chemical equation: Additional Symbols Used in Chemical Equations

There are a number of other symbols used in chemical equations. A symbol written above or below the reaction arrow indicates special reaction conditions. For example, when mercuric oxide is heated it decomposes into mercury metal and oxygen gas; this reaction is shown by the equation 2HgO →Δ 2Hg + O 2↑. The Greek letter delta under the arrow represents the heating. The upward-pointing arrow after the O 2 indicates that this product is gaseous and escapes. When a precipitate is formed by a reaction, the substance that precipitates is often followed by a downward-pointing arrow, e.g., AgNO 3 + NaCl →H2O AgCl↓ + NaNO 3. The H 2O above the arrow shows that the reaction takes place in the presence of water—in this case, in water solution. The formulas AgNO 3, NaCl, and NaNO 3 do not represent molecules, since these substances are almost completely ionized in water solution (see ion ).

When chemical equilibrium occurs in a reaction, the double arrow ( ) is used instead of the single arrow. For example, liquid water dissociates to form hydronium ions (H 3O +) and hydroxide ions (OH ). These ions exist in equilibrium with water molecules. The equation is 2H 2O ⥨H2O H 3O + + OH . The sign = is sometimes used in place of the double arrow.

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