a C=C double bond joining together, with nothing else formed.

Repeat unit

The section of a polymer chain that recurs, drawn in brackets with n outside and bonds passing through the brackets.

Condensation Polymers

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Condensation polymerisation

The joining of many monomers into a chain, in which a small molecule such as water is lost each time a link is formed.

Exam-exact meanings.

Diol

A molecule with an –OH group at both ends. The di- prefix means two.

Dicarboxylic acid

A molecule with a –COOH group at both ends.

Polyester

A condensation polymer whose monomers are joined by ester links.

Biopolyester

A polyester that is biodegradable, meaning microorganisms are able to break it down.