Chemical Equations

Strike a matchstick and something new appears — heat, light, ash — that was not there a moment ago. Chemistry has a compact way of recording that change: a chemical equation. It names the substances that go in (reactants), the substances that come out (products), and — crucially — insists that nothing is created or destroyed along the way. Every atom present at the start must still be present at the end, just rearranged into new combinations. This article stays with the opening idea of CBSE Class 10's "Chemical Reactions and Equations" chapter: spotting that a reaction has happened, writing it first in words and then in symbols, and balancing that symbol equation atom by atom using the Law of Conservation of Mass. No moles, no grams — just careful counting and state symbols. Two fully worked balances anchor the method, and the traps section flags the mistakes that cost the most marks.

The core insight

A chemical equation is balanced when trial-and-error coefficients (never subscripts) make the atom count of every element identical on both sides, because the Law of Conservation of Mass says matter can neither appear nor vanish in a reaction.

The core idea

A chemical reaction converts reactants into products with genuinely different properties — a new colour, a gas fizzing out, a solid precipitating, heat given out or absorbed, or a change of state are the tell-tale signs that something chemical (not just physical) has occurred. Melting ice or dissolving sugar in water changes appearance but not identity, so neither counts.

From words to symbols. A word equation simply names what happens, e.g. "magnesium plus oxygen gives magnesium oxide." A symbol equation replaces those names with chemical formulas, e.g. Mg+O2→MgO\text{Mg} + \text{O}_2 \rightarrow \text{MgO}, and every finished symbol equation also carries state symbols: (s)(s) solid, (l)(l) liquid, (g)(g) gas, (aq)(aq) dissolved in water. Special conditions — heat, light, a catalyst — are written above or below the arrow rather than as part of the formulas themselves.

The governing law. The reason an equation must be balanced at all is the Law of Conservation of Mass:

total mass of reactants=total mass of products.\text{total mass of reactants} = \text{total mass of products}.

where: the reactants are everything written to the left of the arrow and the products everything to the right, both including their state symbols. Because atoms themselves are neither created nor destroyed in an ordinary chemical reaction, this mass law translates directly into an atom-counting rule: for a balanced equation, the number of atoms of each element must match exactly on both sides. An equation that has not yet been fixed up this way — like the raw Mg+O2→MgO\text{Mg} + \text{O}_2 \rightarrow \text{MgO} above, which has 2 oxygen atoms on the left but only 1 on the right — is called a skeletal equation. The figure below shows the balancing habit in miniature: keep adjusting whole-number coefficients in front of formulas until the two sides carry identical baskets of atoms.

Figure
Two balance pans showing atoms of magnesium and oxygen equal on both sides after balancing the skeletal equation for magnesium oxide
A balanced equation keeps the atom count equal on both sides Left side: 2Mg + O2 Mg Mg O O 2 Mg atoms, 2 O atoms Right side: 2MgO Mg O Mg O 2 Mg atoms, 2 O atoms Counts match — the equation is balanced Coefficients (the 2s) were adjusted; the formulas Mg, O2, MgO were never changed inside

Nothing about balancing follows a fixed recipe — it is trial and error, changing the whole-number coefficient placed in front of an entire formula. The subscripts inside a formula must never be touched: changing H2O\text{H}_2\text{O} to H2O2\text{H}_2\text{O}_2 does not balance an equation, it substitutes a completely different substance (hydrogen peroxide) for water.

Why it works

The atom-balancing rule is not an arbitrary classroom convention — it is a direct, testable consequence of conservation of mass, and it is worth seeing why the logic holds together before applying it mechanically. Every atom has a fixed mass (its own atomic mass), and a chemical reaction only breaks and re-forms bonds between existing atoms — it never creates a new atom or destroys one. If the same set of atoms is simply regrouped into new molecules, the total mass contributed by, say, every magnesium atom present must be identical before and after the reaction, and the same must be true for every oxygen atom, separately. Summing over all elements, the total mass of reactants therefore has to equal the total mass of products — that is exactly the Law of Conservation of Mass. This law is not just asserted; it was verified experimentally. When sodium sulphate solution is mixed with barium chloride solution inside a sealed flask, a white precipitate of barium sulphate forms — a clear sign that a chemical reaction (double displacement) has occurred. Weighing the sealed flask before and after shows the mass is unchanged. Because nothing was added or removed and no gas escaped (the flask was sealed), the only way the mass can stay exactly constant is if every atom that went in as a reactant is still present, in some form, as a product. Turning that logic around gives the balancing rule used in practice: since atoms cannot appear or disappear, the count of each element's atoms is conserved separately. So the recipe is to count the atoms of one element at a time, adjust a coefficient to fix any mismatch, recount (a fix for one element can disturb another that was already balanced), and repeat until every element matches on both sides. It is common to balance the element that appears in the fewest formulas last, since it is often affected by every other adjustment made earlier.

