Changes Around Us: Physical and Chemical

The changes could be in the size, shape, smell, or other property of the substance or object.

Changes in which only physical properties like shape, size, and state of substances change are called physical changes. In all these cases, although we observe a change in the appearance, no new substance is formed.

Chemical Changes

Changes in which one or more new substances are formed, are called chemical changes. New substances are formed through a process called chemical reaction. For example, carbon dioxide from reacts with lime water, and forms a new white-coloured substance (calcium carbonate) that is insoluble in water.

The chemical reaction involved in this change can be represented in short form as a chemical equation.

Calcium hydroxide (Lime water) + Carbon dioxide → Calcium carbonate + Water

The turning of lime water milky is also used as a test for carbon dioxide.

Rusting

In the rusting of iron, a new brown-coloured substance called rust is formed. Thus, rusting is a chemical change because it involves the formation of a new substance, iron oxide.

Combustion

When the magnesium ribbon is burnt, a new substance, magnesium oxide, is formed. So, the burning of magnesium ribbon also involves a chemical change. The burning of magnesium ribbon can be represented as

Magnesium (Ribbon) + Oxygen (Air) → Magnesium oxide (White powder) + Heat + Light

A chemical reaction in which a substance reacts with oxygen and produces heat and/or light is called combustion. Substances that undergo combustion reactions are called combustible substances. For example, wood, paper, cotton, kerosene, etc., are combustible substances.

The minimum temperature at which a substance catches fire is called its ignition temperature.

Some Slow Natural Changes

Weathering of Rocks

Sediments are formed by physical changes that break up large rocks into smaller pieces. Temperature changes due to climatic conditions, growing roots of trees, and even freezing of water within cracks in the rocks can cause them to break.

Water or chemicals present in water, can also react with the rocks and cause chemical changes in their composition. For example, an originally black-coloured rock called basalt, which contains iron, chemically changes to produce a red-coloured layer. The red colour is a result of the iron oxide produced when the rock surface is exposed for a long time to water or air containing water vapour. These physical and chemical changes in rocks are collectively called weathering, which eventually leads to the formation of soil.

Erosion

The sand is formed when rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water. This process is called erosion.

Erosion during a landslide is an example of a physical change. River rocks and pebbles often appear smoother due to the constant erosion caused by the flowing water. When the speed of the water or wind decreases, such as in an ocean or a lake, the material transported during erosion settles down at the bottom. These sediments harden over time and become new rocks. Most of these changes take place over thousands of years and cannot be reversed.