*Manitoba Curricular Connection: Grade 2 Science – Properties of Liquids, Solids, and Gases
Here’s how to teach your students how to make a simple salad dressing and a little bit of science as well.
Materials:
Measuring cups and spoons
250 ml Oil
75 ml Balsamic Vinegar
15 ml Water
30ml Dijon Mustard* or Honey
Approximately 25 lettuce leaves
3 shallow bowls
5 jars with lids
2 funnels
Keywords: emulsion, emulsifying agent, soluble, insoluble
Soluble: a substance that is able to be dissolved in liquid. E.g. sugar can be dissolved in water, therefore sugar is soluble.
Insoluble: a substance that is difficult to dissolve in liquid. E.g. oil does not mix well in water, it is insoluble.
Emulsion: a mixture where droplets of one liquid are suspended in another liquid – the two liquids do not mix well.
Emulsifying agent (or emulsifier): this is a substance that helps two liquids to mix together.
For the teacher to set up in advance:
- Make salad dressing by mixing 60 ml of vegetable oil, 30 ml balsamic vinegar and 30 ml Dijon mustard or honey in one of your jars. Put the lid on tightly and shake until mixed. Set aside for the lesson.
- Place 5 lettuce leaves in each of 3 shallow bowls, in a pile. Set aside for the lesson.
- Place each funnel in a glass jar with about 5 leaves in each funnel. Set aside for the lesson.
Method:
Say to the students “People say that oil and water don’t mix, do you know what that means?” After a few give suggestions, tell them that oil and vinegar are the same – they don’t mix. But we mix them together to make salad dressing! Explain what an emulsion is using the definition above.
Pour 60 ml of vegetable oil, and 30 ml of balsamic vinegar into a clean jar and put the lid on tightly. Show the students how the two liquids remain separate in the jar. Now, show them the dressing you made earlier, and tell them that oil and vinegar are mixed together in this jar. Ask them how to get the oil and vinegar in the new jar to mix. (If no one makes any suggestions you can prompt them. You want to have someone suggest shaking the jar up.) Shake the jar and put both jars side by side on your table. It will take about 5 minutes for the oil and vinegar to separate. Set it aside and remind the students to keep looking back at it.
Have you ever noticed what happens to salad that sits out or goes in the fridge after you put the salad dressing on? Students should respond with “it gets soggy” or “the leaves droop” or something like this. You can say “yes, the leaves get droopy – we call that wilting. Why do the leaves wilt?”
Tell the students you are going to add oil to one dish of leaves, water to another dish of leaves, and vinegar to the third dish of leaves. Before you do, have them make and record predictions of what will happen. Pour 15 ml of each liquid onto the leaves – in separate bowls. Let it sit for about ten minutes.
Meanwhile, return to your emulsion of oil and vinegar. The students should notice that the liquids are separating. Explain that this is because oil and vinegar are made of different kinds of molecules that attracted only to their own type of molecule.
Ask students for suggestions to get the oil and vinegar to stay mixed like in the first sample you showed them. Take a few suggestions before explaining that we need to add an emulsifier – define this word using the definition above. An emulsifier will be attracted to both the vinegar molecules and the oil molecules and will help them to stay mixed longer.
Add 60 ml of oil and 30 ml of balsamic vinegar into a third clean jar. Now add 30 ml of either Dijon mustard or liquid honey. Put the lid on tightly and shake it up! Have a careful volunteer shake up the mixture of just oil and vinegar as well. Sit them beside each other and assign some students to observe them and report back what happens.
Direct the students’ attention to the three bowls of lettuce leaves. Hopefully the leaves in the oil have wilted more than the other leaves. Explain that usually, lettuce is grown outside and, like other plants, the leaves need to protect themselves from the weather. Lettuce leaves have a natural thin coating of a wax-like substance called a cuticle to protect the leaves from absorbing too much water. But, oil can easily get through this coating which is why the leaves in the oil have wilted more.
Ask the students if, after doing these two experiments, it makes a difference which dressing we put on salad. After a few give answers, tell them you are all going to figure it out. In the first funnel, add dressing from the mixture that contains the Dijon mustard or honey. In the second funnel add dressing from the mixture that is just oil and vinegar. Put the jars beside each other and ask the students to watch what happens. The dressing without mustard should start dripping vinegar into the jar. The emulsified dressing should stay on the salad.
After finishing the demonstrations, have the class work together to write up the experiments and copy them, with labelled illustrations, into their garden journals. Finish the lesson by sharing a salad with the dressing you have made!
Check out our salad dressing page!
*Canada is the largest mustard producer in the world!
Teaching From the Garden 