Egypt Primary 4 Science, Week 24: Models of two kinds of weathering

Weathering and erosion

Week 24 · Theme 4, Unit 4 Shifting Surfaces, Concept 4.1 Breaking Down and Moving Rocks (B) · 3 lessons

Models of two kinds of weathering

Discovery Education Science P4: Term 2, Unit 4, pp. 110–121

GoalUse chalk to model mechanical and chemical weathering, record the results, and compare the models with real rocks.

New words

dissolve · to mix into a liquid and seem to disappearacid · a liquid that can slowly eat away some rocks; vinegar is a weak acidobservation · what we see, measure or feel during a test

Explanation

Real weathering takes a very long time, so scientists build quick models with materials that change faster. Chalk is a good choice, because it is soft and, like limestone, it is attacked by acid. To model mechanical weathering, put a stick of chalk in a closed plastic jar with a few pebbles and shake it for two minutes. Collisions break it into pieces, but every piece is still chalk. To model chemical weathering, put a second stick in a cup of vinegar. Small bubbles rise and, after hours, the chalk gets smaller and softer. This time the chalk itself is changing. A grown-up does the pouring, and nothing is tasted, sniffed or eaten. A model is never exactly like the real thing, but it helps us see what happens.

Model	What we do	Real weathering
mechanical	shake chalk with pebbles	rocks knock and break
chemical	chalk in vinegar	acid eats rock like limestone

Worked examples

Mina plans a fair test. Both sticks of chalk are the same length, 8 cm, and from the same box. She changes only what is done to them, and she keeps the jar and cup the same size. She predicts the shaken chalk will break more.

Model	Observation
shaken with pebbles	6 pieces and dust
in vinegar for a day	bubbles; 7 cm; soft

Mina records. Shaken chalk: broke into 6 pieces plus dust, all still chalk. Chalk in vinegar: bubbles at once, and after one day it is 7 cm long and soft. It lost 8 − 7 = 1 cm.

Comparing. Both models changed the chalk and both made it smaller. In the shake model the chalk stayed the same material in smaller pieces. In the vinegar model the chalk itself was eaten away. Real rocks follow the same two patterns.

Class activity

Model both kinds (a grown-up pours the vinegar and wears goggles with you). Take two sticks of chalk of the same length and measure them. Shake one in a closed plastic jar with pebbles for two minutes and count the pieces. Put the other in a cup of vinegar; look at it after an hour and after a day and measure it. Do not taste or sniff. Do not eat the chalk. Wash your hands. Fill in a table and say which model changed the chalk more.

Practice

Which model shows chemical weathering: shaking chalk with pebbles, or chalk in vinegar?

A chalk stick is 8 cm long. After a day in vinegar it is 6 cm. How much did it lose?

Why do scientists use models of weathering?

Say one way the vinegar model is different from real weathering.