Lesson 7.1: Chemical reactions alter arrangements of atoms

Lesson Focus

Explore how chemical reactions rearrange atoms to form new substances. We'll discover the difference between powerful chemical changes and simple physical changes, like melting ice, to understand how matter transforms.

Learning Objectives

• Learn about chemical changes and the evidence that signals a reaction has occurred.
• Discover three major types of chemical reactions: synthesis, decomposition, and combustion.
• Understand how to change the rate of a chemical reaction using temperature, concentration, and catalysts.

A chemical reaction occurs when reactant particles collide, breaking their old chemical bonds. Atoms then rearrange to form new bonds, creating entirely new substances called products. This is different from a physical change, like melting ice, where the substance itself doesn't change. Why is dissolving sugar just a physical change?

You can tell a chemical reaction happened by observing clues. Look for a color change, the formation of a solid precipitate from liquids, the creation of gas bubbles, or a change in temperature. These signs indicate that the original reactants have transformed into chemically different products.

Reactions are classified by how atoms move. In synthesis, simple reactants combine to make a complex product (N₂ + 2O₂ → 2NO₂). In decomposition, a reactant breaks into simpler products (2H₂O → 2H₂ + O₂). In combustion, a substance burns with oxygen, producing carbon dioxide and water.

The speed of a reaction depends on how often and how hard reactant particles collide. Increasing concentration (more particles), surface area (more places to collide), and temperature (faster particle movement) all speed up reactions by increasing the frequency and energy of these collisions. Why does a campfire burn faster on a windy day?

A catalyst is a substance that increases a reaction's rate but isn't consumed in the process. It provides an easier pathway for the reaction to happen. In your body, special catalysts called enzymes are essential for life, making reactions happen in seconds instead of years. They are like a helpful shortcut.