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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 &rarrDELTA; 2Hg + O2↑. The Greek letter delta under the arrow represents the heating. The upward-pointing arrow after the O2 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., AgNO3 + NaCl &rarrH2O; AgCl↓ + NaNO3. The H2O above the arrow shows that the reaction takes place in the presence of water : in this case, in water solution. The formulas AgNO3, NaCl, and NaNO3 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 (H3O+) and hydroxide ions (OH-). These ions exist in equilibrium with water molecules. The equation is 2H2O &rlhar2H2O H3O+ + OH-. The sign = is sometimes used in place of the double arrow.
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