Both of the silicon-chlorine single bonds in sicl4 are polar. in which direction should the polarity arrows point?

Тhe arrow should point to the atom with greater electrical energy, in the case of the chlorine atoms

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).