Chemical Changes

Chemical Changes

Middle School

​

Learning Objectives

• Define a chemical change and identify its reactants and products.

• List the common signs that indicate a chemical change has taken place.

• Differentiate between reversible and irreversible chemical changes using examples.

• Identify chemical changes in everyday examples like cooking and rusting.

Key Vocabulary

​

Chemical Change

A process where matter becomes a new substance with different chemical properties.

​

Reactants

The original substances that interact and combine in a chemical reaction.

​

Product

The new substances formed as the final result of a chemical reaction.

​

Precipitate

A solid substance that emerges from a solution during a chemical reaction process.

​

Combustion

A chemical reaction, like burning, that involves oxygen and produces heat and light.

​

​

What Is a Chemical Change?

• A chemical change, or reaction, forms a new substance with different chemical properties.

• This happens when bonds between atoms break and then new bonds are formed.

• Reactants are the starting substances, and products are the new substances created.

• Flammability and rustability are examples of key chemical properties.

​

Multiple Choice

In the chemical reaction where methane (CH₄) and oxygen (O₂) combine to form carbon dioxide (CO₂) and water (H₂O), what are the reactants?

• Oxygen and water

• Methane and oxygen

• Methane and carbon dioxide

• Carbon dioxide and water

​

​

Clues to a Chemical Change

• A way to remember the signs is: City, Girls, Like, To, Order, Pizza.

• C is for Color change, and G is for the production of a Gas (bubbles).

• L is for Light production, and T is for an unexpected Temperature change.

• O is for Odor production, and P is for the formation of a Precipitate.

​

Multiple Choice

According to the mnemonic provided, which of these is a primary sign of a chemical change?

• The substance dissolving

• A change in state (like melting)

• The formation of a precipitate

• A change in shape

Signs of Chemical Changes

​

Unexpected Color Change

• ​New molecules formed in a reaction radiate light differently.

• ​​This difference in light radiation causes us to see new colors.

• ​This is visible in fireworks or when autumn leaves change their color.

​

Temperature Change

• ​Energy is released or absorbed when chemical bonds break and form.

• ​​This makes the surrounding environment feel either hotter or cooler.

• ​Mixing baking soda and vinegar causes the temperature to drop.

​

Combustion or Burning

• ​This is a chemical reaction that gives off heat and light.

• ​​It happens when a substance reacts very quickly with oxygen.

• ​Burning wood or a candle are common examples of combustion.

Multiple Choice

Why does an unexpected temperature change occur during a chemical reaction?

• Because the new molecules radiate light differently.

• Because energy is released or absorbed during chemical bonding.

• Because a solid is formed that changes the heat capacity.

• Because a gas is produced, which cools the surroundings.

​

​

Exploring the Signs: Precipitates and Bubbles

Precipitates Form

• A precipitate is a solid that suddenly forms inside a liquid solution during a reaction.

• This new solid might settle at the bottom or make the entire liquid look cloudy.

• A common example is the formation of solid chunks in milk when it sours.

Bubbles Form

• The formation of bubbles is a clear sign that a gas is being produced.

• This indicates a new substance (a gas) has been created from the chemical reaction.

• You can observe this when an Alka-Seltzer tablet fizzes in a glass of water.

Multiple Choice

If you mix two liquids and the resulting solution becomes cloudy, what has likely formed?

• A solvent

• A reactant

• A gas

• A precipitate

​

​

Examples and Reversibility of Chemical Changes

• Chemical changes happen around us, like iron rusting or gas burning on a stove.

• Frying an egg, a banana rotting, or food spoiling are also chemical changes.

• Most are irreversible; you can’t turn ash from a burned log back into wood.

• Some can be reversed by another reaction, like using vinegar to clean tarnish.

​

Multiple Choice

Based on the text, how can the chemical change of a penny tarnishing be reversed?

• It cannot be reversed

• By using another chemical reaction, like placing it in vinegar

• By polishing it with a cloth

• By melting the penny

​

Common Misconceptions

Misconception	Correction
Any color change is a chemical change.	An unexpected color change signals a reaction. Mixing paints is a physical change.
Boiling water is a chemical change because bubbles form.	The bubbles are steam (gaseous water). This is a physical change of state.
Dissolving salt in water is a chemical change.	Dissolving is a physical change. Salt can be recovered by evaporating the water.
All chemical changes are permanent and can never be undone.	Some can be reversed by causing another chemical reaction.

Multiple Choice

A student notices a bicycle left outside has developed rust. Which statement best explains why this is a chemical change?

• The rain made the bicycle wet.

• The bicycle's shape was altered by the weather.

• The bicycle changed color from silver to reddish-brown.

• The iron in the bicycle frame reacted with oxygen to form a new substance, rust.

Multiple Choice

When baking a cake, numerous chemical changes occur. Which of the following observations from the baking process is the strongest evidence of a chemical change?

• The ingredients being mixed together in a bowl.

• The cake rising as gas bubbles are produced by the baking soda.

• The oven heating up the kitchen.

• The batter turning from a liquid to a solid.

Multiple Choice

A scientist wants to prove that burning natural gas is an irreversible chemical change. What evidence would best support this claim?

• Analyzing the color of the flame.

• Showing that the flame produces light and heat.

• Demonstrating that the products, carbon dioxide and water vapor, cannot be easily turned back into natural gas.

• Measuring the temperature change of the flame.

Multiple Choice

Analyze the process of food spoiling. How can you use the signs of chemical changes to determine that spoiled food is unsafe to eat?

• The food's container has changed shape.

• The food's temperature has changed to match the room.

• The food has been physically moved from the fridge to the counter.

• The food shows a change in odor, color, and potentially has gas production, indicating new, harmful substances have formed.

​

Summary

• A chemical change creates a new substance with different properties.

• Signs include changes in color, temperature, or producing light or gas.

• Combustion, or burning, is a common irreversible chemical change.

• Everyday chemical changes include cooking, rusting, and food spoiling.

• Most chemical changes are irreversible and cannot be easily undone.

• Reversing a change requires another, separate chemical reaction.

Poll

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

• 1 (Not confident)

• 2 (A little confident)

• 3 (Mostly confident)

• 4 (Very confident)