Water isn’t just a solvent; it carries many substances that are vital for life. Two key examples are:
Think of DO as the invisible air that fish and other aquatic organisms need to survive. Just like you need air to breathe, fish need oxygen dissolved in water.
DO is measured in milligrams per litre (mg L⁻¹). A healthy freshwater lake usually has DO levels between 6–8 mg L⁻¹. If DO drops below 5 mg L⁻¹, fish may start to suffocate.
| DO Level (mg L⁻¹) | Fish Health |
|---|---|
| >8 | Excellent |
| 6–8 | Good |
| 4–6 | Poor – fish may show stress |
| < 4 | Critical – fish may die |
Exam Tip: When answering questions about DO, mention the unit (mg L⁻¹) and explain why low DO is harmful to fish.
Many metal ions dissolved in water are essential for life. They act like tiny helpers that perform crucial tasks in organisms.
In aquatic ecosystems, fish eggs need calcium to develop their protective shells. Without enough Ca²⁺, eggs may fail to hatch.
| Mineral | Role in Life |
|---|---|
| Ca²⁺ | Bone & shell formation |
| Mg²⁺ | Enzyme co‑factor, muscle function |
| Fe²⁺/Fe³⁺ | Oxygen transport (haemoglobin) |
Exam Tip: When discussing metal ions, remember to use the correct chemical symbol and charge (e.g., Ca²⁺).
1️⃣ What unit is used to express dissolved oxygen in water?
2️⃣ Name two essential metal ions found in water and one role each plays in living organisms.
??
Answer these in your notebook to check your understanding!