Salt Melts at a Furnace Temperature and Candle Wax Melts in Your Hand
Learn how ionic and covalent compounds differ in melting point, solubility and conductivity, how to identify which is which from observations, and how to calculate molecular and formula unit mass.
Why does salt need a furnace to melt when wax melts in your hand?
Because melting salt means breaking apart a giant lattice of ions, while melting wax means only separating whole molecules from one another.
In common salt every ion attracts every ion around it, throughout the crystal. To melt it you must overcome that attraction everywhere at once, and that takes a great deal of energy.
In wax, the atoms inside each molecule are held by strong covalent bonds — but the attraction between one molecule and the next is weak. Warming it slightly lets the molecules slide past each other, and not one covalent bond is broken in the process.
So the contrast in melting points is not a contrast in bond strength. It is a contrast between a structure held together throughout and a structure held together only within its separate units. Every physical difference on this page follows from that. It covers the third part of the CBSE Class 9 Science chapter on the atomic foundations of matter.
In common salt every ion attracts every ion around it, throughout the crystal. To melt it you must overcome that attraction everywhere at once, and that takes a great deal of energy.
In wax, the atoms inside each molecule are held by strong covalent bonds — but the attraction between one molecule and the next is weak. Warming it slightly lets the molecules slide past each other, and not one covalent bond is broken in the process.
So the contrast in melting points is not a contrast in bond strength. It is a contrast between a structure held together throughout and a structure held together only within its separate units. Every physical difference on this page follows from that. It covers the third part of the CBSE Class 9 Science chapter on the atomic foundations of matter.
How do ionic and covalent compounds differ in their properties?
In three ways that can all be tested in a school laboratory — melting point, solubility and electrical conductivity.
Ionic compounds:
- High melting and boiling points, because the electrostatic attraction runs through the whole lattice
- Usually soluble in water and insoluble in organic solvents such as kerosene or petrol
- Conduct electricity when molten or dissolved in water, but not as solids
- Hard, brittle, crystalline solids at room temperature
Covalent compounds:
- Low melting and boiling points, because only weak forces act between the molecules
- Usually insoluble in water and soluble in organic solvents
- Do not conduct electricity in any state
- Often gases or liquids, and soft solids when solid
Why the conductivity behaves as it does. Conduction needs charged particles free to move. A solid ionic compound has plenty of charged particles, but every ion is locked in position in the lattice, so nothing can move and nothing conducts. Melt it or dissolve it and the lattice breaks up, the ions become free, and the liquid conducts at once. A covalent compound has no ions at all, so there is nothing to conduct with in any state.
Everyday evidence. Dry salt in a container does not conduct, and a salt solution does — which is why wet hands make an electrical appliance dangerous while dry salt on a shelf is harmless. Sugar, a covalent compound, dissolves in water and the solution still does not conduct, because sugar dissolves as whole molecules and produces no ions.
That sugar case is the one worth remembering. Dissolves in water does not by itself mean ionic. Sugar is a covalent compound that happens to be very soluble in water, and the test that separates it from salt is conductivity, not solubility.
Two covalent exceptions are worth knowing.
- Graphite is covalent and does conduct electricity, because its structure leaves some electrons free to move
- Diamond is covalent and has an extremely high melting point, because it is a giant covalent network — every carbon is bonded to its neighbours throughout the crystal, so there are no separate molecules to pull apart
Both are made of carbon alone. They are exceptions because of their structure, not their bonding — which is precisely the point the opening of this page made: the property follows from whether the attraction runs through the whole solid or only within small units.
Ionic compounds:
- High melting and boiling points, because the electrostatic attraction runs through the whole lattice
- Usually soluble in water and insoluble in organic solvents such as kerosene or petrol
- Conduct electricity when molten or dissolved in water, but not as solids
- Hard, brittle, crystalline solids at room temperature
Covalent compounds:
- Low melting and boiling points, because only weak forces act between the molecules
- Usually insoluble in water and soluble in organic solvents
- Do not conduct electricity in any state
- Often gases or liquids, and soft solids when solid
Why the conductivity behaves as it does. Conduction needs charged particles free to move. A solid ionic compound has plenty of charged particles, but every ion is locked in position in the lattice, so nothing can move and nothing conducts. Melt it or dissolve it and the lattice breaks up, the ions become free, and the liquid conducts at once. A covalent compound has no ions at all, so there is nothing to conduct with in any state.
