General Science • Topic 2 of 4

Chemistry Basics

Matter is made of atoms; an element has one kind of atom, a compound two or more chemically combined. Water is H₂O, common salt is NaCl, and CO₂ turns lime water milky. Acids have pH below 7, bases above 7, neutral = 7.

The pH scale

0 7 14 ACID NEUTRAL BASE (Alkali) lemon, vinegar pure water soap, baking soda
pH below 7 = acidic, exactly 7 = neutral, above 7 = basic. Acid + base → salt + water.

Facts SSC repeats

  • Air composition: ~78% nitrogen, ~21% oxygen, ~1% others — nitrogen is the most abundant.
  • Common formulae: water H₂O, common salt NaCl, carbon dioxide CO₂, methane CH₄.
  • Symbols: gold = Au, silver = Ag, iron = Fe, sodium = Na, potassium = K.

✅ Solved examples

1. What is the chemical formula of water?
H₂O.
2. Common salt is chemically known as?
Sodium chloride (NaCl).
3. What is the pH of a neutral solution?
7.
4. Which gas is most abundant in the atmosphere?
Nitrogen (about 78%).

✏️ Practice — try these, take hints as needed

1. The gas that turns lime water milky is?
Exhaled gas.
—
—
Carbon dioxide (CO₂)
2. A substance with pH 3 is an ___.
Below 7.
—
—
Acid
3. The chemical symbol for gold is?
From "aurum".
—
—
Au
4. Acid + base gives salt and ___.
Neutralisation.
—
—
Water
5. Oxygen makes up about ___ % of air.
One-fifth.
—
—
21%

📝 Topic test — 8 questions

Auto-graded with full solutions; saved to your dashboard. Use the calculator and formula sheet (top-right) any time.

Loading questions…
← Physics Basics Biology & Human Body →