An experiment was set up to investigate how quickly water drains through two different substrate mixtures designed for urban green roofs: Pumice Substrate and Silt-Peat Substrate. Each container contained 300 g of dry substrate. First, water was added to make the substrate completely saturated. Then, 150 cm3 of water was poured onto the saturated substrate in each container and allowed to drain into a measuring cylinder. The volume of water collected in the cylinder was recorded every 4 minutes until all 150 cm3 had been collected.
The results are shown below:
Time (minutes)Volume of water collected (Pumice Substrate, cm3)Volume of water collected (Silt-Peat Substrate, cm3)495308135581215085161501122015013524150150 \begin{array}{|c|c|c|} \hline \text{Time (minutes)} & \text{Volume of water collected (Pumice Substrate, cm}^3\text{)} & \text{Volume of water collected (Silt-Peat Substrate, cm}^3\text{)} \\ \hline 4 & 95 & 30 \\ \hline 8 & 135 & 58 \\ \hline 12 & 150 & 85 \\ \hline 16 & 150 & 112 \\ \hline 20 & 150 & 135 \\ \hline 24 & 150 & 150 \\ \hline \end{array} Time (minutes)4812162024Volume of water collected (Pumice Substrate, cm3)95135150150150150Volume of water collected (Silt-Peat Substrate, cm3)305885112135150Name the independent variable in this investigation.
State what is meant by the term dependent variable.
Identify which of the following is a key controlled variable in this investigation:
Calculate the average rate in cm3\text{cm}^3cm3 per minute at which water drained through the Silt-Peat substrate over the entire duration of the drainage (24 minutes). Give your answer to 3 significant figures and show your working.
The student has a choice of two measuring cylinders to measure the drainage volume: Cylinder A has division marks every 0.5 cm3, while Cylinder B has division marks every 1.0 cm3. State which of these two measuring cylinders would give a more precise reading and explain why.
Substrates that drain very rapidly, like coarse perlite, have large air spaces and retain very little water. Suggest why plants find it difficult to grow in such substrates.
Silt-peat substrate has very small air spaces that easily fill with water, displacing air. Explain why plants find it difficult to absorb mineral ions and grow in waterlogged silt-peat substrate.