Chemical equations

A word equation simply names what goes in and what comes out of a reaction, in plain language:

A chemical (symbolic) equation does the same job but replaces every name with its formula, keeping the arrow exactly where it was:

where: the arrow $\rightarrow$ always points from reactants (left) to products (right) and is read "gives" or "produces" — it is never replaced by an equals sign, because a chemical equation shows a one-way transformation, not a balance. Four conventions govern how the symbols are laid out, and each one is separately tested in ICSE papers.

Plus signs separate substances. A $+$ sign sits between two reactants, or between two products, but never between a reactant and a product — that job belongs to the arrow.

State symbols follow the formula, in brackets. Immediately after each formula, a bracketed letter records its physical state at the moment of reaction: $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for aqueous, meaning dissolved in water. These are not optional decoration — a question that specifies the physical state of every substance is explicitly asking you to include them, and marks are lost for leaving them out or misplacing them.

Conditions ride on the arrow. A condition such as heating is written above (or below) the arrow rather than as part of any formula, most often as the Greek letter $\Delta$: $\overset{\Delta}{\longrightarrow}$. If a word equation says "on heating" or "on burning," that instruction must appear on the arrow, or the equation has silently dropped information the question gave you.

Formulae must already be correct. A chemical equation is only as good as the formulae inside it, so every formula should already have been checked using valencies (the criss-cross method from the previous topic) before it goes anywhere near an arrow. An equation built on a wrong formula is wrong from the first symbol, however tidy the rest of the layout looks.

A chemical equation is a description, not a calculation, so this topic has no formula to derive — the skill is a fixed conversion procedure, and getting it right every time is what the ICSE exam rewards. Converting a word equation into a chemical equation always follows the same three moves in the same order, and skipping or reordering them is the single biggest source of lost marks.

Step 1. Identify every reactant and every product named in the word equation, keeping reactants strictly on the "in" side and products strictly on the "out" side of the described change — this separation is what the arrow will later represent.

Step 2. Write the correct formula for each substance using symbols and valencies, exactly as covered in the previous topic; if a formula is wrong here, the finished equation will be wrong no matter how carefully the remaining steps are done.

Step 3. Assemble the formulae into a single line: place $+$ signs between multiple reactants and between multiple products, draw the arrow from the reactant side to the product side, add any condition (such as $\Delta$ for heating) above the arrow, and finally attach a bracketed state symbol immediately after each formula if the question supplies the physical states. Reading a finished equation back into words is the reverse of this same procedure and is tested just as often — every formula is renamed, the arrow becomes "reacts to give" or "produces," and state symbols are read as "solid," "liquid," "gas," or "in solution."

Problem. Convert the word equation "calcium oxide reacts with water to form calcium hydroxide" into a chemical equation with state symbols, given that calcium oxide is a solid, water is a liquid, and calcium hydroxide is formed as a solid (as in slaked lime).

Step 1. List reactants and product in words: the reactants are calcium oxide and water; the product is calcium hydroxide.

Step 2. Write the correct formula for each. Calcium oxide is $\text{CaO}$; water is $\text{H}_2\text{O}$; calcium hydroxide is $\text{Ca(OH)}_2$, obtained by the criss-cross method with valency of $\text{Ca} = 2$ and valency of the hydroxide group $\text{OH} = 1$.

Step 3. Assemble the skeleton equation, with a $+$ sign joining the two reactants and an arrow pointing to the single product:

Step 4. Attach state symbols: $\text{CaO}$ is a solid, so $(s)$; water is a liquid, so $(l)$; calcium hydroxide forms as a solid, so $(s)$.

Final answer:

Competitive stretch

The same four steps apply even when a reaction needs the diatomic form of an element. "Carbon burns in oxygen to form carbon dioxide gas," with carbon a solid and oxygen a gas, gives reactants carbon ($\text{C}$, occurring as single atoms here) and oxygen (occurring as the diatomic molecule $\text{O}_2$, not a lone $\text{O}$), and product carbon dioxide ($\text{CO}_2$). Assembling and adding states gives $\text{C}(s) + \text{O}_2(g) \longrightarrow \text{CO}_2(g)$ — the diatomic subscript on oxygen is the detail examiners most often check.

• Writing the arrow backwards, or replacing it with an equals sign — a chemical equation always shows direction, reactants to products, never a two-way balance.
• Forgetting that oxygen, hydrogen, nitrogen and chlorine exist as diatomic molecules ($\text{O}_2$, $\text{H}_2$, $\text{N}_2$, $\text{Cl}_2$) in their free, uncombined state, and writing a lone "O" or "H" instead.
• Placing a state symbol before the formula instead of immediately after it, or leaving state symbols off altogether when the question explicitly supplies the physical states.
• Carrying forward an incorrect formula from the "formulae of compounds" step — always re-check every formula against valencies before it goes into the equation.
• Dropping a stated condition such as heating: if the word equation says "on heating" or "on burning," the symbol $\Delta$ must appear above the arrow, or the equation loses information the question gave you.

Ask the tutor: If you are unsure which formula to use for a substance or which state symbol fits a given reaction, ask the AI tutor to walk through the conversion with you step by step.