Perhaps this was done to convince the children of Israel, how contemptible must be such gods which could be so reduced to nothing. 1 Corinthians 8:4.
Related Commentaries of Exodus 32:20
Exodus 32:20
20 And he took the calf which they had made, and burnt it in the fire, and ground it to powder, and strawed it upon the water, and made the children of Israel drink of it.