Evidence of a Chemical Change

Evidence of a Chemical Change

Middle School

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Learning Objectives

• Define and differentiate between chemical and physical changes.

• Identify the key signs that indicate a chemical reaction has occurred.

• Distinguish between reactants and products in a chemical reaction.

• Define a precipitate and recognize its formation as a sign of a chemical change.

Key Vocabulary

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Reactants

The starting substances in a chemical reaction that combine to form a new substance.

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Products

The new substances created as a result of a chemical reaction taking place.

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Precipitates

A solid substance that forms from a solution during a chemical reaction between two liquids.

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Chemical vs. Physical Changes

Chemical Change

• A chemical change, or chemical reaction, is a process that forms a completely new substance.

• The original substances are reactants, and the new substances formed at the end are called products.

• These changes, like frying an egg, are very difficult or impossible to reverse.

Physical Change

• A physical change is a type of change where no new substance is actually formed.

• These changes often just involve a change in the state of matter, like ice melting.

• Physical changes, like freezing water back into ice, are usually very easy to reverse.

Multiple Choice

Which statement best describes a key difference between chemical and physical changes?

• A physical change is never reversible, but a chemical change always is.

• A chemical change results in a new substance, while a physical change does not.

• A change in the state of matter, like melting, is an example of a chemical change.

• Both physical and chemical changes always produce a new substance.

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Evidence of a Chemical Change

• Observing certain signs can tell you that a new substance has formed.

• This includes a change in color or the production of gas bubbles.

• Light, a new odor, or a change in temperature are also signs.

• A solid (precipitate) forming or sound being produced are other clues.

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Multiple Choice

If you mix two liquids and notice the container gets warmer and bubbles start to form, what do these signs indicate?

• A chemical reaction has occurred.

• Only a physical change has taken place.

• The liquids have evaporated.

• The container is dissolving.

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Common Misconceptions About Chemical Changes

Misconception	Correction
Any change in appearance is a chemical change.	Changes in shape or state of matter are physical changes.
Chemical changes can always be easily undone.	Most chemical changes are not easily reversible.
Bubbles always mean a chemical reaction is happening.	Bubbles can also be from boiling, which is a physical change.

Multiple Choice

Which of the following observations describes a physical change, not a chemical change?

• A precipitate forming in a beaker.

• Ice melting into water.

• Wood burning and turning into ash.

• A liquid giving off an odor after being mixed with a solid.

Multiple Choice

In the reaction where sodium and chlorine chemically combine to form table salt, what are the reactants?

• Sodium and chlorine

• Chlorine and table salt

• Table salt

• Sodium and table salt

Multiple Choice

When a clear liquid from a flask is poured into another clear liquid in a beaker, a yellow solid forms. How does this observation support the claim that a chemical reaction occurred?

• It shows a precipitate was formed, indicating a new substance.

• It shows that a gas was produced.

• It shows the liquids changed temperature.

• It shows only that the two liquids have mixed together.

Multiple Choice

A student combines clear crystals and a blue liquid. The liquid turns yellow, its temperature rises, and bubbles form. What is the best conclusion from these observations?

• The blue liquid evaporated, causing the color to change.

• The crystals dissolved, which is only a physical change.

• The multiple signs together strongly indicate a new substance was formed through a chemical reaction.

• The change in temperature is the only sign of a chemical reaction.

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Summary

• A chemical change forms a new substance and is not easily reversed.

• A physical change alters a substance's form, but no new substance is made.

• Starting materials are reactants, and new substances formed are products.

• A chemical change may be shown by a change in color, temperature, or odor.

• Producing gas, light, or sound can also signal a chemical reaction.

• A solid that forms from mixing two liquids is called a precipitate.

Poll

On a scale of 1-4, how confident are you about identifying the signs of a chemical change?

• 1

• 2

• 3

• 4