a)
`CaO + H_2O -> Ca(OH)_2`
\(2NaCl+2H_2O\xrightarrow[cmn]{đpdd}2NaOH+Cl_2+H_2\)
b)
\(2Cu+O_2+4HCl\rightarrow2CuCl_2+2H_2O\)
\(Mg+2HCl\rightarrow MgCl_2+H_2\)
\(CuCl_2+2NaOH\rightarrow Cu\left(OH\right)_2\downarrow+2NaCl\\ MgCl_2+Ca\left(OH\right)_2\rightarrow Mg\left(OH\right)_2\downarrow+CaCl_2\)