Flexi Says: Formation of Soap scum which is the white solid result from the addition of soap and other anionic surfactants to hard water is example of double displacement reaction. Hard water contains calcium and magnesium ions, which replace the sodium ions of surfactant and forms scum. Another example of double replacement you can experience while baking when you add baking soda and vinegar to the batter and see vigorous bubbling. Here the replacement reaction results in the formation of CO2 gas bubbles and water.
