Experimental design — practice question

Baking powder helps cakes to expand. If water is mixed with baking powder, carbon dioxide gas is given off. The longer baking powder has been kept, the less carbon dioxide it gives off when water is added. Plan an investigation to determine which of two different samples of baking powder has been kept for a longer time. Your plan should include: • the apparatus needed • the method to use • the measurements to take • the variables to control • how to use the results to work out which sample has been kept for longer. You may draw a diagram to help answer the question.