Chemical Changes | Open Textbooks

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What is a Chemical Change?

A chemical change (also called a chemical reaction) happens when substances combine or break apart to form completely NEW substances with different properties. Unlike physical changes, chemical changes cannot be easily reversed - you can't "un-bake" a cake!

When a chemical change occurs, the atoms in the original substances rearrange themselves to form new substances called products.

Physical vs. Chemical Changes

Physical Change

🧊➡️💧

✓ Same substance, different form
✓ Usually reversible
✓ No new substances formed
✓ Examples: melting, freezing, cutting, dissolving

Chemical Change

🥚➡️🍳

✓ New substance(s) formed
✓ Usually NOT reversible
✓ Different properties than before
✓ Examples: burning, rusting, cooking, digestion

Signs of a Chemical Change

How can you tell if a chemical change has occurred? Look for these clues:

🎨

Color Change

A new color appears that wasn't there before (like a banana turning brown)

💨

Gas Production

Bubbles or fizzing occur (like vinegar and baking soda)

🌡️

Temperature Change

Heat is released or absorbed (like a hand warmer getting hot)

🔮

Precipitate Forms

A solid forms in a liquid mixture

💡

Light Produced

Light is given off (like a glow stick or fireworks)

👃

New Smell

A new odor is produced (like bread baking or milk spoiling)

Examples of Chemical Changes

🔥 Burning Wood

Wood + Oxygen → Ash + Carbon Dioxide + Water + Heat

Signs: Light, heat, color change, smoke (gas), ash (new substance)

🧪 Rust Forming

Iron + Oxygen + Water → Iron Oxide (Rust)

Signs: Color change (gray to orange-brown), new substance forms

🥛 Milk Spoiling

Lactose + Bacteria → Lactic Acid

Signs: New smell, texture change, taste change

🍞 Baking Bread

Dough + Heat → Bread + Carbon Dioxide

Signs: Color change, new smell, gas production (bread rises)

Key Difference: If you can easily reverse the change (like melting ice back to water), it's probably a PHYSICAL change. If you can't reverse it (like unburning wood), it's a CHEMICAL change.

Examples

Physical or Chemical?

Score: 0/8

Is this a physical change or a chemical change?

🧊

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Excellent Work!

You completed the change identification challenge!

Final Score: 0/8

Practice

Test your understanding of chemical changes.

Question 1: Which of the following is a sign that a chemical change has occurred?

A) The substance changes shape

B) A new smell is produced

C) The substance dissolves in water

D) The substance is cut into smaller pieces

Question 2: Why is cooking an egg considered a chemical change?

A) The egg changes from cold to hot

B) The egg changes shape in the pan

C) The proteins in the egg form new substances that cannot be changed back

D) The egg can be broken into pieces

Question 3: When baking soda and vinegar are mixed, bubbles form. The bubbles are evidence of:

A) A physical change only

B) A chemical change producing gas

C) Boiling

D) The substances dissolving

Question 4: Which process is a physical change?

A) A nail rusting

B) Wood burning

C) Ice melting

D) Food digesting

Check Your Understanding

Key Questions to Consider

What is the main difference between physical and chemical changes?
List three signs that indicate a chemical change has occurred.
Why can't you "unbake" a cake?
Is dissolving sugar in water a physical or chemical change? Why?

Real World Connection: Your body performs chemical changes all the time! Digestion is a chemical change - your body breaks down food into new substances that provide energy. Breathing involves chemical changes too, as your body uses oxygen to release energy from food.

Summary

⚗️

Chemical Change

New substance forms, usually irreversible

🔄

Physical Change

Same substance, different form

🔍

Signs

Color, gas, heat, smell, light

⚠️

Key Test

Can you reverse it? If not, likely chemical

Next Steps

Observe chemical changes in your daily life (cooking, rust, etc.)
Try the vinegar and baking soda experiment at home
Move on to learn about the solar system