🔷 What is a Chemical Reaction?
A chemical reaction is a process where one or more substances (reactants) change into new substances (products) with different properties.
🧪 Example:
When hydrogen reacts with oxygen, it forms water:
Hydrogen + Oxygen → Water
This is a chemical change — atoms are rearranged, and new substances form.
🔬 Signs of a Chemical Reaction
You can tell a chemical reaction has happened if you observe:
| Sign | Example |
|---|---|
| Color change | Iron rusting (silver → brown) |
| Temperature change | Hand warmer gets hot |
| Gas formation | Bubbles in vinegar + baking soda |
| Precipitate (solid forms) | Milk + lemon juice curdles |
| Light or sound | Fireworks |
🧮 Chemical Equations
A chemical equation shows what happens in a chemical reaction using symbols and formulas.
🧪 General format:
Reactant(s) → Product(s)
🔁 If the reaction is reversible, we use a double arrow:
⇌
✅ Example of a Simple Chemical Equation:
Hydrogen + Oxygen → Water
In symbols:
H₂ + O₂ → H₂O ❌ (Not balanced!)
👉 Balanced equation:
2H₂ + O₂ → 2H₂O ✅
We need to balance equations so the number of atoms is the same on both sides.
This follows the Law of Conservation of Mass.
⚖️ Balancing Chemical Equations
Why balance? Because:
"Atoms are never created or destroyed in a chemical reaction."
Rules for Balancing:
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Write correct formulas for all reactants and products
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Count atoms of each element
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Add coefficients (numbers in front) to balance atom counts
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Do not change subscripts (e.g., H₂O stays H₂O)
🔢 Example:
Unbalanced:
CH₄ + O₂ → CO₂ + H₂O
Balanced:
CH₄ + 2O₂ → CO₂ + 2H₂O
✅ Now we have:
-
C: 1 on each side
-
H: 4 on each side
-
O: 4 on each side
🔄 Types of Chemical Reactions
| Type | Description | Example |
|---|---|---|
| Combination | A + B → AB | H₂ + Cl₂ → 2HCl |
| Decomposition | AB → A + B | 2H₂O → 2H₂ + O₂ |
| Single replacement | A + BC → AC + B | Zn + HCl → ZnCl₂ + H₂ |
| Double replacement | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl + NaNO₃ |
| Combustion | Substance + O₂ → CO₂ + H₂O (burning) | CH₄ + 2O₂ → CO₂ + 2H₂O |
🧠 Key Concepts to Remember
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Reactants → Substances you start with
-
Products → New substances formed
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Equation → Describes the reaction
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Balance → Make sure atoms match on both sides
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Conservation of Mass → Total mass stays the same
✅ Summary
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Chemical reactions create new substances
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Chemical equations show these reactions using formulas
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Reactions must be balanced
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Different types of reactions have different patterns
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No atoms are lost or gained — they just rearrange