Solving Problems in Science International Junior Science Olympiad

Solving Problems in Science International Junior Science Olympiad (IJSO) Dr. Yu-San Cheung yscheung@cuhk. edu. hk Department of Chemistry The Chinese University of Hong Kong 1

Scientific Method Modifying theory as needed Observation Hypothesis Experiment Analysis Theory Prediction Experiment Law 2

Precision vs. Accuracy Precision(精密度) -representing the agreement between repeated measurements of the same quantity Accuracy(準確度) -representing the agreement between a measured value and the true quantity 3

Significant figure: Multiplication and division: keep the smaller number of significant figure Decimal place: Addition and subtraction: keep the smaller number of decimal place 4

Exercise Determine the answer for (a) to (f) below. Use the correct number of significant digits in your answer. a. 7. 2 mm x 1. 56 mm = b. 99. 5 cm 2 x 0. 0084 cm = c. 63. 6 g/ 2. 7 cm 3 = d. 288 g/16. 9 cm 3= e. 0. 001 g + 5. 3 g + 162 g = f. 32. 58 cm 3 – 5. 2 cm 3 = 5

Control Experiment A method of scientific investigation to demonstrate a cause and effect hypothesis. For example, a phenomenon occurs after a certain treatment is given to a subject, and that the phenomenon does not occur in the absence of the treatment. 6

Variable in Control Experiment Dependent variable • Quantity to be measured; • Quantity which is thought to be affected during the experiment; • Results of the experiment. Independent variable • Quantity to be varied; • Quantity which is thought to affect the dependent variable(s); • Quantity which can be manipulated or controlled. Controlled variable • Quantity kept constant or unchanged. 7

Experiment Set-up Control Group vs Experiment Group Control Group • Set-up • Procedures • Control Variables Experiment Group • Set-up • Procedures • Control Variables • Independent Variables 8

A Photosynthesis(光合作用) Problem The following experiment is designed to find out whether light is necessary of photosynthesis. A. What is the hypothesis of this experiment? B. If starch is present in region A but absent in region B after exposing the plant to sunlight for several hours, what would your conclusion be? C. Which region is the control? Explain briefly. 9