Notes For All Chapters – General Science Class 8
1. Introduction
- Acids and bases are important chemicals found in our daily life.
- Acids taste sour and turn blue litmus red, while bases taste bitter and turn red litmus blue.
- Indicators help identify acids and bases by changing color.
2. Acids
(A) What are Acids?
- Acids are substances that release hydrogen ions (H⁺) when dissolved in water.
- They have a pH less than 7.
(B) Properties of Acids
- Sour taste (e.g., lemon juice, vinegar).
- Turn blue litmus red.
- React with metals to form hydrogen gas (H₂).
- React with carbonates to form carbon dioxide (CO₂).
- React with bases to form salt and water (neutralization).
(C) Types of Acids
| Type | Definition | Examples |
|---|---|---|
| Natural Acids | Found in plants and animals | Citric acid (lemon), Acetic acid (vinegar), Lactic acid (curd) |
| Mineral Acids | Made in laboratories | Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃) |
| Strong Acids | Completely ionize in water | HCl, H₂SO₄, HNO₃ |
| Weak Acids | Partially ionize in water | Acetic acid (vinegar), Carbonic acid (soda water) |
3. Bases
(A) What are Bases?
- Bases are substances that release hydroxide ions (OH⁻) when dissolved in water.
- They have a pH greater than 7.
(B) Properties of Bases
- Bitter taste and soapy texture.
- Turn red litmus blue.
- React with acids to form salt and water (neutralization).
- React with oils and fats to make soaps.
- Some bases are strong, while others are weak.
(C) Types of Bases
| Type | Definition | Examples |
|---|---|---|
| Strong Bases | Completely ionize in water | Sodium hydroxide (NaOH), Potassium hydroxide (KOH) |
| Weak Bases | Partially ionize in water | Ammonium hydroxide (NH₄OH), Magnesium hydroxide (Mg(OH)₂) |
4. Indicators
(A) What are Indicators?
- Indicators are substances that change color in acids and bases.
(B) Types of Indicators and Their Color Changes
| Indicator | Color in Acid | Color in Base |
|---|---|---|
| Litmus Paper | Red | Blue |
| Methyl Orange | Red | Yellow |
| Phenolphthalein | Colorless | Pink |
5. Neutralization Reaction
(A) What is Neutralization?
- When an acid reacts with a base, they cancel each other and form salt and water.
(B) General Reaction
Acid + Base → Salt + Water
(C) Examples of Neutralization
- Hydrochloric acid + Sodium hydroxide → Sodium chloride + WaterHCl + NaOH → NaCl + H₂O
- Sulphuric acid + Potassium hydroxide → Potassium sulphate + WaterH₂SO₄ + KOH → K₂SO₄ + H₂O
6. pH Scale
(A) What is the pH Scale?
- The pH scale measures acidity and basicity on a scale of 0 to 14.
(B) pH Scale Values
| pH Value | Type of Solution | Example |
|---|---|---|
| 0 – 6 | Acidic | Lemon juice (pH 2), Vinegar (pH 3) |
| 7 | Neutral | Water (pH 7) |
| 8 – 14 | Basic | Soap (pH 9), Baking soda (pH 8.5) |
7. Importance of Acids and Bases in Daily Life
(A) Uses of Acids
- Hydrochloric acid (HCl) – Used in digestion, cleaning metal surfaces.
- Sulphuric acid (H₂SO₄) – Used in car batteries, fertilizers, paints.
- Nitric acid (HNO₃) – Used in making explosives, fertilizers.
- Acetic acid (Vinegar) – Used in food preservation and cooking.
(B) Uses of Bases
- Sodium hydroxide (NaOH) – Used in making soaps and detergents.
- Calcium hydroxide (Ca(OH)₂) – Used in whitewashing walls.
- Magnesium hydroxide (Milk of Magnesia) – Used as an antacid to reduce stomach acidity.
8. Everyday Examples of Neutralization
| Situation | Acid | Base | Neutralization Product |
|---|---|---|---|
| Indigestion (Acidity in stomach) | Hydrochloric Acid (HCl) | Magnesium Hydroxide (Mg(OH)₂) | Water + Salt |
| Ant bite treatment | Formic Acid | Baking Soda (NaHCO₃) | Salt + Water |
| Soil treatment for farming | Acidic Soil | Lime (Ca(OH)₂) | Neutral Soil |
| Toothpaste on acidic food | Acids in food | Toothpaste (Basic) | Salt + Water |
9. Corrosion and Rusting
(A) What is Corrosion?
- Corrosion happens when metals react with oxygen, water, or acids.
- Example: Iron reacts with water and oxygen to form rust.
(B) Prevention of Corrosion
- Painting or coating metals with oil.
- Galvanization (coating iron with zinc).
- Using stainless steel instead of iron.
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