LeSa21: primary-school science activities

Launched in 2002 by Astrid Kaiser from the University of Oldenburg, Germany, the LeSa21 website^w1 offers a range of experiments, resources and background information for primary-school science and humanities lessons. Materials for both teachers and their pupils are available on a wide range of topics (currently 93), including ‘The eye’, ‘bicycles’, ‘spring’, ‘health and disease’, ‘girls and boys’, ‘planets, the Moon and the stars’, ‘electricity’, ‘water’ and many others. These materials are developed mainly by university students who are training to be primary-school teachers.

Available in English, German and Spanish, the ‘Learning’ section is suitable for both children and teachers, and includes experiments, stories, poems, pictures, book recommendations and links to relevant websites for each of the topics.

The pages for teachers (in German only) contain information, didactical hints, literature and web links to background information and further resources on each of the topics. Pupils at the six schools involved in the project contribute to further German-language sections. There, they can share their own experiments and ideas on the topics, and discuss a selection of the topics in an online forum.

Below are some activities from the LeSa21 project, which could form the basis of a teaching unit on oil and water, including surface tension and the removal of oil spills, suitable for older primary-school children. The individual experiments and further materials on the topics can be found on the LeSa21 website^w1.

Soap has magic powers – surface tension

This experiment introduces the notion of surface tension.

Materials

• Two bowls of water
• Ground pepper
• A pin
• A spoon (the size is not important)
• Liquid soap or washing-up liquid

Procedure

1. Half fill both bowls with water.
2. Sprinkle a spoonful of pepper onto the water in the first bowl. Take a close look while you are doing this.
3. Let the pin float in the second bowl of water.
4. Add a drop of soap to each bowl.
5. What happens? Draw what you observe.

What happens?

Initially, both the pepper and pin float. One reason is that they are both very light; the other is that water has a kind of thin, invisible ‘skin’ on its surface. Light things do not break this skin, but are supported by it – some animals, e.g. water striders, can even walk on it (see image above). This thin skin is caused by surface tension.

When you added the soap drop, the pepper probably moved away from it and slowly sank. The pin would have sunk immediately.

Why is that? The soap breaks this thin skin of the water – you could also say that soap reduces the surface tension. As the skin disappears, there is nothing left to hold the pepper and the pin on the surface of the water, so they sink. Although this is how it may look, pepper and soap do not repel each other – the tension of the rest of the water (where the soap has not yet reached) pulls the floating pepper away from the soap.

Soap has magic powers – mixing oil and water

Materials

• Two glasses
• Water
• Food colouring or ink
• A spoon (the size is not important)
• Oil
• Liquid soap or washing-up liquid

Procedure

1. Half fill the glasses with water.
2. Add some food colouring or ink to one glass, and mix it with a spoon. Leave it for a while until the water is still again.
3. Add a spoonful of oil to each glass and observe what happens.
4. Add 1-2 drops of soap to each glass, and mix it with a spoon.
5. What do you think will happen? Observe carefully and draw what you can see.

What happens?

Oil and water usually do not mix well. The reason is that water molecules can form hydrogen bonds with each other and with hydrophilic (‘water-loving’) molecules, but not with oil-like hydrophobic (‘afraid of water’) molecules. In addition, oil is lighter than water; it has a lower density, so if you try to mix them, the oil layer stays on top of the water. If you add some soap and stir it for a while, the oil and water can be mixed together. If you leave it for a while, the oil and water will separate again.

Why is that? Soap contains special ingredients called surfactants, which reduce the surface tension of water and allow water and oil to mix. This makes soap a wonderful cleaning agent: not only water-soluble dirt but also oily dirt can be washed away into water.

Surfactant molecules have two different ends, as shown in the diagram (right). One end (the head) is hydrophilic; the other end (the tail) is hydrophobic. Soap contains many surfactant molecules, and each of them binds water at one end and oil at the other end, which is how they help water and oil to mix, and also bind to oily dirt.

After a while, the oil and water separate again, because the bonds between water molecules are more stable than those between the surfactant molecules and the oil or the water.

The oil spill

In April 2010, an explosion on the oil rig Deepwater Horizon in the Gulf of Mexico led to a catastrophic oil spill. As we go to press, an estimated 5000-100 000 standard barrels of oil per day have been leaking out for several months, the spill eventually reaching the southern coasts of the USA. Attempts to remove the oil included burning it and using chemical and mechanical binding agents.

Similar accidents have happened in Europe, too: in Autumn 2002, the oil tanker Prestige was involved in an accident off the Spanish coast and sank. Up to 63 000 tonnes of oil were spilt into the sea, and hundreds of kilometres of the coastline of northern Spain and France were polluted with oil. Many animals died: more than 18 000 oil-contaminated birds were counted. Many volunteers helped to clean the beaches, and special ships tried to stop the growing oil slicks.

Let’s find out what oil does to bird feathers and how we can remove the oil from the water.

Materials

• Several big glasses or china bowls
• Water
• Vegetable oil
• Feathers
• A spoon
• A paper coffee filter
• A straw
• Wooden sticks
• Fine sawdust
  

  
  
  

• Wood shavings
• Cat litter
• Sand
• Oil binding / absorbing powder (used for oil spills, can be bought online or in well-stocked paint shops or chemists)

Procedure

1. Pour some oil into a bowl and add some feathers.

See how they stick together and imagine what effect an oil spill would have on birds and other animals, once the oil reaches the shores.

How might oil spills be cleaned up in open water, to prevent them from reaching the shores?

We will test various methods of removing the oil from the water. Either ask the children to get into groups, with each group testing a different method, or let the whole class watch as you try out all methods together.

2. Take a new bowl, fill it with water, add some oil and stir. What happens?
3. Skim off the oil with a spoon.
   

   
   
   

4. Skim off the oil with a coffee filter.
5. Try to push the oil together with sticks.
6. Mix the oil with wood shavings. Now try to push the oil-soaked wood shavings together.
7. Does it work better with cat litter? If so, why?
8. Bind the oil with sawdust (it will float due to the oil’s hydrophobicity and lower density) and remove it from the water using a spoon or paper coffee filter.
9. Try to weigh down the oil with sand to make it sink.
10. Use oil binding / absorbing powder to bind the oil and remove it using a spoon.

Compare and discuss the results of the experiments. The children should realise that the best results are achieved with a combination of (mechanical and chemical) methods. They may also consider the factors affecting the efficiency of each method, e.g. the volume of removal agent used. Finally, they should notice that there will always be some oil left in the water at the end; in real-life situations, we have to rely on the ecosystem to deal with this remainder over time.

Which other methods of cleaning the water can you imagine? Let the children come up with some ideas for real-life ‘oil spill removal machines’. Ideas might include a ring of cat litter to stop the oil from floating away, plus mechanical diggers to remove the floating oil film.

Comprehension questions

1. What happens if you pour oil into water?
   Answer: oil and water don’t mix – the oil floats, forming iridescent streaks and / or grease drops.
2. How can you separate the oil from water?
   Answer: using a combination of the above methods, but it is difficult.
3. Is the water clean again after your experiment?
   Answer: no, some oil will always remain.
4. What is left in the water?
   Answer: oil, some of which is clumped together with cleaning agents.

Acknowledgements

The two experiments ‘Soap has magic powers’ are part of the book Chemie in der Grundschule (Chemistry in Primary School; Kaiser & Mannel, 2004). Published in German, it contains many other experiments on different topics.

Parts of ‘The oil spill’ experiment are from Kaiser (2009).