Everyday evidence. Dry salt in a container does not conduct, and a salt solution does — which is why wet hands make an electrical appliance dangerous while dry salt on a shelf is harmless. Sugar, a covalent compound, dissolves in water and the solution still does not conduct, because sugar dissolves as whole molecules and produces no ions.
That sugar case is the one worth remembering. Dissolves in water does not by itself mean ionic. Sugar is a covalent compound that happens to be very soluble in water, and the test that separates it from salt is conductivity, not solubility.
Two covalent exceptions are worth knowing.
- Graphite is covalent and does conduct electricity, because its structure leaves some electrons free to move
- Diamond is covalent and has an extremely high melting point, because it is a giant covalent network — every carbon is bonded to its neighbours throughout the crystal, so there are no separate molecules to pull apart
Both are made of carbon alone. They are exceptions because of their structure, not their bonding — which is precisely the point the opening of this page made: the property follows from whether the attraction runs through the whole solid or only within small units.
How do you tell from observations whether a compound is ionic or covalent?
Test the conductivity of the molten or dissolved form. That single test is the most reliable.
Work through the evidence in this order:
- Does the molten or aqueous form conduct electricity? If yes, ionic. If no, covalent.
- Is the melting point high (hundreds of degrees) or low? High suggests ionic; low suggests covalent.
- Is it soluble in water or in an organic solvent? Water suggests ionic; kerosene or petrol suggests covalent.
- Is it a hard brittle crystal, or a gas, liquid or soft solid? Crystalline suggests ionic.
Worked identification 1. A white solid melts only at a very high temperature, dissolves readily in water, and its solution conducts electricity. Every observation points the same way: ionic.
Worked identification 2. A substance is a gas at room temperature and does not conduct electricity. Low boiling point and no conduction: covalent.
Worked identification 3 — the decisive one. A solid does not conduct electricity, but when melted it conducts well. This is ionic, and it is the single clearest diagnosis available. No covalent compound behaves this way, because melting a covalent solid produces molecules, not ions.
Worked identification 4. A substance dissolves easily in kerosene but not in water, and melts at a low temperature. Covalent.
Worked identification 5 — the trap. A white crystalline solid dissolves in water, and the solution does not conduct electricity. Despite looking like salt and dissolving like salt, it is covalent — sugar behaves exactly like this. Conductivity overrules appearance and solubility.
Testing the solid alone can mislead. Both a solid ionic compound and a covalent compound fail to conduct, so a conductivity test on the solid tells you nothing at all. The test must be done on the molten or dissolved form, and a question describing a solid that does not conduct is often setting exactly that trap.
Use the whole set of observations, not one. Graphite would be misidentified as ionic by the conductivity test alone, and diamond by the melting point test alone. Two or three agreeing observations give a confident answer where one may not — which is the same reasoning that applied to identifying a mixture in the earlier chapter of this course.
Work through the evidence in this order:
- Does the molten or aqueous form conduct electricity? If yes, ionic. If no, covalent.
- Is the melting point high (hundreds of degrees) or low? High suggests ionic; low suggests covalent.
- Is it soluble in water or in an organic solvent? Water suggests ionic; kerosene or petrol suggests covalent.
- Is it a hard brittle crystal, or a gas, liquid or soft solid? Crystalline suggests ionic.
Worked identification 1. A white solid melts only at a very high temperature, dissolves readily in water, and its solution conducts electricity. Every observation points the same way: ionic.
Worked identification 2. A substance is a gas at room temperature and does not conduct electricity. Low boiling point and no conduction: covalent.
Worked identification 3 — the decisive one. A solid does not conduct electricity, but when melted it conducts well. This is ionic, and it is the single clearest diagnosis available. No covalent compound behaves this way, because melting a covalent solid produces molecules, not ions.
Worked identification 4. A substance dissolves easily in kerosene but not in water, and melts at a low temperature. Covalent.
Worked identification 5 — the trap. A white crystalline solid dissolves in water, and the solution does not conduct electricity. Despite looking like salt and dissolving like salt, it is covalent — sugar behaves exactly like this. Conductivity overrules appearance and solubility.
Testing the solid alone can mislead. Both a solid ionic compound and a covalent compound fail to conduct, so a conductivity test on the solid tells you nothing at all. The test must be done on the molten or dissolved form, and a question describing a solid that does not conduct is often setting exactly that trap.
