Matter is constantly changing. Some changes are physical changes while others are chemical changes.

Watch this video for a brief introduction on the physical and chemical changes in matter.

The chart below summarizes the differences between the physical and chemical changes in matter.

During a physical change, only the form or appearance of the matter is altered. For example, the melting of ice is a physical change.

Solid	Liquid	Gas
Ice is a solid. The molecules of H2O are packed very close together.	When ice melts, it becomes a liquid. In a liquid, the molecules are not as closely packed as when they are in solid form, but the molecules have not changed in composition. Each molecule is still made up of two atoms of hydrogen and one atom of oxygen.	When water is heated, evaporation occurs. The molecules spread out even further than they were in liquid form. Water vapor is the gaseous form of H2O.

In a chemical change or reaction, the original substances' chemical composition is changed. The atoms in the original substances, called the reactants, are rearranged to form completely new substances, which are called the products. The properties of the products are different from the properties of the reactants.

Chemical changes or reactions have been around for centuries. One example of a chemical change that you are probably familiar with is fireworks. The history of fireworks can be traced back thousands of years to China during the Han dynasty. Fireworks have two essential reactants. One reactant is a chemical that is rich in oxygen. Different types of chemicals produce different colors when they are burned. For example, a compound called lithium carbonate, Li₂CO₃, can be used to produce a red color while a compound called sodium nitrate, NaNO₃, can be used to produce a yellow color. The second reactant is a chemical that acts as a source of fuel. Different fuels burn at different rates and temperatures.

Source: Rusting before and after, Corrosionist.com

Iron, Fe, is a metal on the periodic table. Iron has been used since ancient times by humans. The first iron used by humans probably came from meteorites that fell to Earth. The Iron Age, or the time when iron was used as trade, began about 1300-1200 BC. At this time, iron became cheap and available enough to replace bronze for trade purposes. Artifacts, such as iron beads found in Egyptian graves, date back to 5000 BC. Iron rusts every easily, so artifacts made of iron are more rare than artifacts made of silver or gold. Iron rusting is an example of a chemical reaction. During the process of rusting, oxygen reacts with iron. The rust that is produced is a new substance that has different properties than the properties of the iron and oxygen.

The diagram below is a visual model of what happens to the atoms during the process of rusting.

Chemical changes occur when bonds between atoms are broken and/or made. We cannot see the actual breaking or formation of the bonds between the atoms, but we can see other evidence of chemical changes.

Some signs (or evidence) of chemical change are

• production of a gas (bubble formation),
• change in temperature,
• formation of a precipitate (a solid),
• change in color, and
• production of a new odor.

Watch the following video to see the evidence of chemical changes in a science laboratory.

Practice

Let's see what you have learned.

Analyze the following descriptions and pictures then, decide if a chemical change has taken place. Describe how you knew it was or was not a chemical change.

Change	Chemical Change (Yes or No)	Why or Why Not
When making chocolate milk, the white milk turns brown when the chocolate syrup is added.	YesNo
A glass of milk is left out in the sun, and it curdles. The chunky solid forms at the bottom of the glass.	YesNo
An antacid tablet is dropped into water, and bubbles are formed.	YesNo
	YesNo
Water is heated on the stove and the temperature of the water rises from 22°C to 100°C and bubbles are formed in the pan.	YesNo

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Check Your Answer

Change	Chemical Change (Yes or No)	Why or Why Not
When making chocolate milk, the white milk turns brown when the chocolate syrup is added.	YesNo	Making chocolate milk is not an example of a chemical change. The milk turned brown because the chocolate syrup is brown. Chocolate milk is a mixture.
A glass of milk is left out in the sun, and it curdles. The chunky solid forms at the bottom of the glass.	YesNo	Soured or curdled milk is an example of a chemical change. The solid that forms at the bottom of the glass is a precipitate.
An antacid tablet is dropped into water, and bubbles are formed.	YesNo	An antacid reacts with water and gas is produced. The gas that is produced is carbon dioxide.
	YesNo	The atoms on the left side of the equation, the reactants, were combined to form the compound on the right side of the equation.
Water is heated on the stove and the temperature of the water rises from 22°C to 100°C and bubbles are formed in the pan.	YesNo	Boiling water is a change in state from a liquid to a gas. Changes in state or phase changes are considered physical changes.