Describing Chemical Reactions

Lesson Plan - Get It!

Audio:

Symbols are used in many ways, from communicating with friends to providing driving directions on the roads.

Before continuing, if you need a copy of the periodic table, you can find it under Downloadable Resources in the right-hand sidebar.

Scientists and chemists use symbols to describe how elements bond and change through chemical reactions. A chemical reaction occurs when two or more elements interact and experience a chemical change. Rust forming on an old bicycle is an example of a chemical reaction. When you light a match, you are initiating a chemical reaction. If a molecule or compound participates in a chemical reaction, the bonds are broken and recreated into a new substance. This is how plants turn carbon dioxide into oxygen you breathe, through breaking bonds!

Notice the black carbon atom in the image above. It starts out bonded to four hydrogen ions. After the chemical change has occurred, the carbon atom bonds with two oxygen atoms. This demonstrates how the elements change during a reaction.

Chemical reactions depend on chemical changes. Remember, that is the type of change where the composition of the substance is altered, not just the shape. There are several indicators of a chemical reaction: the formation of a gas, changes in the temperature, color change, and the formation of a precipitate.

Some chemical reactions form a gas as a product of a chemical reaction. When hydrochloric acid — a very strong acid — is added to zinc — a metal — hydrogen gas forms. Hydrogen gas is flammable and very light!

The formation of heat and light is another indicator of a chemical change, examples being lighting a match and a wood-burning fire. Think about how wood looks different as ash left over after a fire.

Many chemical reactions lead to a change in color when the two elements or substances meet.

Some chemical reactions lead to the formation of a precipitate. A precipitate is a solid substance that falls out of a reaction. Watch Lead Iodide Precipitating, by Hans de Grys, to observe this chemical reaction:

Write down three observations from the video to summarize what you learned.

Chemical reactions can occur between two atoms or a blend of atoms and compounds. For example, hydrogen and oxygen generate water, a very simple chemical reaction.

A chemical reaction is written using several symbols. Look back at the reaction above, also pictured below:

The chemical reaction shows CH₄ combined with two atoms of O₂. These are the reactants. Reactants are always written to the left of the arrow, and they are the substances you start with. The arrow represents the reaction running, changing reactants to products. The products are CO₂ and two molecules of H₂0.

There are a couple of different types of chemical reactions, but they always follow the same pattern. The reactants will always be before the arrow, and the products will always be after the arrow.

You rely on chemical reactions in your blood to move oxygen throughout your body. You use chemical reactions to cook dinner and treat medical symptoms.

Summarize what you have learned about chemical reactions by writing three sentences.

Several clues indicate when a chemical reaction has occurred, when gases or solid precipitates are formed after adding materials together. Color changes and the formation of heat and light also occur during chemical reactions. All chemical reactions are written using the same format, with reactants written before the arrow and products written following it. This common language allows scientists and chemists to understand one another's work.

In the Got It? section, you will learn what factors influence how quickly a reaction runs.