NCERT Class 8 Science Curiosity Chapter 07 Particulate Nature of Matter PDF Download

Chapter 7: Particulate Nature of Matter

Probe and ponder

• Why is it possible to pile up stones or sand, but not a liquid like water?
• Why does water take the shape of folded hands but lose that shape when released?
• We cannot see air, so how does it add weight to an inflated balloon?
• Is the air we breathe today the same that existed thousands of years ago?
• Share your questions

You might have collected pebbles and stones from the sand while playing on a riverbank or a beach. Where do these pebbles, stones, and sand come from?

In the mountains, rocks gradually break down due to erosion. Rivers flowing through these regions carry along the eroded rock pieces. As the rivers flow, they continue to break down the rocks further into pebbles, stones, sand; and transport large quantities of them to the plains.

The bigger rocks are eventually broken down into finer grains of sand and clay. Is this grain the smallest unit of a bigger rock or can these grains of sand and clay be broken down further?

Let us find out!

7.1 What Is Matter Composed of?

Activity 7.1: Let us explore

• Take a stick of chalk (Fig. 7.1a) and break it into two pieces (Fig. 7.1b).
• Continue breaking the chalk till it becomes difficult to break it further by hand.
• Grind the small pieces of chalk thus obtained (Fig. 7.1c) using mortar and pestle.
• Observe the fine powder of chalk with a magnifying glass (Fig. 7.1d).
• What do you observe?
• Each tiny grain you observe is still a speck of chalk. Is every speck of this fine chalk powder still composed of the same substance, or has it changed into something else on breaking or grinding?

[Figure 7.1: (a) A stick of chalk; (b) The chalk stick broken into two pieces; (c) A piece of chalk ground into fine powder; (d) A close-up view of chalk powder under a magnifying glass, See in your textbook]

Recall Curiosity, Grade 7 chapter 'Changes Around Us: Physical and Chemical' - is grinding chalk a physical change or a chemical change? You learnt that the chalk does not change into a new substance on grinding. It is a physical change in which only the size of each speck of chalk has reduced further.

Teacher's Note

When you grind chalk, you are breaking it into smaller and smaller pieces, but you are not changing what chalk is made of. The chalk remains chalk all the way down. This is an example of a physical change, not a chemical change. Keep this distinction clear: grinding, cutting, or breaking something is always a physical change because the material stays the same.

These specks of chalk powder can be broken further into smaller particles by further grinding. Let us imagine that this process of grinding continues. Eventually, we would reach a stage where the chalk particles cannot be broken down any further. The tiny units obtained at this stage are the basic building blocks that the chalk was made up of.

This means that one whole piece of chalk was made up of a large number of smaller units. These units are called constituent particles of chalk. A constituent particle is the basic unit that makes up a larger piece of a substance or material. Just like chalk, the grains of sand and clay are not the smallest units of bigger rocks. These are also made up of a large number of their constituent particles.

Let us explore further!

Recall the dissolution of sugar into water to form a solution. What happens to sugar when it is dissolved in water?

Activity 7.2: Let us perform

• Fill a glass tumbler with drinking water.
• Put two teaspoons of sugar into it.
• Do not stir the water. Taste a small spoonful of water from the top layer.

Does the water taste sweet?

• Now, stir the water until the sugar dissolves completely (Fig. 7.2).
• Again taste a spoonful of water from the top layer.

What difference in taste do you notice? Does it taste sweet?

[Figure 7.2: Dissolving sugar in water, See in your textbook]

Safety first

Perform the activity under the supervision of a teacher or an adult. Never eat or drink anything unless asked to.

Since the top layer of water tastes sweet after dissolving sugar, it must be present in the solution. Do you observe any sugar particles in the solution?

Sugar particles can no longer be observed but their presence can be sensed by taste. When sugar dissolves in water, it breaks up into its constituent particles which cannot be broken down further. Each tiny grain of sugar is made up of millions and millions of such constituent particles.

Teacher's Note

When sugar dissolves in water, the sugar crystals break up into invisible particles that are too small to see. You cannot observe these particles with your eyes or even under a magnifying glass, but you know they are there because you can taste the sweetness. This shows you that matter can exist in forms far too tiny for us to see directly.

Key Points

• All matter around us is made up of tiny basic units called constituent particles, which cannot be broken down any further by physical means.
• When we break, grind, or dissolve a substance like chalk or sugar, we are not changing what the substance is - we are only changing its size or form, which is a physical change.
• Constituent particles are so small that they cannot be seen, even with a magnifying glass, but their presence can be detected through other ways, such as taste or the properties of the substance.
• Rocks, sand, clay, and other solid materials are all made up of large numbers of these tiny constituent particles joined together.

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