Core5 mapped NDA questions
Build the idea
Build exam-ready understanding by connecting the core idea, formulas or facts, worked example and previous NDA questions. Focus on why each rule works, then test it under timed conditions.
Concept
Air is a mixture of gases (mainly nitrogen and oxygen); water is a compound essential to life. Their composition and properties are basic but frequently tested.
Water shows an anomalous expansion below 4 °C.
Key Points & Formulas
- Air ≈ 78% nitrogen, 21% oxygen, plus CO₂, argon and water vapour.
- Oxygen supports combustion and respiration; nitrogen is inert and dilutes oxygen.
- Water (H₂O) is a universal solvent; its density is maximum at 4 °C.
- Hard water contains calcium and magnesium salts.
Worked Example
Q. Why does aquatic life survive in frozen lakes during winter?
Water is densest at 4 °C, so the 4 °C layer sinks to the bottom.
Ice (less dense) floats on top and insulates the water below.
Water beneath stays liquid, allowing aquatic life to survive.