Slow Natural Changes
Class 7 Science · Chapter 5: Changes Around Us: Physical and Chemical · NCERT Curiosity section 5.7
Learn
On a trek, Zoya and Kabir notice heaps of sand, soil and broken stones at the foot of a cliff, and smooth round pebbles in a stream. "Rocks look permanent," says Kabir, "but these are pieces of them." Such loose material is called sediment.
The slow breaking of rocks is weathering. Heating by day and cooling by night, water freezing in cracks, and tree roots growing into rocks split them: physical weathering. Water and air can also react with minerals: black basalt rock containing iron slowly gets a red coat of iron oxide, a chemical weathering.
Wind, flowing water and ice then carry the broken pieces away: this is erosion. River pebbles become smooth as they rub against each other. Over thousands of years, weathered rock mixed with the remains of plants and animals forms soil. These changes are very slow, but they shape mountains, valleys and beaches.
Explore: weathering or erosion?
- Sort each one.
Remember
- Weathering breaks rocks, physically (heat, ice, roots) or chemically (iron oxide on basalt).
- Erosion carries pieces away; over ages weathered rock helps form soil.
Quick check
Basalt rock turning red over many years is an example of …
- chemical weathering
- erosion
- combustion
- a reversible change
Answer
chemical weathering. Iron in the rock reacts with air and water to form iron oxide.
Think about it
Why are pebbles in a river round and smooth while rocks on a hillside are sharp?
One answer
River pebbles keep rolling and rubbing against each other and the river bed, so their edges wear away. Hillside rocks have broken recently and have not been rubbed.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.