Chemical Reactions

Lessons

Senior 1 · Chemistry

Chemical Reactions

5 lessons

Lesson 3 of 5

Chemical Equations with State Symbols

22 min

From words to symbols

A word equation tells you what reacts, but not how much, and it only makes sense in one language. A chemist in Kigali, one in Nairobi and one in Beijing may not share a language, but they all read

$\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}$

the same way: solid carbon reacts with oxygen gas to form carbon dioxide gas. This is a chemical equation. It replaces each name with a formula and adds a state symbol to show whether each substance is a solid, a liquid, a gas or a solution.

Definition

Chemical equation: a summary of a reaction using the formulae of the reactants and products. A complete chemical equation is balanced and shows state symbols. Example: $\mathrm{Fe(s) + S(s) \rightarrow FeS(s)}$ .

Step 1: the right formula for every substance

Metals, carbon, silicon and sulfur are written as single symbols: $\mathrm{Fe}$ , $\mathrm{Mg}$ , $\mathrm{C}$ , $\mathrm{S}$ .
Seven elements exist as diatomic molecules and are always written with a 2 (Table 3.1).
The noble gases are single atoms: $\mathrm{He}$ , $\mathrm{Ne}$ , $\mathrm{Ar}$ .
Compounds are written from their valencies, as you learned in Unit 5, or from formulae you know by heart: $\mathrm{H_2O}$ , $\mathrm{CO_2}$ , $\mathrm{NH_3}$ , $\mathrm{CH_4}$ , $\mathrm{HCl}$ , $\mathrm{H_2SO_4}$ .

Table 3.1: The seven diatomic elements

Element	Formula	State at room temperature
Hydrogen	$\mathrm{H_2}$	gas
Nitrogen	$\mathrm{N_2}$	gas
Oxygen	$\mathrm{O_2}$	gas
Fluorine	$\mathrm{F_2}$	gas
Chlorine	$\mathrm{Cl_2}$	gas
Bromine	$\mathrm{Br_2}$	liquid
Iodine	$\mathrm{I_2}$	solid

Step 2: the state symbols

Definition

State symbol: a letter or letters in brackets after a formula, showing the physical state of that substance in the reaction: (s) solid, (l) liquid, (g) gas, (aq) aqueous, meaning dissolved in water. Example: $\mathrm{NaCl(aq)}$ is salt dissolved in water.

Figure 3.1: What each state symbol means at the level of particles.

Table 3.2: The four state symbols

Symbol	Meaning	Examples
(s)	solid	$\mathrm{Zn(s)}$ , $\mathrm{CaCO_3(s)}$ , $\mathrm{C(s)}$
(l)	liquid (a pure liquid, not a solution)	$\mathrm{H_2O(l)}$ , $\mathrm{C_2H_5OH(l)}$ , $\mathrm{Br_2(l)}$
(g)	gas	$\mathrm{O_2(g)}$ , $\mathrm{CO_2(g)}$ , $\mathrm{H_2(g)}$
(aq)	aqueous: dissolved in water	$\mathrm{HCl(aq)}$ , $\mathrm{NaOH(aq)}$ , $\mathrm{ZnSO_4(aq)}$

Worked example

Problem. Write a chemical equation, with state symbols, for zinc granules reacting with dilute sulfuric acid to form zinc sulfate solution and hydrogen gas.

Solution. Word equation: zinc + sulfuric acid → zinc sulfate + hydrogen.

Zinc is a metal: $\mathrm{Zn}$ , a solid.
Dilute sulfuric acid is dissolved in water: $\mathrm{H_2SO_4(aq)}$ .
Zinc sulfate: Zn valency 2, sulfate valency 2, so $\mathrm{ZnSO_4}$ ; it stays dissolved: (aq).
Hydrogen is diatomic: $\mathrm{H_2(g)}$ .

$\mathrm{Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)}$

Check the atoms: left Zn 1, H 2, S 1, O 4; right Zn 1, S 1, O 4, H 2. The equation is already balanced.

