What Are Chemical Reactions?

🔄 Quick Recap

In the introduction, we saw that chemical reactions are happening all around us - from milk spoiling to iron rusting. But what exactly is a chemical reaction?

📚 Understanding Chemical Reactions

A chemical reaction is a process where one or more substances (called reactants) are converted into one or more different substances (called products).

During a chemical reaction:

• Chemical bonds are broken in the reactants
• New bonds are formed to create the products
• The atoms rearrange, but no atoms are created or destroyed
• The resulting product has different properties from the reactants

Physical Change vs. Chemical Change

It's important to understand the difference:

Physical Change	Chemical Change
No new substance forms	New substance forms
Original substance keeps its properties	New properties appear
Usually reversible	Often irreversible
Examples: melting ice, cutting paper	Examples: burning wood, milk souring

🔍 How to Identify a Chemical Reaction

How do we know if a chemical reaction has occurred? Look for these signs:

Let's explore these signs through some simple experiments!

🧪 Activity Time! Observing Chemical Reactions

Activity 1: Burning Magnesium Ribbon

This classic experiment clearly shows a chemical reaction taking place.

Materials Needed:

• Magnesium ribbon
• Sandpaper
• Tongs
• Watch glass
• Spirit lamp or burner
• Safety glasses

Procedure:

1. Clean the magnesium ribbon with sandpaper (this removes the oxide layer)
2. Hold it with tongs
3. Burn it in the flame
4. Collect the ash in a watch glass

Observations:

• The magnesium ribbon burns with a bright white flame
• A white powder forms (magnesium oxide)

What Happened?

The magnesium reacted with oxygen in the air to form magnesium oxide:

Magnesium + Oxygen → Magnesium oxide

Safety Note:

Never look directly at burning magnesium as the bright light can harm your eyes. Always wear safety glasses!

Activity 2: Zinc and Dilute Acid

Materials Needed:

• Zinc granules
• Dilute hydrochloric acid or sulfuric acid
• Test tube or conical flask

Procedure:

1. Place zinc granules in a test tube
2. Add dilute acid
3. Observe what happens
4. Carefully touch the test tube

Observations:

• Bubbles form around the zinc (gas is produced)
• The test tube feels warm (heat is released)

What Happened?

The zinc reacted with the acid to produce hydrogen gas and zinc chloride:

Zinc + Hydrochloric acid → Zinc chloride + Hydrogen gas

⚖️ Quick Comparison: Signs of Chemical Reactions

Sign	What It Means	Example
Color change	New substances with different colors have formed	Copper turning green
Gas formation	A gaseous product is being created	Bubbles when acid reacts with metals
Temperature change	Energy is being released or absorbed	Feeling warmth from a reaction
Precipitate formation	A solid product forms in a liquid solution	Cloudy appearance when mixing solutions
Odor change	New substances with different smells have formed	Rotten egg smell from sulfur compounds

🧠 Memory Trick

Remember "COPS" to identify chemical reactions:

• Color change
• Odor production
• Precipitate formation
• Sound or gas production/temperature change

💡 Key Points to Remember

• Chemical reactions create new substances with different properties
• The total mass of substances doesn't change (atoms are neither created nor destroyed)
• Chemical reactions can be identified by observing changes like color, temperature, gas formation, or precipitate formation
• Chemical reactions involve breaking old bonds and forming new ones

🤔 Think About It!

1. Why do we clean the magnesium ribbon with sandpaper before burning it?
2. What would happen if you performed the zinc and acid experiment in a sealed container?
3. Can you identify three chemical reactions that you've observed today?

🔜 What Next?

Now that we understand what chemical reactions are and how to identify them, we'll learn how to represent them using chemical equations in the next section.