CBSE Class 10 Science

Chemical change and its signs

In a chemical reaction the substances present at the start, the reactants, are changed into different substances, the products, with different properties. The atoms are the same before and after; they are rearranged into new combinations. Some observations show that a reaction has happened:

• a change of state, as when a gas is given off
• a change of colour
• a change of temperature
• the formation of a precipitate, an insoluble solid that appears in a solution

Burning magnesium ribbon in air gives a dazzling white flame and leaves a white powder, magnesium oxide. Dilute sulphuric acid added to zinc granules gives bubbles of hydrogen gas and the zinc slowly disappears. Iron nails left in copper sulphate solution turn the solution from blue towards green, and a brown coating of copper forms on the nails. Each of these signs is evidence, and none is proof by itself, because a change of state such as boiling water also gives off a gas without forming a new substance.

Two preparations are worth doing before a reaction: the magnesium ribbon should be cleaned with sandpaper to remove the layer of magnesium oxide on it, so that it is pure magnesium that burns, and it should be held with tongs, well away from the eyes, because the flame is bright enough to hurt them.

Word equations and chemical equations

The shortest way to record a reaction is a word equation, with an arrow from reactants to products:

A chemical equation replaces each name with a formula, so that it shows the atoms as well:

This is not yet correct, because there are two oxygen atoms on the left and one on the right. Atoms cannot be created or destroyed, so a correct equation must have the same number of atoms of each element on both sides. This is the law of conservation of mass, and an equation that obeys it is a balanced chemical equation.

Balancing an equation

The formulae are fixed and never changed while balancing; only the numbers placed in front of them, the coefficients, can be changed. The method is a sequence of steps.

1. Write the correct formula for every reactant and product.
2. Count the atoms of each element on each side.
3. Pick the element that appears in the fewest formulae, often the one that is hardest to fix last, and adjust a coefficient to make it equal.
4. Repeat for the other elements, leaving oxygen and hydrogen, which tend to be in several formulae, until last.
5. Count every element again on both sides.

For magnesium and oxygen, putting a 2 in front of $\ce{MgO}$ makes oxygen equal (two on each side) but then magnesium is unequal, so a 2 goes in front of $\ce{Mg}$ as well:

A worked example with more elements is the reaction of iron with steam, which gives iron(II,III) oxide and hydrogen:

Iron appears in one formula on each side, so start there: three iron atoms on the right need a 3 in front of $\ce{Fe}$. Oxygen is next; there are four on the right, so the left needs a 4 in front of $\ce{H2O}$, which gives eight hydrogen atoms on the left. Eight hydrogen atoms on the right need a 4 in front of $\ce{H2}$:

Counting shows Fe 3 and 3, H 8 and 8, O 4 and 4. The equation is balanced.

State symbols and conditions

A balanced equation is more informative when it states the physical state of each substance: $\ce{(s)}$ for solid, $\ce{(l)}$ for liquid, $\ce{(g)}$ for gas and $\ce{(aq)}$ for aqueous, meaning dissolved in water. The equation for zinc with dilute sulphuric acid is

An upward arrow after a formula can also show a gas given off, and a downward arrow can show a precipitate. A condition that is needed, such as heat, light, pressure or a catalyst, is written above or below the arrow, as in the reaction of calcium carbonate on heating:

What a balanced equation tells you

A balanced equation gives more than the identity of the substances. The coefficients give the ratio in which the particles react, so $\ce{2H2 + O2 -> 2H2O}$ says that two molecules of hydrogen react with one of oxygen, and equally that two moles do. It also shows that mass is conserved: the total mass of the reactants equals the total mass of the products. Verifying the law of conservation of mass in the laboratory uses a sealed container, such as a flask with a stopper, in which two solutions are mixed, for instance barium chloride and sodium sulphate. The flask is weighed before and after mixing, and the readings match. The container must be sealed in a reaction that produces or takes in a gas, as otherwise the gas escapes or enters and the mass appears to change.

What the equation does not show is the rate of the reaction, whether it is reversible, or whether heat is given out or taken in, unless that is added to the equation as information. It does not show the colours or the smells either.