Worked example

Problem. Write chemical equations with state symbols for (a) heating limestone in a kiln, (b) neutralising sodium hydroxide solution with hydrochloric acid.

Solution. (a) calcium carbonate → calcium oxide + carbon dioxide. Limestone and quicklime are solids; carbon dioxide is a gas:

$\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}$

(b) sodium hydroxide + hydrochloric acid → sodium chloride + water. The acid, the alkali and the salt are all in solution, but the water formed is the solvent itself, a pure liquid:

$\mathrm{NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)}$

Both equations are balanced: count the atoms of each element on each side.

Worked example

Problem. Hydrogen burns in oxygen to form water. Write the formulae with state symbols and check whether the equation is balanced.

Solution. Hydrogen and oxygen are both diatomic gases; the water forms as a liquid when it cools:

$\mathrm{H_2(g) + O_2(g) \rightarrow H_2O(l)} \qquad \text{(not yet balanced)}$

Atom	Left	Right
H	2	2
O	2	1

One oxygen atom seems to have vanished, which is impossible. This equation is a skeleton: the formulae are correct but the numbers are not yet balanced. You will balance it in Lesson 4.

Warning

(l) or (aq)? Use (l) only for a pure liquid, such as water or ethanol. A substance dissolved in water is (aq), even if it is a liquid before it dissolves. Hydrochloric acid is hydrogen chloride dissolved in water, so it is $\mathrm{HCl(aq)}$ .

Tip

To remember the seven diatomic elements, start at element 7 (nitrogen) in the periodic table: N, O, F go across, then Cl, Br, I go down, and hydrogen stands alone. Together they make a "7".

FuseSchool: State Symbols in Chemical Equations (4 min). Watch for the difference between a liquid (l) and an aqueous solution (aq).

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Practice

What do the state symbols (s), (l), (g) and (aq) mean?
Write the formulae of: (a) nitrogen gas, (b) chlorine gas, (c) solid sulfur, (d) liquid water, (e) argon.
Add state symbols: $\mathrm{Mg + 2HCl \rightarrow MgCl_2 + H_2}$ (magnesium ribbon in dilute hydrochloric acid).
Write a chemical equation with state symbols for iron filings heated with powdered sulfur to form solid iron(II) sulfide.
Write a chemical equation with state symbols for carbon dioxide bubbled through limewater, $\mathrm{Ca(OH)_2(aq)}$ , forming a white precipitate of calcium carbonate and water.
Explain why sugar dissolved in tea is written (aq), but ethanol in a bottle is written (l).
Nshuti writes the burning of sulfur as $\mathrm{S(s) + O(g) \rightarrow SO_2(g)}$ . Find his mistake and correct the equation.

Show answersHide answers

(s) solid; (l) pure liquid; (g) gas; (aq) aqueous, dissolved in water.
(a) $\mathrm{N_2(g)}$ (b) $\mathrm{Cl_2(g)}$ (c) $\mathrm{S(s)}$ (d) $\mathrm{H_2O(l)}$ (e) $\mathrm{Ar(g)}$ .
$\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)}$
$\mathrm{Fe(s) + S(s) \rightarrow FeS(s)}$
$\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)}$
The sugar is dissolved in the water of the tea, so it is aqueous. Ethanol in a bottle is a pure liquid on its own, so it is (l).
Oxygen gas is diatomic, $\mathrm{O_2}$ , not $\mathrm{O}$ : $\mathrm{S(s) + O_2(g) \rightarrow SO_2(g)}$ .

Sources and credits

Physical Sciences Grade 10, 14.1: Representing chemical change (writing chemical formulae for reactants and products, and state symbols), Siyavula Education and volunteers, Siyavula, licensed under CC BY 3.0. Adapted by Ganzaa.
Video: State Symbols in Chemical Equations, FuseSchool, embedded from YouTube.
The particle diagram, tables, worked examples, Practice items and exercises were written and drawn by Ganzaa.

1From words to symbols

2Step 1: the right formula for every substance

3Step 2: the state symbols

From words to symbols

Step 1: the right formula for every substance

Step 2: the state symbols