 I have some water in the container and I have this salt. I've added the salt and started dissolving the salt in this water. So, I'll try mixing it, I'll try to dissolve it for some time. And that my salt has dissolved, so I'll add some more. And then dissolve it again. Alright, I've dissolved all the salt. Now, I have switched on the flame and now I will wait for this water to start vaporizing. So, it has started vaporizing, I will wait for it to boil now. Now, notice what starts happening as the water starts boiling. Do you see the salt getting deposited on the sides of the container and also on the top of the container? Just like how you see in front of you. Most of the water has been boiled and see the salt that has been deposited on the sides of the container. And also, what you see around the container, this is also salt. So, the salt that you dissolved, this is the same salt that you see in front of you. Outside the container and also deposited around the container. Alright, at this point, I wanted to think about whether this is a physical change or a chemical change. So, turns out this is a physical change because you started with salt and there was a process that happened and again you ended with salt. You dissolved the salt, then I vaporized the water and then again we got back the salt that we started with. So, no new substance was formed and also this process was reversible. We could obtain the salt that we started from. So, this was a physical change. So, dissolving salt in water is a physical change because we can get back the salt that we started from. And no new, nothing new is formed. Alright, let's look at one more case. Now, over here I have a candle, I have a lighter, I can use this to light up the candle. I have a magnesium ribbon and a pair of tongs. So, I will light up this candle and I will try and burn this magnesium ribbon. And I want you to notice what happens. Oh, look at this, look at how bright, how bright the magnesium ribbon burns. The candle is not even visible. Alright, so what sort of a change was this? Was this a physical change or a chemical change? Okay, so, turns out burning of magnesium is a chemical change because here when magnesium reacts with oxygen and it's reacting with oxygen in the presence of so much heat. When that happens, new substance was formed. That was magnesium oxide. And you can't really go back from the burnt magnesium ribbon to the original magnesium ribbon that there was. So, turns out this is a chemical change. New substances are formed and you cannot reverse the process.