2ºESO. Five signs that indicate there is probably a chemical reaction occurring

Color Change

In the presence of caustic soda and water, potassium permanganate, which is purple in color, reacts with sugar to form potassium manganate, which is green in color. As the reaction progresses, this new substance keeps reacting with sugar, forming manganese(IV) oxide, which is brown in color.

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Production of an odor

Acetic acid (vinegar) is added to iron filings. The two chemicals react to produce hydrogen sulphide, gas which smells like rotten eggs.

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Change of Temperature

Chemical reactions that absorb energy when ocurring (therefore, the temperature around the reaction diminishes) are called endothermic reactions.

Chemical reactions that release energy when ocurring (therefore, the temperature around the reaction increases) are called exothermic reactions.

Evolution of a gas (formation of bubbles)

zinc + hydrochloric acid  -> zinc dichloride + hydrogen (gas)

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Precipitate (formation of a solid)

A solution of lead(II) nitrate, Pb(NO3)2, is dropped into a solution of potassium iodide, KI, forming a brilliant yellow lead(II) iodide, PbI2, precipitate.

Pb(NO3)2 (aq) + 2 KI (aq) -> PbI2 (s) + 2 KNO3 (aq)

When sodium chloride (NaCl) is added to silver nitrate (AgNO3), a white precipitate, silver chloride (AgCl), is formed.

NaCl (aq) + AgNO3 (aq) -> AgCl (s) + NaNO3 (aq)

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