Use the whole set of observations, not one. Graphite would be misidentified as ionic by the conductivity test alone, and diamond by the melting point test alone. Two or three agreeing observations give a confident answer where one may not — which is the same reasoning that applied to identifying a mixture in the earlier chapter of this course.
Formula
How do you calculate the molecular mass of a covalent compound?
Add the atomic masses of every atom in the molecule.
The unit is the atomic mass unit, written u.
The atomic masses used throughout: H , C , N , O , Na , Mg , Al , S , Cl , K , Ca .
**Worked example 1 — water, .**
**Worked example 2 — carbon dioxide, .**
Worked example 3 — methane and ammonia.
Worked example 4 — the diatomic gases.
**Worked example 5 — sulphuric acid, .**
**Worked example 6 — nitric acid, .**
**Worked example 7 — ethanoic acid, .** Count the atoms first: carbons, hydrogens, oxygens.
**Worked example 8 — glucose, .**
Count the atoms before multiplying anything. In the carbons and oxygens appear in two separate places in the formula, and the commonest error is to total only the first group. Writing out *C: , H: , O: * on its own line takes five seconds and removes the risk.
A subscript multiplies only the atom immediately before it. In the applies to hydrogen alone and the to oxygen alone — the sulphur has no subscript, so there is exactly one.
The unit is the atomic mass unit, written u.
The atomic masses used throughout: H , C , N , O , Na , Mg , Al , S , Cl , K , Ca .
**Worked example 1 — water, .**
**Worked example 2 — carbon dioxide, .**
Worked example 3 — methane and ammonia.
Worked example 4 — the diatomic gases.
**Worked example 5 — sulphuric acid, .**
**Worked example 6 — nitric acid, .**
**Worked example 7 — ethanoic acid, .** Count the atoms first: carbons, hydrogens, oxygens.
**Worked example 8 — glucose, .**
Count the atoms before multiplying anything. In the carbons and oxygens appear in two separate places in the formula, and the commonest error is to total only the first group. Writing out *C: , H: , O: * on its own line takes five seconds and removes the risk.
A subscript multiplies only the atom immediately before it. In the applies to hydrogen alone and the to oxygen alone — the sulphur has no subscript, so there is exactly one.
Why is it called formula unit mass for an ionic compound?
Because an ionic compound has no molecules — so there is nothing whose molecular mass could be quoted.
As the previous part of this chapter explained, an ionic solid is a giant lattice of ions, not a collection of separate particles. The formula states the ratio of ions and nothing more. The mass calculated from that formula is therefore called the formula unit mass, and the arithmetic is identical to a molecular mass calculation.
**Worked example 1 — sodium chloride, .**
Notice the half. The value is not a whole number because chlorine's atomic mass is u, which is itself the average over chlorine's two isotopes — exactly as the previous chapter of this course established.
**Worked example 2 — calcium carbonate, .**
**Worked example 3 — magnesium chloride, .**
**Worked example 4 — sodium carbonate, .**
**Worked example 5 — calcium hydroxide, . The bracket's subscript multiplies everything inside**, so there are two oxygens and two hydrogens:
Worked example 6 — sodium hydroxide and calcium oxide.
**Worked example 7 — potassium sulphate, .**
**Worked example 8 — aluminium sulphate, .** Deal with the bracket first: one sulphate is , and there are three of them.
**Worked example 9 — ammonium sulphate, .** One ammonium is , and there are two:
A bracket's subscript multiplies every atom inside it. contains two oxygens and two hydrogens, giving u. Treating the as applying to the hydrogen alone gives u, which is wrong — and it is the same bracket error that spoils the formula itself, now spoiling the mass as well.
Work the bracket out as a single number first. For , finding on its own line and then multiplying by is far safer than trying to keep nine atoms in your head at once. It is the same habit the formula-writing section recommended, applied to the arithmetic.
As the previous part of this chapter explained, an ionic solid is a giant lattice of ions, not a collection of separate particles. The formula states the ratio of ions and nothing more. The mass calculated from that formula is therefore called the formula unit mass, and the arithmetic is identical to a molecular mass calculation.
**Worked example 1 — sodium chloride, .**
Notice the half. The value is not a whole number because chlorine's atomic mass is u, which is itself the average over chlorine's two isotopes — exactly as the previous chapter of this course established.
