Properties and States of Matter

Indicators of Chemical Change

Everything around us is made of matter. Matter can change in different ways. Sometimes a change is only physical, like when ice melts into water. It is still the same substance.

Other times, matter changes into a new substance. This is called a chemical change. Scientists look for clues, called indicators, to decide if a chemical change has happened.

In this lesson, you will learn the main signs of a chemical change:

• Unexpected color change
• Temperature change
• Gas production
• Formation of a precipitate

These clues do not mean we can always see the tiny particles that make up matter, but they help us notice that something new may have formed.

1. Unexpected Color Change

One sign of a chemical change is an unexpected color change. This means a material changes color in a way that is not caused by simply mixing colors together.

For example, when iron rusts, it changes from gray to reddish-brown. Rust is a new substance. That color change is a clue that a chemical change happened.

Be careful: not every color change is chemical. If you mix blue paint and yellow paint to make green, that is usually just mixing colors, not making a new substance.

2. Temperature Change

Another sign is a temperature change. During some chemical changes, energy is released or taken in. This can make something feel warmer or cooler.

For example, some hand warmers get hot when the materials inside react. That warming is evidence that a chemical change is taking place.

If two substances are mixed and the container suddenly feels cold or warm, that may be a clue that a chemical reaction happened.

3. Gas Production

A third sign is gas production. Sometimes a chemical change makes a gas, and we can see bubbles or fizzing.

For example, when vinegar and baking soda are mixed, they fizz and bubble. Those bubbles are a gas being produced. Gas production is a strong clue that a new substance formed.

But remember: bubbles do not always mean a chemical change. When water boils, it also makes bubbles, but that is a physical change because the water is still water.

4. Formation of a Precipitate

A precipitate is a solid that forms when two liquids are mixed together. The new solid was not there at first. Its formation can be a sign of a chemical change.

For example, if two clear liquids are mixed and a cloudy solid appears, that solid is a precipitate. This tells us a new substance may have formed.

You can think of a precipitate as a surprise solid showing up during a reaction.

How Chemical Changes Are Different from Physical Changes

It is important to tell the difference between a physical change and a chemical change.

• Physical change: The substance stays the same, even if its size, shape, or state changes.
• Chemical change: A new substance is formed.

Here are some examples of physical changes:

• Ice melting
• Tearing paper
• Boiling water
• Breaking a crayon

Here are some examples of chemical changes:

• Rust forming on metal
• Baking a cake
• Wood burning
• Vinegar and baking soda fizzing

Important Idea

Sometimes one clue by itself is not enough. Scientists often look for more than one indicator before deciding that a chemical change happened.

For example, if a mixture changes color and gets warmer, that is stronger evidence than just one clue alone.

Worked Example 1: Easy

Question: A student mixes vinegar and baking soda. The cup starts to fizz and bubbles rise. What indicator of chemical change does the student observe?

Step 1: Look for a clue from the reaction.

The clue is fizzing and bubbles.

Step 2: Match the clue to an indicator.

Bubbles and fizzing mean gas production.

Answer: The student observes gas production, which is an indicator of chemical change.

Worked Example 2: Medium

Question: Two clear liquids are mixed. After a few seconds, a cloudy solid appears. What indicator of chemical change is this?

Step 1: Notice what changed.

A solid formed even though both starting materials were liquids.

Step 2: Name the indicator.

This is the formation of a precipitate.

Answer: The indicator is formation of a precipitate.

Worked Example 3: Medium-Hard

Question: A nail is left outside in the rain. After many days, it becomes reddish-brown. Is this a possible sign of chemical change? Why?

Step 1: Look for one of the indicators.

The nail changed from gray to reddish-brown.

Step 2: Decide if the color change is unexpected.

Yes. The nail did not just get wet. It formed rust, which is a new substance.

Answer: Yes. The unexpected color change is a sign of chemical change.

Worked Example 4: Challenge

Question: A pot of water is heated on the stove. Bubbles form as the water boils. Is this definitely a chemical change?

Step 1: Notice the clue.

There are bubbles.

Step 2: Think carefully about what is happening.

Boiling changes liquid water into water vapor. It is still the same substance: water.

Step 3: Decide whether it is chemical or physical.

This is a physical change, not a chemical change.

Answer: No. Even though bubbles appear, boiling water is a physical change because no new substance is formed.

Tips for Identifying Chemical Change

• Ask: Did a new substance form?
• Look for unexpected color change.
• Check for a temperature change without heating or cooling from outside.
• Watch for gas production, like fizzing or bubbling.
• See if a precipitate forms when liquids are mixed.
• Remember that some signs can also happen during physical changes, so use more than one clue when possible.

Let’s Review

The main indicators of chemical change are:

1. Unexpected color change
2. Temperature change
3. Gas production
4. Formation of a precipitate

When you see these signs, a chemical reaction may have happened. That means the matter may have changed into a new substance.

Brief Summary

A chemical change happens when a new substance forms. We can look for clues such as an unexpected color change, a temperature change, gas production, and the formation of a precipitate. These indicators help us tell chemical changes apart from physical changes.