Chemical Reactions and Equations

Physical and chemical changes

A chemical reaction is a chemical change. Bonds between atoms break and new bonds form, making new substances.

Signs of a chemical reaction

Look for evidence that something new has formed:

• Gas produced: bubbles or fizzing.
• Colour change.
• Temperature change: gets hot or cold.
• Light or sound given off.
• Precipitate: a solid forms when two solutions are mixed, making the liquid cloudy.
• Smell changes.

Reactants and products

• Reactants are the substances you start with.
• Products are the new substances made.
• The arrow means "react to make" or "produces".

$$\text{reactants}\to \text{products}$$

Word equations

A word equation names the reactants and products.

$$\text{magnesium}+\text{oxygen}\to \text{magnesium oxide}$$

$$\text{hydrochloric acid}+\text{sodium hydroxide}\to \text{sodium chloride}+\text{water}$$

Conservation of mass

Atoms are not created or destroyed in a reaction. They are only rearranged. So:

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

Formula equations and state symbols

A formula equation uses chemical formulas instead of names.

$$2Mg(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to 2MgO(\mathrm{s})$$

State symbols show the state of each substance:

The big numbers in front (like the 2 in $2MgO$) are coefficients. They show how many units of each substance react. You will learn how to choose them in the next chapter.

Common types of reactions

Quick summary

• A chemical change makes a new substance. A physical change does not.
• Signs of a reaction: gas, colour change, temperature change, light, precipitate, smell.
• Reactants → products. Write reactions as word equations or formula equations.
• Conservation of mass: total mass of reactants = total mass of products.
• State symbols: (s), (l), (g), (aq).
• Gas tests: hydrogen pops, oxygen relights a glowing splint, carbon dioxide turns limewater milky.

Quiz