In the Chat tab of WhatsApp, you need to scroll to the bottom to see if the archived chats. If you need to hide chat of a particular contact, then you have to chat with him/her from a different whatsapp number. Additionally may I suggest you to use a Applock. There could be many reasons for one to hide their personal conversation with someone. Now I have another solution for you. You can also add lock on your WhatsApp without any third party software. Please note, I have another unofficial solution for you, which can hide your chat as you asked, so keep reading.