Recognizing Chemical Reactions — Common Signs & Examples

What is a Chemical Reaction?

Chemical reactions occur when substances are changed at a chemical or molecular level. Because we cannot see chemical structure with our eyes, we need to rely on other detectable changes.

What to Look For

Visible changes can indicate a chemical reaction. Two of the main things we can see are a color change or change of state.

Color changes are a useful way to recognize chemical reactions. The browning of an apple is an example of this. If you cut a piece of apple and leave it sit out, it will brown over time. The color changes because oxygen is reacting with chemicals in the apple.

Changes of state

There are three main states substances exist in: solid, liquid, gas. Some reactions create new substances in a different state.

A great example of this is the homemade volcano! A homemade volcano "erupts" due to reaction between two household chemicals: vinegar (acetic acid) and baking soda (sodium bicarbonate). When these two chemicals react together they create sodium acetate, water, and carbon dioxide. Carbon dioxide is a gas, while the original chemicals are a liquid and solid. The formation of a new chemical indicates a chemical reaction occurred.

Keep in mind, not all changes of state indicate a chemical reaction. Boiling liquid water to create gaseous water or steam, is a physical reaction. It is important to remember chemical reactions cause the formation of new substances due to change in the chemical molecules.

How else can we detect chemical reactions?

Sometimes chemical reactions occur and we cannot tell using our eyes. Chemists will use instruments to check the chemical composition for changes. These instruments provide data on the chemical make-up of the substance which can be used to determine if a change occurred. A few examples of different types of instruments are: mass-spectrometer (MS), ultra violet-visible spectrophotometer (UV-Vis), infrared spectrophotometer (IR), and nuclear magnetic resonance spectrometer (NMR).

Recognizing Chemical Reactions

• There are a few different ways to recognize whether or not a chemical reaction is occurring

• Visual clues are the easiest way to tell if a chemical reaction is occurring

Visual Clues for Recognizing Chemical Reactions

• Change of color

• Change in state

• Formation of a precipitate (formation of a solid)

• Formation of a gas (sometimes difficult to see)

• A release of light

Other Clues for Recognizing Chemical Reactions

• Change in temperature

• Change in smell

• Note: When checking odors from chemical reactions, safety is the first priority, so make sure to waft the odor to the nose rather than smell straight from the container

(Video) 7 Signs a Chemical Reaction Is Taking Place

by Wayne Breslyn

This video describes several ways you can tell if a chemical reaction is happening. You might find:

• a change in temperature.
• a change in color.
• the release of a gas.
• the formation of a precipitate.
• a release of light.
• a change in smell.