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Chemically Pure Substances…Samacheer, Grade VIII, Chapter : Elements and Compounds around us


The discourse on pure substances and mixtures have been a long-standing one among scientists. Different people interpret the term ‘pure’ differently. A pure substance can be defined as any single type of material that has not been contaminated by another substance. A scientist does not share a common person’s notion of unadulterated substances as pure substances. A scientist is more concerned about the molecular level arrangement of substances. For students, this is a bit abstract and a cause of confusion, as they would hold their native understanding. In order to make sense of this concept, it needs explicit and elaborate discussions. In chemistry, a pure substance has a definite composition. It can be a compound or a single element. An element is a pure substance that cannot be separated into simpler substances by chemical or physical means.

Pure Substances

Display a set of ten objects with a combination of elements, compounds and mixtures in all three physical states. Example: Inflated balloon, distilled water, tap water, salt, sugar, milk, charcoal, graphite powder, iron fillings, iodine flakes, aluminium sheet and copper wire. 

Ask students to classify the objects into three categories (solids, liquids and gases) – this can be used to check their understanding on how materials are classified based on their states. What properties of materials make it solid, liquid and gas (based on the arrangement of atoms – closely packed, loosely arranged). Discuss and arrive at the conclusion that all substances are made of atoms and molecules. 

There are millions of different substances in the world. All substances have one important thing in common: they are all made of tiny building blocks of matter known as atoms. To build on to the next concept, discuss which atoms make up each of these substances and write them on the board. 

E.g., water – hydrogen, oxygen; sugar – carbon, hydrogen and oxygen, etc.

Investigating Questions:

• What according to you (students) are pure among these? Give your reasons. (The answers may vary from adulteration, contamination to poisonous substances).
• How do you think a scientist would classify these substances?
• Would they have the same parameters as those you have considered?

A substance that has the same kind of molecules is considered a pure substance. Ask the students to have a look at these substances again and let them classify and separate the pure substances. There is a possibility that students may not put water, salt, sugar in the pure substance category. Hence stress on the word ‘molecule’.

Once they have separated pure substances, let them understand that there are two types of pure substances. Elements have only one type of atom throughout the substance. Discuss other examples of elements commonly found around us – gold, silver, copper, iron, etc. 

The other type of pure substances are compounds that have two or more elements combined in a fixed ratio. Salt – NaCl, sugar – C12H22O11, water - H2O are examples.

Compounds and Mixtures

A. Exploration - Separation of substances

Give students samples of salt solution, graphite and iron filling mixture, sugar and distilled water. Ask them to separate it using any physical method like magnetic separation or hand picking. Let the groups explore. Discuss on how they think they can split the above samples.

Once they have explored, ask them what they have observed and inferred. Mixtures are separated into its components by physical methods of separation – like boiling, magnetic separation, hand picking, filtration etc. This is because they are not chemically bound to each other.

B. Can we distinguish a pure substance from a mixture using its physical properties?

Provide each group with two samples of water – distilled and salt solution. Insert a thermometer into the boiling tube placed under a spirit lamp and ask students to observe.

The boiling point of both the solutions would be different.

Ask them to predict the reason for it and to arrive at a conclusion as to which is a mixture and compound. Adding a substance changes the physical properties of the compound. The boiling
point changes due to the addition of salt. 

Pure substances have a fixed property which is alter when another substance is mixed with it.

C. Card Game - Element, Compound and Mixture

To test the understanding of students, present them with picture cards and asking them to segregate it into elements, compounds and mixtures.
Pass out a set of symbol cards to each partner group of students. Tell students to sort the cards into two groups based on the symbols shown. Guide students to sort cards into pure substances and mixtures.
What makes pure substances different from mixtures? Students’ responses will vary but should include an explanation that pure substances have just one kind of shape whereas mixtures have several different types of clusters. Elements have just one kind of shape whereas compounds have several different shapes.

Classifying Elements based on its properties

Metals and Non-metals

Give them a sample of a few elements and ask them to list their observation… Based on the observations, classify substances into metals and non-metals.

Grade: 
8

Subject: 
Science

Term: Term 1