Chemical Reactions and Equations

Karnataka · KSEEB · Class 10 · Science

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Central idea

A chemical reaction never creates or destroys a single atom; it only rearranges them into new groups, the way rearranging the same set of alphabet blocks can spell a completely different word. And exactly which atom moved to where, gaining or losing oxygen along the way, is what oxidation and reduction are really counting.

Central idea

Class 7 identified rusting and burning as chemical changes because new substances appear.

Remembering Class 7 and 9: new substances, made of rearranged atoms

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Clean a 3-4 cm strip of magnesium ribbon with sandpaper to remove its dull oxide coating, then hold it with tongs, well away from your eyes, and burn it over a flame, catching the ash in a watch-glass.

Catching a reaction in the act: burning magnesium

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Magnesium's dazzling flame is not the only kind of evidence.

More proof: colour, gas, and heat as clues

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Describing the magnesium reaction in a full sentence every time gets tedious, so chemists shorten it.

Writing it down: word and chemical equations

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Since atoms are only rearranged, never created or destroyed, a correct equation needs exactly the same number of atoms of each element on both sides.

Balancing: making the atom count match exactly, and saying what state everything is in

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Add water slowly to calcium oxide (quicklime) in a beaker, and the mixture doesn't just get wet, it grows hot enough to feel through the glass almost immediately, as calcium oxide reacts vigorously with water to produce calcium hydroxide (slaked lime) and a large amount of heat: CaO(s) + H2O(l) leads to Ca(OH)2(aq) + heat.

Combination reactions: two or more become one

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The heat given off when calcium oxide meets water is not a side detail, it has its own name. A reaction that releases heat as it forms its products is called an exothermic reaction.

Naming the heat: exothermic reactions

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Pale green ferrous sulfate heptahydrate crystals, before heating: on heating, they lose water, change colour, and decompose further, releasing sulfur dioxide and sulfur trioxide gas.

See it in a diagram

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Exothermic vs endothermic, side by side: heat flows out and the temperature rises for exothermic; energy flows in, needing heat, light or electricity, for endothermic.

Naming the opposite: endothermic reactions

The exact experiment described here: a clean iron nail (right) next to one dipped in blue copper sulfate solution (left) - now coated brownish-red with deposited copper.

Displacement reactions: a stronger element pushes out a weaker one

The exact reaction described here: mixing sodium sulfate and barium chloride solutions instantly produces a dense white precipitate of barium sulfate, while sodium and chloride stay behind in solution.

Double displacement: two compounds swap partners

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