**Worked example 2 — calcium carbonate, .**
**Worked example 3 — magnesium chloride, .**
**Worked example 4 — sodium carbonate, .**
**Worked example 5 — calcium hydroxide, . The bracket's subscript multiplies everything inside**, so there are two oxygens and two hydrogens:
Worked example 6 — sodium hydroxide and calcium oxide.
**Worked example 7 — potassium sulphate, .**
**Worked example 8 — aluminium sulphate, .** Deal with the bracket first: one sulphate is , and there are three of them.
**Worked example 9 — ammonium sulphate, .** One ammonium is , and there are two:
A bracket's subscript multiplies every atom inside it. contains two oxygens and two hydrogens, giving u. Treating the as applying to the hydrogen alone gives u, which is wrong — and it is the same bracket error that spoils the formula itself, now spoiling the mass as well.
Work the bracket out as a single number first. For , finding on its own line and then multiplying by is far safer than trying to keep nine atoms in your head at once. It is the same habit the formula-writing section recommended, applied to the arithmetic.
Exam tip
Exam tip: test the molten form, and resolve the bracket first
The decisive test is conductivity of the molten or dissolved form. A solid that does not conduct but conducts when melted is ionic. Testing the solid alone tells you nothing, because neither type conducts as a solid.
Solubility in water does not prove ionic. Sugar is covalent, dissolves readily, and its solution does not conduct.
Remember the pattern: ionic means high melting point, water-soluble, conducts when molten or dissolved, crystalline. Covalent means low melting point, organic-solvent-soluble, non-conducting.
Say why ionic compounds conduct only when molten or dissolved — the ions must be free to move, and in the solid they are locked in the lattice.
Name the two exceptions: graphite conducts, and diamond has a very high melting point, both because of structure.
For a molecular mass, count the atoms on a separate line first — is C , H , O , giving u.
A subscript multiplies only the atom before it; a bracket's subscript multiplies everything inside. is u, not u.
Resolve a bracket to a single number first: , so u.
Use formula unit mass for ionic compounds and molecular mass for covalent ones, and say why when asked.
And write u on every answer.
Solubility in water does not prove ionic. Sugar is covalent, dissolves readily, and its solution does not conduct.
Remember the pattern: ionic means high melting point, water-soluble, conducts when molten or dissolved, crystalline. Covalent means low melting point, organic-solvent-soluble, non-conducting.
Say why ionic compounds conduct only when molten or dissolved — the ions must be free to move, and in the solid they are locked in the lattice.
Name the two exceptions: graphite conducts, and diamond has a very high melting point, both because of structure.
For a molecular mass, count the atoms on a separate line first — is C , H , O , giving u.
A subscript multiplies only the atom before it; a bracket's subscript multiplies everything inside. is u, not u.
Resolve a bracket to a single number first: , so u.
Use formula unit mass for ionic compounds and molecular mass for covalent ones, and say why when asked.
And write u on every answer.
Did you know
Why the sea conducts electricity and rainwater barely does
Pure water is a surprisingly poor conductor of electricity. Sea water is a good one. The difference is not the water — it is what is dissolved in it.
Water molecules are held together by covalent bonds and carry no charge, so pure water has almost nothing free to move and conducts very little. Dissolve salt in it and the lattice breaks up into and ions, free to drift through the liquid. Charged particles free to move are exactly what conduction requires, and the solution conducts well.
Rainwater has fallen straight from the sky and has dissolved very little, so it conducts poorly. Water that has run over rocks and through soil has picked up dissolved salts along the way, which is why river and well water conduct better. Sea water has the most dissolved salt of all.
The same reasoning explains a familiar safety warning. Dry hands on a switch are reasonably safe; damp hands are not. The danger is not water as a substance but the dissolved salts in it — sweat included — turning a poor conductor into a good one across your skin.
It also explains why a doctor's saline drip and the fluid inside every one of your cells are salt solutions. Nerve impulses depend on ions moving across membranes, so an organism needs dissolved ions to work at all, and water is the solvent that supplies them.
So the conductivity test taught on this page is not merely a way of labelling compounds in a laboratory. It is the same physics that makes a monsoon puddle near a live wire dangerous, and the same chemistry that makes a nerve signal possible.
