Explain the following observations in terms of shifting equilibria by le Chatelier's principle. Write the equation for the equilibrium involved in each case.
a) BaCrO4 is insoluble in basic solution, but dissolves readily in acidic solution.
b) The addition of NaOH to the Fe3+ + SCN-=====FeSCN2+ equilibrium causes color change and a precipitate to appear.
c) The solubility of PbCl2 in .10M HCl is much lower than in pure water.
d) CaC03 is an insoluble salt, but will dissolve in acid.