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Chapter 7 — Particulate Nature of Matter 99 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 z Take a stick of chalk (Fig. 7.1a) and break it into two pieces (Fig. 7.1b). z Continue breaking the chalk till it becomes difficult (a) to break it further by hand. z Grind the small pieces of chalk thus obtained (Fig. 7.1c) using mortar and pestle. z Observe the fine powder of chalk with a magnifying (b) glass (Fig. 7.1d). z What do you observe? z Each tiny grain you observe is still a speck of chalk. (c) Is every speck of this fine chalkpowderstillcomposed of the same substance, or has it changed into something else on breaking (d) or grinding? Fig. 7.1: (a) A stick of chalk; (b) The chalk stick Recall Curiosity, Grade 7 chapter ‘Changes Around broken into two pieces; Us: Physical and Chemical’— is grinding chalk a physical (c) A piece of chalk change or a chemical change? You learnt that the chalk ground into fine powder; (d) A close-up view of does not change into a new substance on grinding. It is a chalk powder under a physical change in which only the size of each speck of magnifying glass chalk has reduced further. Reprint 2026-27