Worked example

Balance the skeletal equation for aluminium reacting with oxygen to form aluminium oxide, and add state symbols (aluminium and the oxide are both solids, oxygen is a gas).

Step 1. Write the skeletal equation: Al+O2→Al2O3\text{Al} + \text{O}_2 \rightarrow \text{Al}_2\text{O}_3.

Step 2. Count atoms on each side. Left: 1 Al, 2 O. Right: 2 Al, 3 O. Nothing matches yet.

Step 3. Balance oxygen first, since it appears with different subscripts on each side (2 versus 3). The lowest common multiple of 2 and 3 is 6, so place a coefficient of 3 in front of O2\text{O}_2 (giving 6 O on the left) and a coefficient of 2 in front of Al2O3\text{Al}_2\text{O}_3 (giving 6 O on the right): Al+3O2→2Al2O3\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3.

Step 4. Recheck aluminium, which the last step disturbed. Right side now has 2×2=42 \times 2 = 4 Al, so place a coefficient of 4 in front of Al on the left: 4Al+3O2→2Al2O34\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3.

Step 5. Verify every element. Left: 4 Al, 3×2=63 \times 2 = 6 O. Right: 2×2=42 \times 2 = 4 Al, 2×3=62 \times 3 = 6 O. Both sides carry 4 Al and 6 O — balanced.

Step 6. Add state symbols. Answer: 4Al(s)+3O2(g)→2Al2O3(s)4\text{Al}(s) + 3\text{O}_2(g) \rightarrow 2\text{Al}_2\text{O}_3(s). This burn is also accompanied by a bright light, the same type of observable clue (heat and light released) that confirmed the magnesium-oxygen reaction was chemical rather than physical.

Common traps

  • Balancing by editing a subscript instead of adding a coefficient — this silently changes the substance itself and is never allowed.
  • Fixing one element's count, then forgetting to recheck the elements that adjustment disturbed; always recount every element after each change.
  • Skipping state symbols, or assigning the wrong one (e.g. writing (s)(s) for a gas) — these are marked separately from the atom balance in exams.
  • Treating a purely physical change, like ice melting or sugar dissolving, as a chemical reaction simply because "something changed."

Ask the tutor: if a skeletal equation is not balancing no matter what you try, ask the AI tutor to walk through the atom count element by element with you.

Why can we not just balance an equation by changing the subscripts to whatever numbers make it work?

Because a subscript is part of a formula's identity, not a balancing knob. H2O\text{H}_2\text{O} and H2O2\text{H}_2\text{O}_2 are different substances — water and hydrogen peroxide — with different properties, so changing one into the other to "fix the count" would describe a reaction that never actually happened. Balancing only ever uses whole-number coefficients written in front of a complete, unchanged formula, because a coefficient just means "this many molecules," which is exactly the freedom conservation of mass allows.

If atoms are just rearranged and never destroyed, why does a burning candle seem to lose mass?

Nothing about conservation of mass is violated — the mass has not vanished, it has left in a form you did not weigh. Burning wax combines with oxygen from the air and produces carbon dioxide and water vapour, both of which escape as gases into the surrounding air. If you burned the candle inside a sealed container that also held enough oxygen and captured every product gas, the total mass before and after would come out exactly equal, just as it does with the sealed sodium sulphate and barium chloride flask.

How do I decide which element to balance first when there are several?

There is no single rule, but a useful habit is to leave elements that appear in only one formula on each side for later, and start with an element whose atom counts differ in a simple ratio, often the one that is easiest to fix with a small coefficient, such as oxygen in the aluminium oxide example. Balance one element, recount all elements (since fixing one can unbalance another), and repeat. With practice on a handful of common reactions, the right order becomes intuitive rather than something you have to plan out.

Before this

Where this leads