Chemical Reaction and Chemical Equation

Something is changing around us at every moment. Ice left in a dish melts, water on a stove boils away as steam, a stick of chalk falls and breaks into pieces. One thing is the same in all of these changes. Only the shape or the state of the substance has altered, while what the substance is made of stays exactly as it was. Melted ice is still water, and cooling that water gives ice again. Broken chalk is still chalk.

Now look at a different kind of change. Paper that is set alight turns into ash and smoke, and no amount of care will turn that ash and smoke back into paper. Iron left damp grows a layer of rust in a few days, and the rust cannot be turned back into bright iron. Milk that has set into curd never becomes milk again. Here the substance has not merely changed its shape. The substance itself has become something new.

What actually happens inside a substance during a chemical change? Substances are built out of atoms. During a chemical change the atoms of the substances join together, break apart and change places with one another. Joining means separate atoms coming together and sticking as one group. Breaking apart means atoms that were held together coming loose. Changing places means an atom of one substance swapping its partner with an atom of another substance. Because the atoms end up arranged in a new way, a new substance is formed.

Describing a chemical reaction in a long sentence every single time is slow and clumsy. So chemists write it in a short, fixed form instead. A chemical reaction can be expressed through an equation, and that is what a chemical equation is. Reading one tells you at a glance which substances went in and which substances came out.

Every chemical reaction has two kinds of substance in it. First, the substances that take part in the reaction. Second, the substances that are formed once the reaction is over. A chemical equation puts these two kinds in two different places, and the place itself tells you which is which.

An arrow is placed between the reactants and the products. That arrow is the sign that a chemical change has taken place. It always points from left to right, because the change runs from the reactants towards the products. The same substance does not appear on both sides of one equation, because the products are different substances from the reactants.

A chemical reaction can be written out in words. The names of the substances are written as they are, with a plus sign between them and an arrow in the middle. The reaction in which sodium and chlorine give sodium chloride, for example, can be shown as sodium + chlorine → sodium chloride. Writing it this way shows at one glance which substance stands on which side, but it says nothing at all about how many atoms are inside each substance.

A chemical equation can also be written using the symbols of the elements or the molecular formulae of the compounds. Instead of the name of a substance, its symbol is written. The symbol of sodium is Na, the molecular formula of chlorine is \( \mathrm{Cl}_2 \), and the formula of sodium chloride is NaCl. So the word equation above can be written in symbols like this:

\[ \mathrm{Na} + \mathrm{Cl}_2 \rightarrow \mathrm{NaCl} \]

Look carefully at the formula equation above. There are two chlorine atoms on the left, but only one chlorine atom on the right. Where did the other one go? In a chemical reaction atoms are neither made nor lost, only rearranged. So the number of atoms of each element on the left of a formula equation must equal the number on the right.

To make the numbers equal, suitable numbers are written in front of the elements or compounds on the left and on the right. Writing 2 in front of sodium chloride gives two chlorine atoms on the right as well, and then sodium has to be matched by writing 2 in front of sodium on the left:

\[ 2\mathrm{Na} + \mathrm{Cl}_2 \rightarrow 2\mathrm{NaCl} \]

Hydrogen and oxygen together give water. Its word equation is hydrogen + oxygen → water. Let us take that as far as a balanced formula equation.

Step 1: Write the formula of each substance and join the two sides with an arrow:

\[ \mathrm{H}_2 + \mathrm{O}_2 \rightarrow \mathrm{H}_2\mathrm{O} \]

Step 2: Count the atoms on each side. The left has 2 hydrogen atoms and 2 oxygen atoms, while the right has 2 hydrogen atoms and only 1 oxygen atom. Oxygen does not match.

Step 3: Write 2 in front of water on the right so that oxygen matches:

\[ \mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O} \]

Step 4: The right now holds 4 hydrogen atoms while the left holds only 2. Write 2 in front of hydrogen on the left:

\[ 2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O} \]

Step 5: Count once more. Both sides now hold 4 hydrogen atoms and 2 oxygen atoms. The equation is balanced.

A few chemical reactions are set out below in both forms, as a word equation and as a balanced formula equation. In every line the substances on the left are the reactants and those on the right are the products.

In the calcium carbonate equation there is a single substance on the left and two on the right. That shows how one substance can break apart into two new ones. In the zinc equation, zinc has changed places with hydrogen, and out of that exchange come zinc chloride and hydrogen gas.