Weathering: physical, chemical, and biological

Weathering means rocks break or change at their original place. Exam tip: remember the difference between weathering and erosion.

Direct answer: Option A, change only in size and pieces. Physical weathering is mechanical disintegration: a large rock is broken into smaller fragments without a major change in its chemical composition. Temperature expansion and contraction, pressure release, salt growth and frost action can produce cracks and pieces. Option A is correct because the main change is physical size, shape or number of fragments. Option B is wrong because complete chemical change belongs to chemical weathering, such as oxidation or carbonation. Option C is wrong because fossil formation is a biological and geological preservation process, not the defining result of physical weathering. Option D is wrong because new layered soil does not form immediately and soil formation involves weathering, organisms, organic matter, water and time. Physical weathering may help soil formation later, but it does not itself mean instant layered soil. Memory cue: physical weathering changes form, not chemical identity.

Chemical weathering involves reaction of minerals with water and gases. Exam tip: connect the change with composition.

Roots, organisms and microbes help break or alter rocks. Exam tip: identify biological weathering with living organisms.

Temperature change makes rocks expand and contract, causing breakage. Exam tip: treat thermal action as physical weathering.

The direct answer is A, frost weathering. When water enters a crack and freezes, it expands. The expanding ice presses against the sides of the crack. Repeated freezing and thawing gradually widens the crack until pieces of rock break away. This is a physical or mechanical weathering process because the rock is broken into smaller pieces without a necessary change in its chemical composition. Option A is correct because it specifically names the action of freezing water in rock cracks. Option B, oxidation, is a chemical reaction commonly involving oxygen and minerals such as iron; it may produce rust-like changes, not crack expansion by ice. Option C, carbonation, occurs when carbonic acid reacts with minerals, especially limestone; it is also chemical weathering. Option D, humus formation, is the production of dark organic matter from decayed plants and animals in soil, not the breaking of rock by ice. The exact chain is water enters crack, temperature falls, water freezes and expands, pressure increases, crack widens, and rock fragments detach. Memory cue: freeze, expand, crack, break.

Salt crystals exert pressure and weaken rocks. Exam tip: connect it with arid and coastal areas.

In oxidation, minerals react with oxygen. Exam tip: remember colour change in iron-rich minerals.

Iron minerals react with oxygen and become rust-like. Exam tip: reddish-brown colour is a sign of oxidation.

Water and carbon dioxide form a weak carbonic acid. Exam tip: remember its effect on limestone.

Direct answer: A is correct. Carbonation is a chemical weathering process involving carbon dioxide and water. Carbon dioxide dissolves in rainwater or soil water and forms weak carbonic acid, represented simply as carbon dioxide plus water forming carbonic acid. This weak acid reacts readily with calcium carbonate, the main mineral in limestone, and can dissolve it, especially where water moves through cracks and joints. Repeated dissolution may create caves, sinkholes and other karst features. Option A is correct because limestone is comparatively soluble in weak carbonic acid. Option B is wrong because limestone does react chemically. Option C is wrong because limestone is not made only of sand; it is mainly calcium carbonate, often formed from shells or other carbonate material. Option D is wrong because constant heat is not the essential cause of carbonation. Memory cue: carbon dioxide plus water attacks calcium carbonate, so carbonation strongly affects limestone.

Some minerals dissolve in water and weaken the rock. Exam tip: connect solution with chemical weathering.

In hydration, minerals absorb water and may expand. Exam tip: understand water absorption as chemical change.

In hydrolysis, water changes minerals into new forms. Exam tip: remember feldspar changing into clay.

Roots exert pressure while growing and enlarge cracks. Exam tip: treat root action as biological weathering.

Burrowing increases entry of air and water into rock and soil. Exam tip: remember the role of small organisms too.

Mosses and lichens release chemicals that weaken the surface. Exam tip: write them as examples of biological weathering.

In deserts, day-night temperature range causes rocks to expand and contract. Exam tip: link high temperature range with physical weathering.

Repeated freezing and thawing enlarges cracks. Exam tip: remember water and below-freezing temperature for frost action.

Water is a major medium for chemical reactions. Exam tip: link humid climate with chemical weathering.

Heat and moisture speed up chemical reactions. Exam tip: remember tropical humid areas for chemical weathering.

Dry and cold regions favour temperature range and freeze-thaw action. Exam tip: identify weathering type from climate.

Weathering prepares the parent material of soil. Exam tip: treat weathering as the first base of soil formation.

Parent rock gives rise to soil and residues after weathering. Exam tip: connect parent rock with the source of soil.

Water in cracks can freeze or react with minerals. Exam tip: cracks are weak points for weathering.