During an investigation into the effect of temperature on the rate of reaction between sodium thiosulfate and hydrochloric acid, a student measures the time taken for a black cross marked under the reaction flask to become obscured by the precipitate of sulfur.
State the piece of apparatus that should be used to measure this time.
141 exam-style questions on Edexcel GCSE Chemistry 8.1 Rates of reaction, covering 8.1.1 Measuring and interpreting rates of reaction, 8.1.2 Collision theory and the factors affecting rate, and 8.1.3 Catalysts and enzymes. Each one has a worked solution and a mark scheme showing where the marks go.