State three characteristics that show a chemical reaction has occurred.

• Chemical reaction- A chemical reaction is the change of a substance into a new one that has a different chemical identity.
• Three Characteristics that show a chemical reaction has occurred are the following.
• Change of color
• Formation of a precipitate
• Evolution of a gas

• Change of color-
• When red lead oxide gets heated, yellow lead monoxide is formed.
• $\underset{\mathrm{Lead}\mathrm{oxide}\left(\mathrm{red}\right)}{2{\mathrm{Pb}}_3{\mathrm{O}}_4\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\mathrm{Lead}\mathrm{monoxide}\left(\mathrm{yellow}\right)}{6\mathrm{PbO}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}$

• Formation of a precipitate-
• $\underset{\mathrm{Silver}\mathrm{nitrate}}{\mathrm{AgNO}3\left(\mathrm{aq}\right)}+\underset{\mathrm{Sodium}\mathrm{chloride}}{\mathrm{NaCl}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Silver}\mathrm{chloride}\left(\mathrm{white}\right)}{\mathrm{AgCl}(\downarrow )}+\underset{\mathrm{Sodium}\mathrm{nitrate}}{\mathrm{NaNO}3\left(\mathrm{aq}\right)}$
• In the above reaction silver nitrate is reacting with sodium chloride to form sodium nitrate and silver chloride as a precipitate.

• Evolution of a gas-
• In the above example, electricity is passed through Sodium chloride solution and Sodium chloride decomposes into Sodium and Chlorine gas is evolved.
• $\underset{\mathrm{Sodium}\mathrm{chloride}\mathrm{solution}}{2\mathrm{NaCl}\left(\mathrm{aq}\right)}\stackrel{\mathrm{electricity}}{\to }2\underset{\mathrm{Sodium}}{\mathrm{Na}\left(\mathrm{s}\right)}+\underset{\mathrm{Chlorine}}{{\mathrm{Cl}}_2\left(\mathrm{g}\right)}$