The science of separation – 2
Avneesh Shukla
In the first part of this series (Separation Techniques – Part I – November 2016), we discussed some techniques that happen on a routine basis. In this article we will explore and learn about other techniques of separation of various substances by using easily available and local resources.
Can you think of an idea or method to separate water and oil?
Before going to the school, I worked on designing an alternate model/apparatus for separating water and oil. This reminded me of my college days where we separated two liquids using a separating funnel1. After a discussion with colleagues, I finally designed a model using a water bottle, a straw, some clay (for sealing hole) and a needle as shown in the picture.
When I arrived at the school the next day, the school teacher said, “I was also thinking about the separation of oil and water but I didn’t get anything.” What the teacher said motivated me since I realized that she was also trying to find out the process or method of separation. This showed that she was eager and curious to learn. Without losing time I showed her my apparatus and then both of us entered the classroom. The students were then asked, “Can we separate oil and water?” They responded that it is impossible to separate the two. Then I asked them to list out other substances that cannot be separated. They responded, “Kerosene oil and water, mustard oil and water, refined oil and water, petrol and water, honey and water.”
In an earlier lesson on separation techniques, the students had reflected on how it was not possible to separate chalk powder and soil. Taking that further, I asked the students if they could separate
• Soil and salt
• Sand and salt
• Salt from salty water (mixture of salt in water)
• Sugar from sugar water (sugar solution in water)
• Salt and Naphthalene ball
Almost in unison they said, “We can’t separate them.” At this point, the class teacher referred to the process of salt formation from sea water by asking, “How is salt separated from salty water of sea?” When I did not receive any concrete answer, I concluded this discussion by giving a clue:
• If we continuously boil the salty water or sugar solution, what will happen?
Separation of oil and water
We then moved ahead and repeated our question: “Can we separate oil and water?” The answers had changed but were still not clear. Some said it was impossible, others said it may be possible but that they did not know how. Then I introduced my apparatus and gave them oil (refined oil) and water and asked them to fill the apparatus with the substances. They were asked to shake it and then rest it for a minute. Soon, two layers were visible, i.e., one of water and other of oil (with oil forming the upper layer and water, the bottom layer). At the same time I raised a question, “Why do oil and water form two separate layers?”
They responded that….
• since oil is lighter than water, it forms the upper layer
Then we took some iron nails, some small-sized iron nuts and placed them in a beaker filled with water. We saw that both sank. Then I asked the students why even if the nuts were lighter than nails, did they both sink? At this point, the class teacher asked the students if they had seen huge blocks of ice float in sea/water. If we compare the weight of nails with that of the ice blocks, it will definitely be less than the latter, then why do nails sink? The students were quite confused and this was clearly visible on their faces. I explained to them that there is a simple rule that “objects/substances whose density2 is lower than water, float and objects whose density is greater than water, sink in it.” Since oil and ice have lower density than water, they float in water but nails and nuts have higher density, therefore they sink.”
After this digression, I came back again to the separation of oil and water. When we got the two clear layers of water and oil, I asked the students to open the cap of the bottle. They opened the cap and separated the upper layer of oil (after opening the cap of the bottle oil comes out from the straw and we collected it in a beaker) and only the lower layer of water was left inside the bottle. During the separation process, a spark was clearly visible in the eyes and faces of the students. Then I told them that in the laboratory we use an apparatus called “separating funnel” for separating the two liquids.
While demonstrating this technique I learnt that since water and oil are not fully immiscible, complete separation of oil and water could not happen. At that time, if we had used two immiscible liquids, i.e., one polar and one non-polar liquid (polar and non-polar liquids are immiscible in each other, i.e., water and diethyl ether, water and dichloro methane etc., could have played a better role.
In order that students understand this better, I divided the students into three groups and distributed material in groups as follows:
Group 1 – iron nails + iron nuts + Al foil + 2 beakers
Group 2 – sand + water + funnel + 2 beakers
Group 3 – soil + water + stones pieces + sand + funnel + 2 beakers
I then told them to mix all the given material in beakers and try to separate them. The first group separated the given material within half a minute and then I gave them some other materials (mentioned below) and told them to first mix and then separate them.
Group 1 – iron nails + iron nuts + Al foil + soil + sand + stone pieces + water + magnets + funnel + 2 beakers
The second and third groups prepared the mixture. I asked them to allow it to rest for a minute for sedimentation. When the impurities settled down, they could carefully separate the water by ensuring the presence of sediments inside the beaker. They separated the layer of water leaving behind the sediment inside the beaker (I explained that this process is known as “Decantation3”) but we found that the water still remained dirty with a mixture of sand and soil in it. The colour of the water too was brownish and black. The students however had no idea how to tackle this problem when the class teacher asked them what they did when they wanted clean water from a tap. The students replied that they tied a fine cloth to the tap.
