Chemical equations — writing, balancing, and interpreting with state symbols

Question

Balance the following equation and add state symbols:

\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2

(CBSE Class 10 — Chemical Reactions and Equations)

Balancing Algorithm

flowchart TD
    A["Write the skeleton equation"] --> B["List atoms on each side"]
    B --> C{"All balanced?"}
    C -->|No| D["Pick the atom with highest difference"]
    D --> E["Adjust coefficient of compound containing it"]
    E --> F["Recount all atoms"]
    F --> C
    C -->|Yes| G["Add state symbols: s, l, g, aq"]
    G --> H["Verify: atoms equal on both sides"]

Solution — Step by Step

\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2

Count atoms on each side:

Atom	Left	Right
Fe	1	3
H	2	2
O	1	4

Fe and O are unbalanced.

We need 3 Fe on the left: put coefficient 3 before Fe.

3\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2

Fe is now balanced (3 = 3).

We need 4 O on the left: put coefficient 4 before H $_2$ O.

3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2

O is balanced (4 = 4). But now H on left = 8, H on right = 2. Put 4 before H $_2$ .

3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2

Atom	Left	Right
Fe	3	3
H	8	8
O	4	4

All balanced. Adding state symbols:

\boxed{3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightarrow \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)}

Steam (g) is used here because iron reacts with steam, not liquid water.

Why This Works

A chemical equation must obey the law of conservation of mass — atoms are neither created nor destroyed in a chemical reaction. Balancing ensures the same number of each type of atom appears on both sides. We only adjust coefficients (the numbers before formulas), never subscripts (which would change the compounds themselves).

State symbols tell us the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water (aqueous). They provide additional information about the reaction conditions.

Alternative Method — Hit and Trial with Fractions

Some students find it easier to use fractions first, then clear them:

Start with Fe $_3$ O $_4$ having 4 oxygens. Put 4 before H $_2$ O. This gives 8 H on left, so put 4 before H $_2$ . Then 3 before Fe.

This “work backwards from the most complex compound” approach often finishes faster.

For CBSE exams, always add state symbols — they carry separate marks. The common state symbols to remember: metals are (s), water is (l), gases like H $_2$ , O $_2$ , CO $_2$ are (g), acids and salts in solution are (aq). Steam is written as H $_2$ O(g), not H $_2$ O(l).

Common Mistake

The biggest error: changing subscripts instead of coefficients. Writing H $_3$ instead of 3H $_2$ changes the molecule itself — H $_3$ does not exist as a stable molecule. We can only place whole numbers in front of the formulas. Another common slip: forgetting to recheck all atoms after adjusting one coefficient. Fixing oxygen might unbalance hydrogen — always do a final count of every element.