8.1: Evidence of a Chemical Reaction
- Page ID
- 105316
Learning Objectives
- Identify the evidence for chemical reactions
In a chemical change, new substances are formed. In order for this to occur, the chemical bonds of the substances break, and the atoms that compose them separate and rearrange themselves into new substances with new chemical bonds. When this process occurs, we call it a chemical reaction. A chemical reaction is the process in which one or more substances are changed into one or more new substances.
To identify a chemical reaction, we look for a chemical change. A chemical change always produces one or more types of matter that differ from the matter present before the change. The formation of rust is a chemical change because rust is a different kind of matter than the iron, oxygen, and water present before the rust formed. The explosion of nitroglycerin is a chemical change because the gases produced are very different kinds of matter from the original substance. Other examples of chemical changes include reactions that are performed in a lab (such as copper reacting with nitric acid), all forms of combustion (burning), and food being cooked, digested, or rotting (Figure \(\PageIndex{1}\)).
Video\(\PageIndex{1}\): Evidence of a Chemical Reaction
Example \(\PageIndex{1}\): Evidence of a Chemical Reaction
Which of the following is a chemical reaction?
- Freezing liquid Mercury
- Adding yellow to blue to make green
- Cutting a piece of paper into two pieces
- Dropping a sliced orange into a vat of sodium dydroxide
- Filling a balloon with natural air
Solution
a,b,c, & e involve only physical changes. A sliced orange has acid (citric acid) that can react with sodium hydroxide, so the answer is d
Exercise \(\PageIndex{1}\)
Which of the following is a chemical reaction?
- Painting a wall blue
- A bicycle rusting
- Ice cream melting
- Scratching a key across a desk
- Making a sand castle
Answer
b
Example \(\PageIndex{2}\): Evidence of a Chemical Reaction
Which of the following is not a chemical reaction?
- Shattering Glass with a baseball
- Corroding Metal
- Fireworks Exploding
- Lighting a match
- Baking a cake
Solution
Shattering glass with a baseball results in glass broken into many pieces but no chemical change happens, so the answer is a.
Exercise \(\PageIndex{2}\)
Which of the following is NOT a chemical reaction?
- Frying an egg
- Slicing carrots
- A Macbook falling out of a window
- Creating ATP in the human body
- Dropping a fizzy tablet into a glass of water
Answer
b and c
Summary
Chemical reactions can be identified via a range of different observables including change in color, energy change (temperature change or light produced), gas production, formation of precipitate and change in properties among others.
Contributors and Attributions
Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110).
Henry Agnew (UC Davis)