 I have a porcelain bowl kept on top of a wire gaze and tripod stand. Inside the bowl there is some wax and I will heat the wax from this candle and let it melt. Now I can extinguish the candle and let the melted wax cool. So we see that as it cools it turns back to solid. Now wax started from solid, changed to liquid. Upon heating then turned back to solid upon cooling. What kind of a change is this? Is this a physical change or a chemical change? Think about it. Turns out melting and cooling wax is a physical change. Wax is still wax but it has changed its state from solid to liquid and back to solid again. This changes physical because no new substance is formed. Wax atoms have only rearranged themselves in response to the heat from the candle. But the wax atoms are still wax atoms. No new molecule or substance is formed. So this wax can be melted and cooled many times and it will still be the same wax with the same color and smell. And that's why melting and cooling wax is a physical change as no new substance was formed. Just like how melting ice and then cooling water back to ice is also a physical change. As no new substance forms you are only changing the state of water. So in a physical change the state of a substance can change but no new substance is formed. And generally physical changes are reversible as well. Just like how we changed the liquid wax back to solid by cooling it. So we reversed the process of melting. Alright now let's look at an example of a burning candle. What type of a change is this? Physical or chemical? So first let's see what a candle is made up of. There is this part which is called a wick which is highly absorbent. Just like a towel. A towel can absorb water. Similarly the material that the wick is made up of it has the ability to absorb all the wax that is melting around it. And then of course there is all this solid wax that the candle is made up of. So when you light a candle you melt the wax in and near the wick. And the wick absorbs the liquid wax and pulls it upward. The heat of the flame vaporizes this wax and it is the wax vapor that burns. So as the liquid wax moves up the wick it vaporizes and reacts with the oxygen in the air to produce heat, light, water vapor and even carbon dioxide gas. So this changes chemicals because the wax molecules have been broken down into different molecules and new substances have been created as a result of the chemical reaction that is happening. The wax vapor reacting with oxygen to form carbon dioxide and water vapor. And also once the wax has been burned it cannot be turned back into its original form. And therefore you see the height of the candle reducing right? You cannot get back the original height. So here we see that the wax vapor reacts with oxygen, wax vapor burns and it forms new substances. And in fact we can show that it is the vapors of the wax that burns and not liquid wax. So when we extinguish the candle we see white smoke leaving the wick. These are really vapors of wax and now notice what happens when we bring a lit match or a lighter near the white smoke. It immediately relights the wick and candle starts burning again. The wax vapor pulls the flame of the lighter. So burning of a candle is a chemical change.