This filtered the water.
The first group was playing with a magnet when I asked them the use of the magnet and why it was given to them. They had no answer to that. I explained that a magnet attracts nails and nuts. So, how can we separate the nails and nuts from the mixture of various substances having magnetic and non-magnetic substances or impurities? They replied that a magnet could be used to separate them because a magnet will attract only magnetic substances. I explained that this method was known as ‘magnetic separation’ since a magnet was used to separate the magnetic impurities.
Then I gave students some salt and asked them to mix it in the prepared mixture of sand, stone, soil in water. The students were asked to separate the contents, but the colour of the mixture was still brownish. Then I gave them a strainer. The students tried separation again but the impurities still remained. I introduced the simple filter paper and asked them to complete the separation. But the students found it difficult to arrange the filter paper in the funnel. Then I demonstrated the method of using a filter paper.
After using the simple filter paper three times, the students responded that the mixture still had some impurities since the colour remained brownish. I introduced the Whatman filter paper (it has finer pores than a simple filter paper) and told them to use that. After separating the mixture nearly three times, they got a colourless filtered sample.
Separation of salt from salt mixture (Evaporation)
I pointed out that the mixture also contained salt. How could we separate that? The students suggested that the water could be boiled till it evaporates. Then we arranged the following materials:
• Spirit & spirit lamp
• Matchstick
• Beakers
• Wire gauge
After arranging the material I asked the students to take a small amount of mixture (salt mixture in water) in a beaker and then evaporate it. When the entire quantity evaporated, the salt remained at the bottom and on the sides of the beaker. I explained, “We heat the salt mixture until the entire water evaporates. A similar method is used in the manufacture of simple salt or table salt. Sea water contains lots of salts and table salt is one of them. We store the water in big containers and keep them in the sun light. The water continuously changes to vapour from liquid because of the sunlight. The process of changing substances from liquid state to gaseous state is known as vaporization/evaporation. When all the water evaporates, the mixture of salt remains. By purifying and separating impurities through various methods, we get table salt.”
Separation of salt and naphthalene ball
Sublimation
The next step was to ask students if they could separate salt and naphthalene ball. Most of the students answered in the negative, while a few who said it could be done were not sure how. Then we arranged the following materials:
• Naphthalene ball salt • Funnel • Beaker • Cotton
After arranging the materials, I prepared a mixture of salt and naphthalene ball in its powdered form.
I asked the students to take some of this mixture in a beaker and heat it while keeping the beaker fully covered with the funnel. The corners were covered by cotton. After a few minutes, the cotton (which was inside the funnel) had the crystal of the naphthalene ball on them. A wave of happiness and excitement came over them. I explained, “The naphthalene ball converts directly into the gaseous state from the solid without converting into the intermediate liquid state. When its vapour cools, it changes again into solid state. This is known as Sublimation.”
Precautions during activity/experiment
• Evaporation, sublimation activities should be conducted in the presence of a guide or supervisor.
• Glass beakers can break so don’t keep them over the flame continuously.
• Don’t taste any mixture – it can be harmful to health or can react with our body.
• Before an experiment, clean all the apparatus and your hands to minimize the errors.
• After an experiment, wash your hands with soap or detergent.
• If it is possible use gloves and spectacles.
References
• SCERT textbook
• NCERT textbook
• School Rasayan-2, sandarbh article
• http://www.wikipedia.org
• https://www.youtube.com/watch?v=JeaxMKPM8Z8
• https://www.youtube.com/watch?v=tLmh_rMQu7M
• https://www.youtube.com/watch?v=O6s6Q3oWsgY
• https://www.youtube.com/watch?v=mP4Hgui-g6U
• https://www.youtube.com/watch?v=-HpRdpk1aXU
• https://www.youtube.com/watch?v=8bZl7mcG0Ew
- A separating funnel or separation funnel is a laboratory equipment which is mostly made of glass and used for separation of two liquids. It is cone shaped having a half circle on top. It has a stopper at the top and a tap at the bottom.
- Mass per unit volume is known as Density. Every matter has unique density. Refi ned oil has 0.913 to 0.920 gram per cm3 at 15°C and water has 0.9999 gram per cm3 at 0°C.
- Decantation is the process for the separation of mixtures, by removing a layer of liquid, generally one from which a precipitate has settled. The purpose may be either to produce a clean decant, or to remove undesirable liquid from sediment.
The author is with the Azim Premji Foundation. He can be reached at avneesh.shukla@azimpremjifoundation.org.
