Evidence of Chemical Changes

General Chemistry Index

Where are we going with this? The point of this deck is to give the background information and examples so that we can differentiate between substances (pure and mixtures) based on physical and chemical properties.

Evidence of Chemical Changes

A chemical change occurs when a reaction takes place and you end up with substances that are different from those with which you started.

When chemical changes occur, there are usually physical changes and often other signs of the change. Some of those signs are…

A Change in Color

Many chemical changes result in the new substance showing a different color than the original.

https://youtu.be/Q91TWOcfP9w

 

Production of Gas

Some reactions release gas, and can be observed as bubbles or an odor.

Formation of a Precipitate

In liquid mixtures, chemical reactions will often cause the newly formed substances to take on a solid form and suspend more obviously in the liquid. This process is called formation of a precipitate.

The solid that forms in a liquid mixture is called a precipitate. The precipitate may remain suspended in the liquid, may settle to the bottom, or may float to the top.

Temperature Change

It is not uncommon for chemical changes to involve a noticeable temperature change. When a chemical change gives off heat (the system gets warmer), it is said to be exothermic. When a chemical change takes in heat (the system cools off), it is said to be endothermic.

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It is not always easy to tell if an observed change is physical or chemical. A heated piece of iron will change color, but still be iron. Gas bubbles will form when water's temperature approaches its boiling point, but it is still water.

For a chemical change to take place, two or more substances must change at the molecular level to become one or more NEW substances.