Evidence of Chemical Reaction

Chemical reactions show observable signs called evidences or indicators. One indicator is color change. For example, iron turns reddish-brown when rust forms, and sliced apples become brown after exposure to air. Another indicator is the formation of a precipitate, which is a solid produced when two liquids react. Mixing silver nitrate and sodium chloride forms a white solid called silver chloride. The release of gas is another sign of chemical reaction and is often seen as bubbles or fizzing. Vinegar reacting with baking soda releases carbon dioxide gas. Temperature change is also an indicator because some reactions release heat while others absorb heat. Burning fuel releases heat, while instant cold packs absorb heat. Production of odor can also indicate a chemical reaction, such as the foul smell of spoiled eggs or rotten food. 

Types of Chemical Reaction

Chemical reactions can occur in several forms. In a synthesis reaction, two or more substances combine to form one product, such as when iron reacts with oxygen to produce rust. In a decomposition reaction, one substance breaks down into simpler substances, such as when water separates into hydrogen and oxygen gases. A single replacement reaction happens when one element replaces another element in a compound, like zinc replacing hydrogen in hydrochloric acid. In a double replacement reaction, two compounds exchange ions to form new compounds, such as when silver nitrate reacts with sodium chloride to produce a white precipitate. Combustion reactions occur when substances react with oxygen and release heat and light, such as the burning of methane gas. 

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