Water molecules are held together by covalent bonds and carry no charge, so pure water has almost nothing free to move and conducts very little. Dissolve salt in it and the lattice breaks up into and ions, free to drift through the liquid. Charged particles free to move are exactly what conduction requires, and the solution conducts well.
Rainwater has fallen straight from the sky and has dissolved very little, so it conducts poorly. Water that has run over rocks and through soil has picked up dissolved salts along the way, which is why river and well water conduct better. Sea water has the most dissolved salt of all.
The same reasoning explains a familiar safety warning. Dry hands on a switch are reasonably safe; damp hands are not. The danger is not water as a substance but the dissolved salts in it — sweat included — turning a poor conductor into a good one across your skin.
It also explains why a doctor's saline drip and the fluid inside every one of your cells are salt solutions. Nerve impulses depend on ions moving across membranes, so an organism needs dissolved ions to work at all, and water is the solvent that supplies them.
So the conductivity test taught on this page is not merely a way of labelling compounds in a laboratory. It is the same physics that makes a monsoon puddle near a live wire dangerous, and the same chemistry that makes a nerve signal possible.
Exam relevance
How do ionic and covalent properties feed into JEE and NEET Chemistry?
This page is the foundation for two Class 11 chapters and one arithmetic skill used in every chemistry calculation for the next three years.
Chemical Bonding and Molecular Structure in Class 11, examined in both JEE Main and NEET, explains the property comparisons made here. Lattice energy quantifies why an ionic melting point is high, Fajans' rules describe compounds that are partly ionic and partly covalent, and the giant covalent network of diamond becomes a formal structural type alongside molecular solids. The Class 9 insight that the property follows from whether the attraction runs through the whole solid is what that chapter makes quantitative.
The mass calculation feeds directly into Class 11 Some Basic Concepts of Chemistry, where the molecular mass becomes the molar mass in grams per mole and every stoichiometry, concentration and gas-law calculation begins with it. An error in a formula unit mass propagates through an entire numerical answer, which is why the bracket habit matters far beyond this page.
Conductivity leads into Class 12 Electrochemistry, where the conduction of molten and aqueous ionic compounds becomes electrolysis, with molar conductivity and Kohlrausch's law. The Class 9 statement that ions must be free to move is the whole basis of that chapter.
Solubility leads into Class 12 Solutions, and the like dissolves like pattern noticed here — ionic in water, covalent in organic solvents — is explained there by polarity.
What the questions look like. Numericals on molecular and formula unit mass are quick and frequent, and the versions that catch students out involve brackets or a formula written in condensed organic form such as . Assertion-reason items favour the statement that ionic solids do not conduct while their melts do, and the statement that graphite conducts although it is covalent. Match-the-column questions pair a compound with a property, and property-to-identity questions give three observations and ask which bond type they indicate.
How board and competitive emphasis differ. A board paper asks you to give three differences between ionic and covalent compounds and to compute two or three masses. A competitive paper embeds the mass inside a mole or concentration calculation, and asks the property question as a comparison — which of these four has the highest melting point — so ranking matters more than listing.
The single trap that costs the most marks. Mishandling a bracket in a mass calculation. is u and not u, and is u. Resolve the bracket to one number before multiplying, every time.
A second trap worth naming. Concluding ionic from solubility in water. Sugar dissolves beautifully and is covalent, and its solution does not conduct. The conductivity of the molten or aqueous form is the test that decides, and a question that mentions dissolving without mentioning conduction has usually left out the decisive evidence on purpose.
Chemical Bonding and Molecular Structure in Class 11, examined in both JEE Main and NEET, explains the property comparisons made here. Lattice energy quantifies why an ionic melting point is high, Fajans' rules describe compounds that are partly ionic and partly covalent, and the giant covalent network of diamond becomes a formal structural type alongside molecular solids. The Class 9 insight that the property follows from whether the attraction runs through the whole solid is what that chapter makes quantitative.
The mass calculation feeds directly into Class 11 Some Basic Concepts of Chemistry, where the molecular mass becomes the molar mass in grams per mole and every stoichiometry, concentration and gas-law calculation begins with it. An error in a formula unit mass propagates through an entire numerical answer, which is why the bracket habit matters far beyond this page.
Conductivity leads into Class 12 Electrochemistry, where the conduction of molten and aqueous ionic compounds becomes electrolysis, with molar conductivity and Kohlrausch's law. The Class 9 statement that ions must be free to move is the whole basis of that chapter.
