Light is excluded from a water culture experiment in order to________

Analyze the question: In a water culture experiment, plants are grown in a nutrient solution (without soil) to find out which mineral elements the plant needs. The question asks why light is kept away from the solution, which is usually done by covering the container with black paper or wrapping it in foil.

Concept: The nutrient solution contains dissolved minerals such as nitrates, phosphates and potassium. If light reaches this solution, algae (simple photosynthetic organisms) can grow in it. Algae would:

• compete with the plant for the mineral nutrients,
• use up the oxygen in the solution at night,
• change the composition of the solution, which makes the results unreliable.

Checking the options:

• A. Prevent overheating of the roots: Overheating is not the main purpose. The aim is to control algae, not temperature.
• B. Enable roots to grow straight down: Roots respond to gravity (geotropism), not to the absence of light, so this is not the reason.
• C. Prevent ionization of water molecules: Light does not ionize water in this setup, so this is wrong.
• D. Prevent the growth of algae: Correct. Without light the algae cannot photosynthesize, so they cannot grow in the solution.

Tip: Remember the phrase "Water culture = Dark bottle, No algae." Whenever you see a nutrient solution in an experiment, think: light + minerals = algae, so the container is covered to keep the experiment fair and accurate.