ChemistryClass 7ICSE

No Atom Is Allowed to Disappear in a Chemical Equation

Learn to turn a word equation into a skeletal one with state symbols, balance it by hit and trial using conservation of mass, list what a balanced equation cannot tell you, and classify reaction types.

Why must a chemical equation have the same atoms on both sides?

Because atoms are neither created nor destroyed in a reaction — they are only rearranged. Whatever went in must come out, so the count of every element has to match on the two sides.

That principle is the law of conservation of mass, and balancing is simply how we honour it on paper. This page covers everything in the ICSE Class 7 Chemistry chapter's second part: writing skeletal equations, balancing them, what a balanced equation does and does not tell you, and the four types of reaction.

How do you turn a word equation into a skeletal equation?

Replace each name with its formula, keep the reactants on the left and the products on the right, and join them with an arrow.

A word equation first: zinc plus dilute sulphuric acid gives zinc sulphate plus hydrogen.

The skeletal equation — correct formulae, but not yet balanced:

Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}

The arrow means "gives" or "produces", and it always points from reactants to products.

State symbols are added in brackets after each formula:

- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous, meaning dissolved in water

So the equation becomes

Z n (s) + H_{2} S O_{4} (a q) \to Z n S O_{4} (a q) + H_{2} (g)

Two extra notations appear in ICSE answers. An upward arrow or (g) marks a gas evolved, and a downward arrow or the word precipitate marks an insoluble solid formed.

The step that must come first is the formula itself. Writing

Z n S O_{4}

Z n S O

Z n (S O_{4})_{2}

makes the equation impossible to balance correctly, so get the valencies right before touching any coefficient.

How do you balance an equation by hit and trial?

Count the atoms of each element on both sides, then adjust the coefficients in front of the formulae until every count matches. Never change a subscript — that would change the substance itself.

Worked example 1. Start from the skeletal equation for burning hydrogen:

H_{2} + O_{2} \to H_{2} O

Count: left has 2 H and 2 O; right has 2 H and 1 O. Oxygen does not match. Put a 2 before the water:

H_{2} + O_{2} \to 2 H_{2} O

Now the right has 4 H and 2 O, so hydrogen is short on the left. Put a 2 before the hydrogen:

2 H_{2} + O_{2} \to 2 H_{2} O

Final count: 4 H and 2 O on each side. Balanced.

Worked example 2. Iron reacting with steam:

3 F e + 4 H_{2} O \to F e_{3} O_{4} + 4 H_{2}

Check each element: Fe is 3 and 3; H is

4 \times 2 = 8

on the left and

4 \times 2 = 8

on the right; O is 4 and 4. Balanced.

The justification is the law of conservation of mass — the total mass of the reactants equals the total mass of the products, because the same atoms are present throughout.

That is why changing a subscript is forbidden. Rewriting

H_{2} O

H_{2} O_{2}

would balance the oxygen, but

H_{2} O_{2}

is hydrogen peroxide, a completely different substance.

What can a balanced equation not tell you?

A balanced equation conveys a great deal, but it leaves several things out — and the limitations are examined as often as the information.

It tells you:

- Which substances react and which are formed.
- The number of molecules of each, from the coefficients.
- That the atoms and the mass are conserved.
- The physical states, when state symbols are included.

It does not tell you:

- The conditions needed — heat, light, pressure or a catalyst — unless they are written above the arrow.
- Whether heat is given out or absorbed during the reaction.
- The speed of the reaction, or how long it takes.
- The concentration of the solutions used.
- Whether the reaction is complete or reversible.

So the equation for cooking gas burning says nothing about needing a spark to start, and nothing about the heat that makes it useful at all.

That is the reasoning to reproduce: an equation is a statement of what changes into what, in fixed proportions. Everything about how the change happens has to be written in separately.

What are the four types of chemical reaction?

Reactions are classified by how the reactants rearrange, and each type has a standard balanced example.

Combination — two or more substances join to form a single product. Magnesium ribbon burns in air with a bright light:

2 M g + O_{2} \to 2 M g O

Decomposition — a single substance breaks into two or more simpler ones, usually on heating. Limestone heated in a kiln gives quicklime:

C a C O_{3} \to C a O + C O_{2}

Displacement — a more reactive element pushes out a less reactive one from its compound. Zinc granules in dilute sulphuric acid release hydrogen:

Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}

Double decomposition — two compounds exchange their radicals. Common salt solution added to silver nitrate solution gives a white precipitate:

N a C l + A g N O_{3} \to A g C l + N a N O_{3}

Lime being slaked for whitewash is a combination reaction, and the lime kiln that made it ran a decomposition.

The two that get mixed up are displacement and double decomposition, and the test is simple. In displacement, one of the reactants is a free element. In double decomposition, both reactants are compounds and they simply swap partners.

Exam tip

Exam tip: counting element by element before you stop

Balancing is marked on the final equation, so verify it rather than trusting it.

After balancing, write a short atom count under each element on both sides and confirm they match. Most wrong answers are one element away from correct.

Change coefficients only, never subscripts, and balance the element that appears in the fewest formulae first. Leave hydrogen and oxygen for last, since they usually appear in several.

Include state symbols if the question uses them, and remember (aq) for anything dissolved in water.

When classifying a reaction, count the products: one product suggests combination, several from one reactant suggests decomposition. Then check whether a free element is involved to separate displacement from double decomposition.

And if asked to justify balancing, name the law of conservation of mass explicitly. The name itself carries a mark.

Did you know

Why can you change the big number in front but not the small one below?

Because the two numbers answer different questions — how many molecules and what the molecule is.

A coefficient in front simply says how many copies of a molecule take part, so changing it changes the amount and nothing else. A subscript is part of the molecule's identity.

So

2 H_{2} O

is two glasses of the same water, while

H_{2} O_{2}

is a different liquid altogether. Balancing is allowed to adjust how much of each substance reacts; it is never allowed to quietly swap one substance for another.

Key takeaways

Chemical equations and reaction types: quick revision

- A skeletal equation puts correct formulae on each side of an arrow; state symbols (s), (l), (g) and (aq) show the physical states.
- Balance by adjusting coefficients only — changing a subscript changes the substance, turning

H_{2} O

into hydrogen peroxide.
- Balancing is required by the law of conservation of mass:

2 H_{2} + O_{2} \to 2 H_{2} O

has 4 H and 2 O on both sides.
- A balanced equation gives the substances, the number of molecules and the states, but not the conditions, heat change, speed or concentration.
- Combination gives one product (

2 M g + O_{2} \to 2 M g O

); decomposition breaks one into several (

C a C O_{3} \to C a O + C O_{2}

).
- Displacement involves a free element pushing out another; double decomposition has two compounds exchanging radicals.

You will remember all of this far better after answering five questions on it than after reading it twice.