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Indicators of a Chemical Change

Indicators of a Chemical Change

Author: Michael Geyer
Description:

This tutorial will help students identify the most common indicators that indicate a chemical change (reaction) has taken place and not some physical change.

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Tutorial

Introduction

A chemical change is the process by which one or more substances are changed into one or more new, different substances. We call this change a chemical reaction, and rarely refer to it as a chemical change. The details of chemical reactions will be a topic for study later in the school year. For now you need to be able to identify when a chemical change (reaction) is taking place. This tutorial will present the five most common, reliable indicators of a chemical change. You need to familiarize yourself with these indicators. They will be grouped according the their reliability at predicting chemical changes.

1) Formation of a gas

When gas bubbles are formed it is often a sign that a chemical reaction is taking place. This isn't always the case. Two examples of situations where you will see gas bubbles and not have a chemical reaction are when a liquid boils or when you are blowing bubbles in a liquid through a straw. However, the presence of gas bubbles is a very strong indicator of a chemical reaction.

Formation of a gas (a)

Magnesium ribbon (Mg) is dropped into 1M hydrochloric acid (HCl).

Formation of a gas (b)

Calcium carbonate (CaCO3) is dropped into 1M hydrochloric acid (HCl).

2) Formation of heat and light together

When heat and light are given off together it is often a sign that a chemical reaction is taking place. Again, there are situations where you will see heat and light together and a chemical reaction will not be taking place (such as with an incandescent lightbulb), but this is another very strong indicator of a chemical reaction. The most common observation of this is a flame.

Heat and light together (a)

A burning candle

Heat and light together (b)

Burning a wood splint.

3) Formation of a precipitate

A precipitate is a solid that appears after two liquids are mixed together. If a solid appears when you mix two liquids together, you have formed a new substance and a chemical reaction has taken place. Precipitates can be any color, but will always only be a solid. If allowed to sit for a while, the precipitate will likey settle to the bottom of the container in which it was formed. This is another very strong indicator of a chemical reaction.

Formation of a precipitate

A 1.0 M solution of silver nitrate (AgNO3) is added to a solution of sodium chloride (NaCl).

This picture was taken about a half hour after the precipitate was formed. Notice the white solid at the bottom of the test tube and the nearly clear appearance at the top of the liquid level.

4) Color change

A color change can indicate a chemical change. However, this is not a very strong indicator of a chemical reaction alone. There are quite a few examples of situations where you will see a color change and not have a chemical reaction (painting or coloring something, etc). But, when seen with one of the above indicators, it adds proof that a chemical change is occurring.

Color change

A few drops of a colorless solution of phenolphthalein is added to a colorless solution of 0.2 M sodium hydroxide (NaOH).

5) Temperature change

A temperature change can also indicate a chemical change. However, this is also not a very strong indicator of a chemical reaction alone. There are quite a few examples of situations where you will observe a temperature change and not have a chemical reaction (warming water in a microwave, a light bulb getting hot, water freezing, etc). But, when seen with one of the three strong indicators above, it adds proof that a chemical change is occurring.

Going beyond...

If you would like to pursue this topic a bit more, then the following web link will take you to the Department of Chemistry at Frostburg State University for a more extensive list of indicators of chemical change.

http://antoine.frostburg.edu/chem/senese/101/reactions/symptoms.shtml