Separation Techniques – Filtration, Evaporation and Distillation - Science Lesson for Grade 7 Students

One day, Jordan accidentally poured oil into water. He was fascinated to see that oil was on top of water. “So, these two don’t mix. It is easy to separate them. I wonder how to separate things in a mixture if they are well combined,” he said to himself. Being a curious boy, he took his problem to his science teacher, Mr. Peter.

Jordan: Mr. Peter, air, sea water and petroleum are all mixtures of things, right?

Mr. Peter: Yes Jordan.

Jordan: Are there any methods to separate the ingredients of mixtures?

Mr. Peter: Oh yes. Good that you ask. There are indeed methods of separation. These methods of separation actually depend on the physical properties of the components.

Jordan: What are some of the common separation methods?

Mr. Peter: Well, there are some methods known as physical methods. You know why they are called physical methods? Because these processes do not involve chemical changes. Filtration, evaporation, and distillation are some methods to separate mixtures.

Jordan: Wow! Those are big words.

Mr. Peter: Don’t worry. I will explain. Filtration is used to separate an insoluble solid from the liquid in a mixture. This method uses a filter paper. This filter paper has tiny pores that allow the liquid to pass through. The insoluble solid particles that cannot pass through the filter paper are left behind.

Jordan: Oh! How interesting

Mr. Peter: Yes. The insoluble solid is called the residue. What is a filtrate? The liquid that passes through the filter paper is called the filtrate.

Jordan: I see. Are there any uses of filtration?

Mr. Peter: Oh yes. Filtration is a very useful process. We use it in our everyday lives. Also, in industries. In most countries, filtration is one of the stages of water treatment.

Jordan: Really? I didn’t know that. How can filtration be used in water treatment?

Mr. Peter: The raw water is filtered through layers of sand beds, which contain sand grains of different sizes. Filtration is an economical method used to remove solid silt and sand particles from water.

Jordan: Ok. I guess the sand filters will help to trap suspended particles while allowing clear water to run through.

Mr. Peter: Yes. Many industries such as the textile industry treat their waste water before recycling for further use. Water treatment filters out the chemicals, organic substances and minerals.

Jordan: Very interesting stuff. You also mentioned evaporation earlier.

Mr. Peter: Yes, evaporation is used to separate a dissolved solid (solute) from a liquid. We can use evaporation to separate salt from sea water. When the liquid in the mixture has completely evaporated, the residue is the solid salt.

Jordan: Ok. Are there any uses of evaporation?

Mr. Peter: In some parts of the world, salt is extracted from sea water. The sea water is allowed to flow into large flat areas called salt pans. The sun helps to evaporate the water. Salt is left behind.

Jordan: Some day I want to see these salt pans. How about distillation?

Mr. Peter: Distillation is used to separate a liquid from a solution. The liquid collected from distillation is called the distillate.

Jordan: Ok. How does it actually happen?

Mr. Peter: The mixture is heated in a round-bottomed flask until it boils. The distillate is collected in the conical flask. You know, sometimes boiling chips are used in distillation and heating. Boiling chips are small pieces of material such as porcelain chips or pieces of broken ceramic pots.

Jordan: What are the uses of distillation?

Mr. Peter: Distillation is a water purification process. It removes impurities such as heavy metals, bacteria and viruses.

Jordan: At home, my mother always buys distilled water from the super market.

Mr. Peter: There is also desalination. It is a process that removes salts and minerals from sea water. Thus undrinkable sea water becomes drinking water.

Jordan: One of my friends was stranded at sea when he went for fishing. They suffered a lot because all they could drink was the sea water. Imagine being surrounded by water and not having water to drink!

Mr. Peter: Yes. In desalination, first the sea water is boiled. The water will evaporate and condense leaving the salt behind.

Jordan: All very interesting information. Tomorrow I will come to ask about paper chromatography. Is that ok?

Mr. Peter: Of course, Jordan. You are welcome any time.