Teaching sciences normally follows one specific pattern. First students listen to lectures and they do the practical work in a lab course afterwards. Therefore, students are expected to be well prepared for lab work and to know exactly what is expected to happen during the experiments. All the teaching I experienced as a student at high school and university was structured this way, so were many courses I taught myself at university level.
With this kind of structure lab teaching is a bit like watching a movie you already know the end of. The lectures spoiler what is supposed to happen during the lab. Therefore, it might be a good idea to sometimes turn this model upside down.
What if students would get to carry out experiments, without knowing all the theory beforehand? I believe it could fuel a lot of curiosity and scientific thinking among them. With their experimental results students could try to come up with their own theories, which could later be discussed in a lecture or seminar where the true mechanism is revealed.
The iodine clock reaction is a good example that could be easily turned upside down. The students know what chemicals they start with and the color changes observed due to the presence of iodine and starch give them a hint about what is happening. Thus, they can try to work out the chemical reactions themselves.
Also many organic synthesis experiments could be carried out this way. Students should be told the starting materials, the synthesis procedure and possible hazards. With the help of chemical analysis of the products, they can try to figure out the chemical reaction afterwards.
Another simple example could be the demonstration of the existance of exothermic and endothermic reactions. Students could measure the temperature changes caused by different known chemical reactions in a calorimeter and draw conclusions about the occurrence of endothermic and exothermic reactions from their results.
Of course not all experiments are suitable to be turned upside down like this, especially when many hazards are involved. But it is worth to try this where it is possible. This way lab courses can become more similar to actual scientific work, where the outcome of an experiment is not always known in detail either beforehand. And most of all, their outcome is not spoilered.