## Chemistry and Chemical Reactivity (9th Edition)

a) Air is a mixture of many compounds, predominantly nitrogen ($N_2$) and oxygen ($O_2$). b) Mineral form of Calcium Fluoride ($CaF_2$). c) Brass is a metal alloy of copper (Cu) and zinc (Zn); therefore, it is a mixture. d) 18-carat gold is an alloy of gold: 18 out of 24 parts in mass with the rest being impurities and other metals. Therefore, it's a mixture.