Which of the following instruments is used to determine the turbidity of water?

Step 1: Analyze the question The question asks which instrument measures turbidity. Turbidity is how cloudy or murky water is because of suspended particles such as silt, clay, and plankton.

Step 2: Explain the concept The more suspended particles in the water, the less light penetrates, and the less clearly you can see into it. Ecologists use this to study water quality and productivity in ponds, lakes, and rivers.

Step 3: Examine each option

• A. Hygrometer: measures humidity (water vapour in the air). It is not for water turbidity.
• B. Hydrometer: measures the density or specific gravity of liquids, for example in a battery or milk. It does not measure cloudiness.
• C. Secchi disc: a circular disc, usually black and white, lowered into water on a rope. The depth at which it just disappears from view shows how turbid the water is. A shallow disappearance depth means high turbidity, and a deep one means clear water.
• D. Rain gauge: measures the amount of rainfall.

Step 4: Answer The instrument used to determine turbidity is the secchi disc, which is option C.

Exam tip: Link each instrument to what it measures:

• Hygrometer → humidity (think "hygro" = moisture in air)
• Hydrometer → density of liquids
• Secchi disc → water clarity (turbidity)
• Rain gauge → rainfall
• Anemometer → wind speed
• Barometer → air pressure

Whenever you see "turbidity" or "water transparency", think Secchi disc.