Solubility leads into Class 12 Solutions, and the like dissolves like pattern noticed here — ionic in water, covalent in organic solvents — is explained there by polarity.
What the questions look like. Numericals on molecular and formula unit mass are quick and frequent, and the versions that catch students out involve brackets or a formula written in condensed organic form such as . Assertion-reason items favour the statement that ionic solids do not conduct while their melts do, and the statement that graphite conducts although it is covalent. Match-the-column questions pair a compound with a property, and property-to-identity questions give three observations and ask which bond type they indicate.
How board and competitive emphasis differ. A board paper asks you to give three differences between ionic and covalent compounds and to compute two or three masses. A competitive paper embeds the mass inside a mole or concentration calculation, and asks the property question as a comparison — which of these four has the highest melting point — so ranking matters more than listing.
The single trap that costs the most marks. Mishandling a bracket in a mass calculation. is u and not u, and is u. Resolve the bracket to one number before multiplying, every time.
A second trap worth naming. Concluding ionic from solubility in water. Sugar dissolves beautifully and is covalent, and its solution does not conduct. The conductivity of the molten or aqueous form is the test that decides, and a question that mentions dissolving without mentioning conduction has usually left out the decisive evidence on purpose.
Key takeaways
Ionic and covalent compounds and their masses: quick revision
- Ionic: high melting point, usually water-soluble, conducts when molten or dissolved but not as a solid, hard brittle crystals.
- Covalent: low melting point, usually organic-solvent-soluble, non-conducting in every state, often gases, liquids or soft solids.
- Melting an ionic solid breaks a giant lattice; melting a covalent solid only separates whole molecules, leaving the bonds inside them intact.
- Conduction needs charged particles free to move. In a solid ionic compound the ions are locked in place; melting or dissolving frees them.
- Sugar dissolves in water and does not conduct — solubility alone never proves ionic.
- Exceptions from structure: graphite conducts because some electrons are free; diamond has a very high melting point because it is a giant covalent network.
- Identify by the molten or aqueous conductivity test — a solid that does not conduct but conducts when melted is ionic. Testing the solid alone decides nothing.
- Molecular mass sum of the atomic masses of all atoms, in u. Use H , C , N , O , Na , Mg , Al , S , Cl , K , Ca .
- , , , , , u.
- u; u.
- is C , H , O , giving u. u.
- Count the atoms on a separate line first, and note that a subscript multiplies only the atom before it.
- Formula unit mass is used for ionic compounds, because they have no molecules — the formula gives only the ratio of ions.
- u, the half coming from chlorine's average atomic mass.
- , , , , , u.
- A bracket's subscript multiplies everything inside: u, not u.
- u and u — resolve the bracket first.
Work out the formula unit mass of three compounds containing brackets, then check each by counting every atom individually — if the two routes agree, brackets will never cost you a mark.
- Covalent: low melting point, usually organic-solvent-soluble, non-conducting in every state, often gases, liquids or soft solids.
- Melting an ionic solid breaks a giant lattice; melting a covalent solid only separates whole molecules, leaving the bonds inside them intact.
- Conduction needs charged particles free to move. In a solid ionic compound the ions are locked in place; melting or dissolving frees them.
- Sugar dissolves in water and does not conduct — solubility alone never proves ionic.
- Exceptions from structure: graphite conducts because some electrons are free; diamond has a very high melting point because it is a giant covalent network.
- Identify by the molten or aqueous conductivity test — a solid that does not conduct but conducts when melted is ionic. Testing the solid alone decides nothing.
- Molecular mass sum of the atomic masses of all atoms, in u. Use H , C , N , O , Na , Mg , Al , S , Cl , K , Ca .
- , , , , , u.
- u; u.
- is C , H , O , giving u. u.
- Count the atoms on a separate line first, and note that a subscript multiplies only the atom before it.
- Formula unit mass is used for ionic compounds, because they have no molecules — the formula gives only the ratio of ions.
- u, the half coming from chlorine's average atomic mass.
- , , , , , u.
- A bracket's subscript multiplies everything inside: u, not u.
- u and u — resolve the bracket first.
Work out the formula unit mass of three compounds containing brackets, then check each by counting every atom individually — if the two routes agree, brackets will never cost you a mark.