Can I make instant profits in this way? Different exchanges have different rates to buy BTC. 

So, how about buying in an exchange which has a lower BTC rate, send that BTC to an exchange which has higher sell rate, and sell to make a profit?
Yes, you can do that, but it may be difficult to do. This process is called arbitrage and is something that occurs in other markets, not just in cryptocurrency trading.

Keep in mind that if you do this, the price difference might be gone by the time that the other exchange accepts the deposit.

Alternatively, if you have enough capital, you can keep money on multiple exchanges and make same sized trades (e.g. buy 1 BTC on one exchange and sell 1 BTC on the other). However you will still need to rebalance by making transactions, and during that time, you will not be able to perform arbitrage.