Biological washing liquids are solutions that contain enzymes to help remove stains caused by biological molecules. One of these enzymes is lipase. These washing liquids are concentrated solutions and need to be diluted with water to reach their 'working' concentrations. A group of students was asked to find the 'working' concentration of the enzyme lipase for two different biological washing liquids, A and B. The recommended dilutions for washing fabrics using these biological washing liquids are: A – 20 cm³ of washing liquid in 5 dm³ water B – 35 cm³ of washing liquid in 10 dm³ water. The students found a website that lists the concentration of lipase in different biological washing liquids before they are diluted. These concentrations vary between 85 mg cm⁻³ and 155 mg cm⁻³. The students simulated fat-stained fabric by cutting 20 mm × 20 mm pieces of cotton fabric and soaking them in cooking oil. The fabric pieces were then allowed to dry. The students diluted both washing liquids according to the recommended dilutions, added a piece of fat-stained cotton fabric to each and measured the activity of lipase using a pH probe and stop-clock. The students recorded the time taken for the pH to decrease from 7.9 to 6.5. The pH decreases because fatty acids are produced from the hydrolysis of fats by lipase. [Figure 1.1] shows the apparatus the students used. Table 1.1 shows the results of two trials using each of the diluted washing liquids at the recommended temperature of 30°C. This is lower than the optimum temperature of lipase, which is 60°C. [Table 1.1]