Maharashtra Board Class 8 Science Chapter 6 Composition of matter PDF Download

Composition Of Matter

Can You Recall?

1. What are the various states of matter?

2. What is the difference in ice, water and steam?

3. What are the smallest particles of matter called?

4. What are the types of matter?

Can You Tell?

1. Classify the following matter into three groups. Cold drink, air, sherbat, soil, water, wood, cement.

2. What are the states of matter that you used as criteria for the above classification?

Try This

Take some mustard seeds in a transparent plastic jar. Thread a long thread at the centre of a big balloon by means of a needle and tie it tight.

Stretch this rubber diaphragm and fix it on the mouth of the jar by means of a rubber band. Pull the diaphragm up and down with the help of the thread first slowly, then with moderate force and then vigorously. Record your observation in the following table.

We saw in the previous standards that all the objects that we see around us and also those which cannot be seen are made of same or the other matter.

On the above experiment we give energy, less or more, to the mustard seeds by pulling the diaphragm up and down, making them move differently. The particles in the solid, liquid and gaseous states of matter have movement some what similar to that.

An intermolecular force of attraction acts between the particles (atom or molecules) of matter. The extent of the particle movement is determined by the strength of this force. The particles of solid are very close to each other and vibrate at their fixed positions. Due to this, solids get properties like definite shape and volume and also high density and non-compressibility.

The strength of intermolecular force is moderate in the liquid state. Though it is not strong enough to fix the particles in definite position, it is strong enough to hold them together. As a result, liquids have definite volume. However they have fluidity and their shape is not definite but changes in accordance with the container. The intermolecular force is very weak in gases. Therefore the constituent particles of gases move freely and occupy all the available space. Consequently gases have neither definite shape nor definite volume.

6.2 Physical State Of Matter: Submicroscopic Picture

Teacher's Note

In ice, water and steam, only the space between particles changes. The particles stay the same. Like how air takes the shape of any balloon.

Exam Trick

Remember: Solid particles are stuck together like students in a line. Liquid particles move freely like students in a classroom. Gas particles fly everywhere like birds in the sky.

Points To Remember

Solids have definite shape and volume because particles are very close.

Liquids have definite volume but no definite shape because particles are far apart.

Gases have no definite shape or volume because particles are very far apart.

Intermolecular force is strongest in solids and weakest in gases.

Teacher's Note

When ice melts to water, the particles just move farther apart. No new matter is made. Just like when you spread out toys that were in a box.

Exam Trick

Remember the phrase: Solid = Fixed, Liquid = Fills Container, Gas = Fills Room. This tells you the key property of each state.

Points To Remember

Compressibility means you can squeeze something into less space.

Solids cannot be compressed because particles are already tightly packed.

Gases can be compressed easily because particles are far apart.

Liquids can be compressed a little bit.

The state of matter depends on how much energy the particles have.

6.3 Characteristics Of States Of Matter

Can You Tell?

Write the composition of the following materials by means of chemical formulae and classify them accordingly.

This is the second method of classification of matter. In this method the criterion used for classification of matter is chemical composition of matter. We have seen in the previous standard that matter is classified into three types element, compound and mixture by considering whether the smallest particles of matter are similar or different and what are they made of. All the smallest particles (atoms/molecules) in an element or a compound are alike, however, the smallest particles in a mixture are of two or more types.

The smallest particles of an element contain identical atoms. For example, each molecule of oxygen contains two oxygen atoms in bonded state. The smallest particles (molecules) of a compound are formed by joining two or more types of atoms to each other. For example, each molecule of water contain two hydrogen atoms joined to one atom of oxygen. The smallest particles of a mixture are atoms/molecules of two or more elements/compounds. For example, the main constituent molecules of the mixture namely, air are N2, O2, Ar, H2O, CO2. Similarly, the mixture (an alloy) brass contain atoms of copper (Cu) and zinc (Zn), while bronze contain atoms of the elements copper (Cu) and tin (Sn).

The figure 6.4 shows a schematic submicroscopic picture of the types of matter, namely, element, compound and mixture and also their characteristics.

Teacher's Note

Air is a mixture, not a compound. This is why air can have different amounts of water on different days. But water is always the same.

Exam Trick

Remember: Element = one substance only. Compound = two or more joined together. Mixture = two or more can be separated easily.

Points To Remember

An element has only one kind of atom or molecule.

A compound has atoms of two or more elements joined by chemical bonds.

A mixture has two or more substances that are not chemically joined.

In a compound, the parts are always in the same ratio.

In a mixture, the parts can be in different ratios.

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