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The Science of Soap Bubbles

One of the major objectives of our ‘Science and Scientific Enquiry’ track is to ingrain the practices and processes of the Scientific Method:

  • Observation

  • Building hypotheses that might explain what was observed

  • Designing experiments to validate our hypotheses

  • Conducting the experiments, recording and analysing the data

  • Building a model that explains what was observed

Today's session on Sep 9, 2018 at the APL Global School, facilitated by Dr Utpal Chattopadhyay (and supported by Dr Hari Krishna), started with a video of water striders skipping across the surface of water. This was followed by a discussion around how the striders managed to stay afloat, and what mechanisms could be at work. Students divided themselves into three groups and a challenge was given to the class: Using available materials (straws, balloons, string, rubber bands, paper clips and toothpicks), design a structure that can float on water.

Students presented their plans for the design and each plan was discussed and debated in the class. After the discussions, the teams worked on building the structures.

The students quickly realized that all their experimental arrangements/models floated on water easily and that they needed do work with something that would ordinarily sink in water to investigate if something else was helping the striders stay afloat. Through discussions they realized that steel paper clips could be used for this purpose. They verified that if the clips are put inside water, then these clips sink. When placed with care, they figured it was possible to make them float on the water surface.

Thus were the students guided towards understanding that flotation can happen because of buoyancy, as also due to surface tension. The rest of the day’s activities and discussions were focused on this newly discovered phenomenon of surface tension.

One manifestation of surface tension can be seen where the water surface acts like a delicate skin that is holding the water inside a drop or in a container. To illustrate this, we created a simple demo. A glass is filled with water to the brim. The top is levelled off using a coaster. Rupee coins were dropped gently into the glass. At the beginning of the demonstration, students were asked about their estimates about the number of coins that can be dropped into the glass before the water starts overflowing. Everyone said a value below five. Once the first five coins were dropped, students were surprised that water level started to rise above the edges of the glass, but there was no overflow. More and more coins were added, and the water level rose precariously above the edges of the glass. In the end about 30 coins were added before the water spilled from the glass.

This demo seemed to challenge the intuitions of all the students and further piqued their curiosity about this phenomenon.

This was followed by an activity undertaken by each student. The students were given a 2 rupee coin, affixed using tac-it to the table in front of them. They were given droppers. The challenge was to add as many drops of water onto the surface of the coin without causing overflow. Each student was asked to repeat the experiment multiple times. At the end of the activity, all the readings from each student were recorded. In total, there were 50 data points, and the mean value was 43 drops of water. Through this activity students understood how to carry out controlled experiments and collect/ analyse the data. For example, they could clearly identify patterns in the experimental data that they had gathered. Some data, they concluded, were “out of range”. They could figure out - without being told about it - the idea of the average number of drops.

Once it was evident to the students that surface tension is a force, the origins of this force was explained to them. Although the atomistic and molecular view of matter is not within their grade levels they understood the molecular origins of the surface tension with ease. Further discussions about the effect of temperature and mixing other liquids to water on the surface tension was designed to reinforce this understanding.

During lunch, some opted to play in the grounds while others opted to stay back in class for some lively and friendly discussions. The post-lunch activities started from where the discussion ended before lunch. Students were given soap solutions (instead of water) and were asked to repeat the earlier experiment. They figured that the surface tension varies with the type of the solution. They found that while the average number of drops with water was 43, that with soap water was around 30. They concluded that the surface tension of soap water is less than that of water.

Surface tension exploration thus moved seamlessly to soap films and bubbles. Soap bubbles are not new to the students. But the vast amount of scientific concepts and phenomena involved in soap bubbles was new to them. The exploration of this science behind soap bubbles started with two simple questions:

  • Why do bubble always form spherical shapes while floating in air?

  • What causes bubbles to pop/burst?

Using different demonstrations and activities, the students understood the science behind soap bubbles.

All through the session, students were paying attention, asking questions and challenging the assumptions and hypothesis under discussion. Here is an example: On the question of whether bubbles pop due to evaporation of water in the soap film, one of the students challenged that hypothesis by pointing out that when bubbles burst after landing on a table, one can see a small liquid patch. Is it right then, he asked, to say bubbles burst due to evaporation? This resulted in a great discussion on a detailed mechanism of popping of bubbles. It was further discussed that if bubbles were bursting due to evaporation, then will they survive for a longer time in humid conditions? It was left to the students to try an experiment in their bathrooms to test their hypothesis on this aspect!

Towards the end of the session students were able to apply their understanding of the concepts and attempted to predict the behaviour of soap films under “new” conditions. Many predicted the behaviour correctly.

By exploring different facets of surface tension, students understood what it means to think like a scientist and experienced the processes which are essential to the Scientific Method.

All participants also received a Diary of sorts to record their thoughts and observations over the course of the Program. We also briefly introduced the skill of Estimation/ Guesstimation, which we will pick up in subsequent sessions...

Below is a gallery with additional pictures from the session!