Question

melting of water and milk in sunshine​

Answer 1

You're right that milk is more dense than water, but this is not why it melts more quickly, although it is indirectly related. The reason that milk is dense is because it contains a lot of impurities - raw milk is about 3.7% fat and 3.2% protein (for Holstein cows; other breeds actually have more).

★ᴘʟᴇᴀsᴇ ᴍᴀʀᴋ ᴀs ʙʀᴀɪɴʟɪᴇsᴛ ᴀɴsᴡᴇʀ ᴀɴᴅ ғᴏʟʟᴏᴡ ᴍᴇ!!

Answer 2

Physical change as there is no change in composition. Only the separation of components takes place by the physical phenomenon, centrifugation.