New Substance Formation Indicates a Chemical Reaction
Not every visible change is a chemical reaction. Sugar dissolved in water is still sugar. Ice that melts is still water. Neither example forms a new substance.
A chemical reaction happens when starting substances change into new substances. The starting substances are called reactants, and the substances formed are called products.
Without analysis equipment, visible changes can provide initial clues that a chemical reaction has occurred. These clues include gas formation, a precipitate, a color change, or an energy change. Reactions can also be classified as precipitation, acid-base, and oxidation-reduction reactions.
Observing Reaction Evidence in a Model
Choose one clue, then compare the before and after states. Observe what appears and infer what may have happened to the substances.
Rising bubbles can indicate that a gas is forming from the mixture.
- What appears
- Bubbles or gas leave the mixture.
- Meaning of the change
- One product may be a gas, such as .
- Limit of the evidence
- Boiling bubbles are not automatically a chemical reaction.
Four Common Clues
Gas formation may appear as bubbles in a liquid or as gas leaving the mixture. Smoke can accompany combustion, but the US Environmental Protection Agency classifies smoke as airborne particulate matter, so smoke is not itself proof of a gaseous product. Never deliberately smell an unknown substance. Waft vapors only when specifically instructed.
A precipitate is a new solid formed from a solution. For example, two clear solutions can produce a white solid because their ions form a compound that does not dissolve well.
A color change can be a strong clue when no dye is added. The color changes because a new product can interact with light differently.
An energy change can feel warm or cold, or appear as light. Breaking the reactants' bonds requires energy, while forming product bonds releases energy. The overall reaction warms the surroundings when more energy is released than absorbed, and cools them when the reverse is true.
The key distinction between a chemical and physical change is whether a new substance forms.
Checking Multiple Signs of a Chemical Reaction
One sign is not always enough. Boiling water produces bubbles, but it is a physical change because the substance is still . Dissolved salt also looks different, but it can be recovered by evaporating the water.
The table shows how several events can be assessed from the signs that are observed.
| Event | Conclusion | Reason |
|---|---|---|
| Ice melts | Physical change | The substance is still . |
| Paper burns | Chemical reaction | Gaseous products and ash form, while energy is released as heat and light. |
| Two clear solutions produce a solid | Likely chemical reaction | A precipitate appears that was not there before. |
| Red dye is mixed with water | Physical change | The dye spreads through the water and changes its color. The dye remains the same substance. |
To be sure, we need to check whether the products are truly different from the reactants. In the laboratory, that can be done by measuring mass, testing a gas, observing a precipitate, or analyzing composition.
Reaction Evidence and Basic Chemical Laws
The basic laws of chemistry describe consistent mass relationships during reactions, including conservation of mass and fixed composition. Before applying one of these laws, first confirm that the observed event is a chemical reaction.
Take metal oxides as an example. Nickel can form one of its oxides with the formula , and cobalt can also form one of its oxides with the formula . Each formula says that a metal and oxygen combine in a definite ratio.
After the evidence confirms a chemical reaction, measure the masses or composition of the substances. The appropriate chemical law can then relate the measured reactants and products.