exothermic reaction of plaster of Paris in water

Plaster of Paris is white powder you mix with water and pour into a mould. You can make models with it while hospitals use it on broken limbs. When you mix it with water it gives off heat – which is a sign that a chemical reaction is taking place. In that case, maybe the amount of water affects the rate of the reaction.

What we did

We took three identical amounts of Plaster of Paris and placed a temperature probe in each. The probes were covered with ‘cling film’ to protect them. We added three different amounts of water to each, mixed them and recorded the temperature changes over the next hour.

Our results

This graph shows the changing temperatures of three different water-plaster mixtures. The blue trace had the least water added. The red trace had the most.

You will find some of the questions easier if you put these results into your data logging software. Click here to get the results.

Looking at the results

  • Choose one of the graph traces. Describe how the temperature changes over time.
  • The blue trace had the least water added. The red trace had the most. Which do you think is the optimum (or ‘best’) mixture for Plaster and water?
  • How does the amount of water added to the Plaster affect the highest temperature reached?
  • Would you say that the reactions are getting faster or slower over time?
  • Why might this reaction start off slow and speed up over time?At what times do the graphs peak?
  • Why might this be happening at different points?
  • The total volume of each experiment was different. How might this have spoiled our results?

What you can do

Is there a best amount of water to use to make plaster? Try our experiment using measured amounts of water. Record not just the temperatures but also the time the plaster took to set.

This page written up by Roger Frost. Idea and results by Laurence Rogers, Leicester University. Questions suggested by a Leeds teachers’ panel.

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