Give an example each of a desirable and an undesirable chemical change? 

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Chemical changes involve changes in chemical composition. A desirable chemical change is the conversion of carbon dioxide and water to glucose and oxygen, in presence of sunlight. This reaction is more commonly known as photosynthesis and is carried out by green plants (which contain the chlorophyll pigment). This reaction maintains the balances of gases in our atmosphere, provides us with food (directly or indirectly), is the source of fuels (either fresh such as wood, or fossilized, such as coal or petroleum) and generates oxygen for our sustenance. 

An undesirable chemical change is rusting of iron and steel. Exposure to atmospheric elements, such as moisture, causes the formation of ferric oxide, which results in weakening of metal components, loss of luster and electrical conductivity. Rusting results in economic losses around the world.

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