The introduction will be 2 or more sentences long. The first sentence should state the purpose. Think to yourself.... what will the lab data determine? If you look at the data you are collecting along with the calculations or analysis questions you are answering, then you will be able to write a good purpose. The next sentences should focus on summarizing the method. It includes the specific chemicals you are using and a summary of the specific technique(s). If there is a special equation you are using, include that too. In other words, show the link between what you will do in the lab to what you are supposed to be determining/attempting to support by doing the lab.
Sample Introduction:
Determine the effect of temperature on the rate of the reaction between hydrochloric acid and magnesium. The reaction of 1.0 M hydrochloric acid and magnesium will be run at 4 different temperatures and the rate of production of hydrogen gas will be measured using a pneumatic trough and a stopwatch.
Two key points:
1. Do NOT include learning goals (e.g., unacceptable to use “to learn” or “to become familiar” or "to understand")
2. Revisit your purpose after completing the lab. Make any changes or revise/rewrite at the end of your lab. You will hopefully have a better understanding of the lab as you finish writing up the lab, so you will probably be able to fine tune the purpose.