What is wrong state symbol for a dissolved substance?

A substance dissolved in water is (aq), not (l).

What are not simplifying coefficients?

Reduce to the smallest whole-number ratio (for example divide all by 2 if possible).

Write the formula of magnesium nitrate, given Mg^2+ and NO_3^-. [2 marks]

Balance: Fe + O_2 → Fe_2O_3. [2 marks]

SingaporeChemistrySyllabus dot point

How are chemical formulae built from ionic charges, and how is a chemical equation balanced?

Write formulae of ionic compounds from the charges on their ions, construct and balance chemical equations, and add state symbols

A focused answer to the O-Level Chemistry outcome on formulae and equations. Building ionic formulae by balancing charges, balancing chemical equations atom by atom, and adding the correct state symbols.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to write the formula of an ionic compound by balancing the charges on its ions, construct and balance chemical equations so that atoms are conserved, and add the correct state symbols. These are the writing skills of chemistry: every calculation, every reaction and every exam answer depends on a correct formula and a balanced equation.

The answer

Writing ionic formulae from charges

An ionic compound is electrically neutral, so the total positive charge must equal the total negative charge. To write the formula:

Write the symbol of each ion with its charge (for example $\text{Ca}^{2+}$ and $\text{Cl}^-$ ).
Find how many of each ion are needed so the charges cancel. Here one $\text{Ca}^{2+}$ ( $+2$ ) needs two $\text{Cl}^-$ ( $-2$ total).
Write the numbers as subscripts: $\text{CaCl}_2$ .

A quick method is to swap the charge numbers as subscripts (the "cross-over" rule), then simplify to the smallest whole-number ratio. Compound ions such as sulfate ( $\text{SO}_4^{2-}$ ), nitrate ( $\text{NO}_3^-$ ) and carbonate ( $\text{CO}_3^{2-}$ ) keep their group together; if you need more than one, put it in brackets, as in calcium nitrate $\text{Ca(NO}_3)_2$ .

Balancing chemical equations

A chemical equation must have the same number of atoms of each element on both sides, because atoms are not created or destroyed (conservation of mass). You balance by putting whole-number coefficients in front of the formulae. You may never change a formula (the subscripts) to balance, only the big numbers in front.

A reliable order is:

Write the correct formulae of all reactants and products (do not change these).
Balance the metals first, then the non-metals, leaving oxygen and hydrogen toward the end.
Balance oxygen and hydrogen last, adjusting coefficients.
Check every element, and simplify the coefficients to the smallest whole-number ratio.

State symbols

State symbols are added in brackets after each formula to show the physical state:

$(s)$ solid
$(l)$ liquid
$(g)$ gas
$(aq)$ aqueous (dissolved in water)

For example, the neutralisation of hydrochloric acid by sodium hydroxide is written $\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$ .

Worked example

Write a balanced equation, with state symbols, for the reaction of solid magnesium with dilute hydrochloric acid to give aqueous magnesium chloride and hydrogen gas.

Step 1: Write the correct formulae

Magnesium is $\text{Mg}$ . Hydrochloric acid is $\text{HCl}$ . Magnesium chloride: $\text{Mg}^{2+}$ with two $\text{Cl}^-$ gives $\text{MgCl}_2$ . Hydrogen gas is $\text{H}_2$ .

Unbalanced: $\text{Mg} + \text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$ .

Step 2: Balance chlorine and hydrogen

The right side has two chlorine atoms, so put $2\text{HCl}$ on the left. This also gives two hydrogen atoms on each side.

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$ .

Step 3: Check every element

Magnesium: $1 = 1$ . Hydrogen: $2 = 2$ . Chlorine: $2 = 2$ . Balanced.

Step 4: Add state symbols

\text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g)

Examples in context

Example 1. Combustion equations. Burning a fuel such as propane is balanced by doing carbon first, then hydrogen, then oxygen, giving $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$ . The order (carbon, hydrogen, oxygen last) makes balancing organic combustion straightforward and recurs throughout the organic topic.

Example 2. Ionic equations in qualitative analysis. When silver nitrate solution forms a precipitate with chloride ions, the ionic equation $\text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s)$ shows just the ions that react, with state symbols making clear that a solid forms from two dissolved ions. Correct formulae and state symbols are essential here.

Try this

Q1. Write the formula of magnesium nitrate, given $\text{Mg}^{2+}$ and $\text{NO}_3^-$ . [2 marks]

Cue. One $\text{Mg}^{2+}$ needs two $\text{NO}_3^-$ , so the formula is $\text{Mg(NO}_3)_2$ (brackets around the nitrate).

Q2. Balance: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$ . [2 marks]

Cue. $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$ (iron $4 = 4$ , oxygen $6 = 6$ ).

Q3. State what state symbol is used for a substance dissolved in water, and give an example. [1 mark]

Cue. $(aq)$ , aqueous; for example $\text{NaCl}(aq)$ for salt dissolved in water.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marks(a) Write the formula of aluminium oxide, given the ions

\text{Al}^{3+}

and

\text{O}^{2-}

. (b) Balance the equation for the reaction of aluminium with oxygen:

\text{Al} + \text{O}_2 \rightarrow \text{Al}_2\text{O}_3

. (c) Add state symbols, given all species are solid except oxygen.

Show worked answer →

(a) Balance the charges: the lowest common multiple of $3$ and $2$ is $6$ , needing two $\text{Al}^{3+}$ ( $+6$ ) and three $\text{O}^{2-}$ ( $-6$ ). The formula is $\text{Al}_2\text{O}_3$ .

(b) Balance: $4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3$ . Check: aluminium $4 = 2 \times 2$ ; oxygen $3 \times 2 = 6 = 2 \times 3$ .

Markers reward the formula from charge balancing, a correctly balanced equation with whole-number coefficients, and the right state symbols.

Original3 marksBalance the following equation and explain what balancing achieves:

\text{C}_3\text{H}_8 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}

(the combustion of propane).

Show worked answer →

Balance carbon first ( $3\text{CO}_2$ ), then hydrogen ( $4\text{H}_2\text{O}$ ), then oxygen (right-hand side has $6 + 4 = 10$ oxygen atoms, so $5\text{O}_2$ on the left):

$\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$ .

Balancing ensures the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (conservation of mass).

Markers reward the correct coefficients and the statement that balancing conserves atoms (mass) because none are created or destroyed.