In this chapter, you will learn
- —Understand what acids and bases are and their properties
- —Learn how to test acids and bases using indicators like litmus paper
- —Understand the neutralization reaction and its products
- —Explore real-life applications of acids, bases, and neutralization
- —Learn how a fire extinguisher works using acid-carbonate reaction
What are Acids?
Acids are substances that taste sour. The word 'acid' comes from the Latin word 'acidus' meaning sour.
When acids are dissolved in water, they release hydrogen ions (H⁺). This is what gives acids their characteristic properties.
Properties of Acids:
- Taste sour (like lemon or vinegar)
- Turn blue litmus paper red
- Release H⁺ ions in water
- Have pH less than 7
Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Citric acid (in lemon), Acetic acid (in vinegar)
Exam Tip
Remember: Blue litmus turns RED in ACID. Memory trick: B-R-A (Blue-Red-Acid)
Types of Acids
Acids are classified into two main types:
1. Mineral Acids (Inorganic Acids):
- Prepared artificially from minerals
- Generally strong acids
- Examples: HCl (Hydrochloric acid), H₂SO₄ (Sulphuric acid), HNO₃ (Nitric acid)
- Cannot be consumed - dangerous!
2. Organic Acids:
- Found naturally in plants and animals
- Generally weak acids
- Examples: Citric acid (lemon), Acetic acid (vinegar), Lactic acid (curd)
- Safe in diluted form - found in food
Exam Tip
CBSE often asks to differentiate between mineral and organic acids. Remember: Mineral = Strong & Artificial, Organic = Weak & Natural
What are Bases?
Bases are substances that taste bitter and feel slippery or soapy to touch.
When bases are dissolved in water, they release hydroxide ions (OH⁻).
Properties of Bases:
- Taste bitter
- Feel slippery or soapy
- Turn red litmus paper blue
- Release OH⁻ ions in water
- Have pH more than 7
Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide Ca(OH)₂
Important: Water-soluble bases are called ALKALIS. All alkalis are bases, but all bases are NOT alkalis!
Exam Tip
Remember: Red litmus turns BLUE in BASE. Memory trick: R-B-B (Red-Blue-Base)
Common Mistake
Students often confuse alkalis and bases. Remember: Only bases that DISSOLVE in water are called alkalis.
Indicators - Testing Acids and Bases
Indicators are substances that show different colours in acids and bases. They help us identify whether a substance is acidic or basic.
Common Indicators:
| Indicator | In Acid | In Base |
|---|---|---|
| Blue Litmus | Turns RED | No change |
| Red Litmus | No change | Turns BLUE |
| Phenolphthalein | Colourless | Turns PINK |
| Turmeric | Yellow (no change) | Turns Red/Brown |
Exam Tip
Litmus paper is the most common indicator asked in exams. Blue→Red in Acid, Red→Blue in Base.
Neutralization Reaction
When an acid reacts with a base, they neutralize each other. This reaction is called neutralization.
ACID + BASE → SALT + WATER + HEAT
Example:
HCl + NaOH → NaCl + H₂O
(Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water)
Key Points:
- Acid and base cancel out each other's properties
- Products are always salt and water
- Heat is released (exothermic reaction)
- The resulting solution is neutral (pH = 7)
Exam Tip
Always write the complete equation with products (salt + water) in your answers. Don't forget to mention that heat is released!
Applications of Neutralization
Neutralization has many important applications in daily life:
| Application | Problem | Solution |
|---|---|---|
| Antacids | Excess HCl in stomach (acidity) | Take antacid (base) |
| Bee Sting | Formic acid injected (acidic) | Apply baking soda (base) |
| Wasp Sting | Alkaline substance (basic) | Apply vinegar (acid) |
| Acidic Soil | Soil too acidic for crops | Add lime/calcium hydroxide |
| Tooth Decay | Acid from bacteria | Toothpaste (mild base) |
Exam Tip
Bee sting vs Wasp sting is a very common exam question. Bee = Acidic (use base), Wasp = Basic (use acid)
Common Mistake
Students often confuse bee sting and wasp sting treatment. Remember: Bee starts with B, treat with Baking soda (Base)!
Fire Extinguisher - Acid + Carbonate Reaction
A soda-acid fire extinguisher uses the reaction between acid and carbonate to produce carbon dioxide (CO₂).
How it works:
- Fire extinguisher has two chambers
- Inner chamber: Sulphuric acid (H₂SO₄)
- Outer chamber: Sodium carbonate (Na₂CO₃)
- When inverted, acid mixes with carbonate
- Carbon dioxide (CO₂) is produced
- CO₂ does not support burning → Fire goes out!
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + CO₂ + H₂O
(Sodium Carbonate + Sulphuric Acid → Sodium Sulphate + Carbon Dioxide + Water)
Exam Tip
Remember the equation for fire extinguisher reaction. CO₂ is the key - it does not support burning.