What is a chemical reaction?
A chemical reaction is a transformation in which substances disappear (the reactants) and new substances are formed (the products). Matter is not created or destroyed: it is simply rearranged.
Signs that a chemical reaction has taken place include: - a change of colour; - a gas being released (bubbles); - formation of a precipitate (a solid forming in a solution); - release of heat or light.
Identifying reactants and products
In a chemical reaction written as:
Reactant A + Reactant B → Product C + Product D
- Reactants are the starting substances, to the left of the arrow.
- Products are the substances formed, to the right of the arrow.
Example: combustion of carbon in dioxygen Carbon + Dioxygen → Carbon dioxide C + O₂ → CO₂
The reactants are carbon and dioxygen; the product is carbon dioxide.
Characteristic tests to identify species
Chemists use characteristic tests to identify certain reactants or products.
| Species to detect | Test | Positive result |
|---|---|---|
| CO₂ (carbon dioxide) | Limewater | Solution turns cloudy (white) |
| H₂O (water) | Anhydrous copper sulfate | White powder turns blue |
| H⁺ ions (acid) | pH paper / indicator | Red colour / pH < 7 |
| OH⁻ ions (base) | pH paper / indicator | Blue colour / pH > 7 |
The concept of limiting reactant
Reactants are often not present in exactly the right proportions. The limiting reactant is the one that is completely consumed first: it determines how much product forms. The other reactant is in excess.
Example: if 1 g of charcoal burns but only a small amount of dioxygen is available, dioxygen is the limiting reactant, even if there is plenty of charcoal.
Conservation of matter
In a chemical reaction, the total mass is conserved: the mass of reactants equals the mass of products. This is the law of conservation of mass (Lavoisier's law).
"Nothing is lost, nothing is created, everything is transformed."