Students investigated the factors affecting diffusion rate using agar blocks containing sodium hydroxide and phenolphthalein indicator, which makes the blocks pink. When these blocks are submerged in dilute hydrochloric acid, the acid diffuses inward, neutralising the alkali and turning the agar colourless.
Four agar blocks of different shapes and sizes were completely submerged in a beaker containing a high concentration of dilute hydrochloric acid.
Which of the following blocks will be the first to turn entirely colourless?
A cylinder with diameter 2.0 cm2.0\text{ cm}2.0 cm and length 6.0 cm6.0\text{ cm}6.0 cm
A cuboid with dimensions 1.2 cm×4.0 cm×8.0 cm1.2\text{ cm} \times 4.0\text{ cm} \times 8.0\text{ cm}1.2 cm×4.0 cm×8.0 cm
A sphere with diameter 1.8 cm1.8\text{ cm}1.8 cm
A cube with side length 3.0 cm3.0\text{ cm}3.0 cm
34 exam-style questions on OCR A A Level Biology Exchange surfaces. Each one has a worked solution and a mark scheme showing where the marks go.