Water expands when it freezes because it absorbs heat

Hydrogen bonds hold ice in an open hexagonal lattice that is less dense than the liquid, where molecules pack closer

Ice contains trapped air

Ice is made of lighter isotopes of hydrogen

Water molecules are bigger

Water is a liquid and H₂S is a gas, so they cannot be compared

Hydrogen sulfide has weaker covalent bonds

Water molecules hydrogen-bond to each other; sulfur is not electronegative enough for H₂S to do so

The added electrons go into an inner shell, which is smaller

The atoms do not get smaller; only their ions do

Each added proton pulls the same outer shell in more strongly; the added electrons go into that same shell and shield each other poorly

Heavier atoms are compressed by their own mass

The atom: the ion has lost its entire outer shell

The ion: positive charge repels the remaining electrons outwards

They are the same size: only one electron is missing

The ion: it has more protons per electron

They are all the same size

Mg²⁺

Na⁺

O²⁻

Beryllium's full 2s subshell makes it a noble-gas-like atom

Boron's outer electron is in a 2p orbital, which is higher in energy and better shielded than beryllium's 2s

Boron's outer electron is in the third shell

Boron has fewer protons than beryllium

Oxygen's fourth p electron must pair up in an orbital already holding one; the repulsion makes it easier to remove

Nitrogen's outer electron is in a 2s orbital

Oxygen's extra proton is cancelled by its extra electron

Oxygen has a larger atomic radius than nitrogen

The ion becomes too small to hold an orbital

The third electron comes from the inner second shell, much closer to the nucleus and far less shielded

The third electron is a proton in disguise

Magnesium can only ever lose two electrons

Fluorine

Chlorine

Oxygen

Neon

Chlorine has a higher nuclear charge, so it always wins

Fluorine's 2p shell is so small that the incoming electron is strongly repelled by the electrons already there

Chlorine's electron goes into a 3d orbital

Fluorine already has a full outer shell

They have no electrons in their outer shell

They are gases, and gases do not react

They are too heavy to react

Their outer shell is full, so they have no tendency to gain, lose or share electrons

Its outer electron is further from the nucleus and better shielded, so it is lost more easily

Potassium has more outer electrons to give away

Potassium is denser, so it sinks and reacts under water

Potassium's nucleus is less stable

Chlorine is the stronger oxidising agent: its smaller atom attracts an extra electron more strongly than bromine does

Chloride ions are heavier than bromide ions

Chlorine has more electrons than bromine

Chlorine is a gas and bromine is a liquid

Their ions are larger and scatter light

Their partly filled d orbitals are split in energy by surrounding ligands, and electrons absorb visible light jumping between them

Their nuclei emit visible light

Their compounds contain traces of coloured impurities

Metals are shinier, and shiny surfaces conduct

Ionic compounds have no charged particles

Metals contain more electrons per atom

Metals have delocalised electrons that move freely; in solid NaCl the ions are charged but locked in place

The Na–Cl bond is stronger than the Cl–Cl bond

NaCl is a giant lattice held by strong electrostatic attractions between ions; Cl₂ molecules are held to each other only by weak intermolecular forces

Sodium chloride is heavier

Chlorine gas has no bonds at all

Diamond's carbon atoms have more electrons

Graphite contains hydrogen between its layers

Diamond forms at higher pressure, which squeezes its bonds shorter

In diamond every carbon is bonded to four others in a rigid 3D network; graphite has strong layers held together only weakly

Nitrogen is heavier than air and sinks away from reactions

Nitrogen has a full outer shell like a noble gas

The two atoms share three pairs of electrons; breaking this triple bond takes about 945 kJ/mol

Nitrogen molecules are too small to collide with anything

Copper's bonds are weaker, so they bend rather than break

Copper is a single giant molecule

Metal layers can slide while the electron sea keeps holding them; shifting an ionic lattice brings like charges together, which repel and split the crystal

Ionic bonds are directional, metallic bonds are stronger

Double bonds are never polar

The molecule is linear, so the two bond dipoles point in opposite directions and cancel

Oxygen and carbon have the same electronegativity

CO₂ is a gas, and gases cannot be polar

Nitrogen and oxygen atoms are smaller than carbon

The angles are the same; the difference is a measurement error

Lone pairs repel more strongly than bonding pairs and squeeze the bonds closer together

Hydrogen atoms repel each other less when there are fewer of them

SF₆ — sulfur has twelve electrons around it

NH₃ — nitrogen has eight

CH₄ — carbon has eight

H₂O — oxygen has eight

Both electrons in the fourth bond came from nitrogen's lone pair; the H⁺ brought none

Both electrons came from the hydrogen

The fourth hydrogen is bonded to another hydrogen

The fourth bond is ionic, the other three covalent

The bonds get stronger down the group

Larger molecules have more electrons, so their temporary dipoles — London dispersion forces — are stronger

Heavier molecules fall to the bottom of the container and pack into a solid

Iodine molecules are polar

Butane has fewer carbon atoms

Butane is branched, so it packs badly

The alcohol is heavier

The alcohol's O–H group hydrogen-bonds between molecules; butane has only dispersion forces

A straight chain has more surface contact with its neighbours, so dispersion forces are stronger

Branched molecules have fewer electrons

Straight chains are more polar

The branched isomer has weaker C–C bonds

Oil molecules are too large to fit between ions

Salt is heavier than oil and sinks

Water's polar molecules surround each ion and the ion–dipole attractions repay the energy of breaking up the lattice; oil offers nothing to attract ions

Oil is too thick for ions to move through

Water is too dense for iodine to enter

Iodine reacts with hexane

Hexane molecules are polar

Iodine is non-polar: mixing it into hexane costs no hydrogen bonds, but inserting it into water would break hydrogen bonds without replacing them

Mg²⁺ and O²⁻ carry twice the charge and are smaller, so the electrostatic attraction in the lattice is far stronger

MgO has more ions per formula unit

MgO is a covalent network solid

Oxygen is more electronegative than chlorine

Dissolved ions disrupt the formation of the ice lattice, so the mixture freezes only at a lower temperature

Salt reacts with ice and gives out heat

Salt crystals are sharp and cut the ice

Salt is dark and absorbs sunlight

No: more water molecules dissociate at higher temperature, but H⁺ and OH⁻ rise equally, so it is still neutral — neutral pH is simply below 7 at that temperature

No: the pH meter is wrong at high temperature

Yes, slightly: dissolved CO₂ increases with heat

Yes: heat produces acid in water

It is a concentrated acid

It is dangerous, while weak acids are safe

It is fully dissociated into ions in water, whatever its concentration

It has a very low pH, always below 1

Dissolving a gas in water releases heat, so raising the temperature shifts the equilibrium towards undissolved gas

Carbonic acid decomposes only above room temperature

Warm water is less dense, so bubbles rise faster

Heat breaks the CO₂ molecules apart

It shifts the equilibrium towards the reactants

It doubles the yield but halves the rate

Nothing: it speeds the forward and reverse reactions equally, so equilibrium is reached faster but sits in the same place

It shifts the equilibrium towards the products, giving more product

Bonds must be broken before new ones form, and that initial energy barrier is not crossed at room temperature until a spark supplies it

Oxygen is unreactive unless heated

The reaction is endothermic until it is hot

The spark provides the electrons needed for the reaction

Aluminium's oxide forms a thin, tough, adherent layer that seals the metal; iron's oxide is flaky and lets air and water keep reaching the metal

Aluminium is less reactive than iron

Aluminium is too light to hold water on its surface

Aluminium does not react with oxygen

It is the weighted average of two isotopes: about 75% chlorine-35 and 25% chlorine-37

Chlorine atoms lose half a neutron

Measurement is only accurate to the nearest half unit

The value includes the mass of the electrons

Ozone is heavier and sinks into the lungs

Ozone is less stable: it readily gives up its third atom, which is highly reactive

Ozone is charged

Ozone has more electrons per molecule

Aluminium oxide has a lower melting point

Iron oxide conducts electricity and aluminium oxide does not

Aluminium is more reactive than carbon, so carbon cannot take the oxygen from it; iron is less reactive than carbon, so it can

Carbon reacts with aluminium to form a carbide

Oxygen dissolves in the water and is lost

Each water molecule contains two hydrogen atoms and one oxygen atom, and both gases are diatomic, so 2 H₂O gives 2 H₂ and 1 O₂

Oxygen is produced as O₃

Hydrogen is lighter and escapes more easily

Copper has crystallised out because the solution was supersaturated

Zinc has attracted copper by magnetism

Copper sulfate has decomposed on the zinc surface

Zinc is more reactive than copper, so it gives its electrons to Cu²⁺ ions, which deposit as copper metal while zinc dissolves as Zn²⁺

Hot engines split CO₂ into CO and oxygen

Nitrogen in the air turns CO₂ into CO

Carbon monoxide is what fuel is made of

With too little oxygen, the fuel's carbon cannot all be oxidised to CO₂, so some stops at CO

Sodium chloride is a physical mixture of the two, diluted

The compound is made of Na⁺ and Cl⁻ ions, which have completely different electron arrangements from the atoms

The chlorine is trapped inside the sodium

The two poisons cancel each other out

The 3d subshell fills after 4s, adding ten elements — the first transition series — between calcium and gallium

Elements 21 to 30 were discovered later and inserted

Period 4 atoms have room for two outer shells

The 4s subshell holds ten electrons

They have the same number of protons in the outer shell

They have similar atomic masses

They are all found in seawater

They all have one electron in their outer shell, and the number of outer electrons determines chemical behaviour