IS THERE ANY SUBSTANCE WHICH IS A CHEMICAL CHAGE AND A PHYSICAL CHANGE?
Your friend given correct answer.
Simply breaking of food particle with help of teeth is physical change and its oxidation to produce energy is a chemical change.
The melting of Wax from candle is physical change.But reaction of hydrocarbon of wax with atmospheric O2 to produce CO2 is a chemical change.
There are many examples where physical and chemical change takes place simultaneously.The most common and simplest is digestion of food.when we eat food, two process will involve for complete digestion.one is breaking of bigger food particles into smaller ones which takes place with the help of teeth.this is physical process as till now the basic constituent of food does not change.The smaller food particle reacts with different enzymes of mouth and stomach to break down the complex food into its monomer which is readily absorbed in our body is a chemical change. Example if we eat rice the major constituent is sugar is broken down into simpler molecule glucose which is absorbed in body. Sugar is polymer and glucose is monomer hence change of constituents that is chemical change.
One more example is of burning candle.
a) Physical Change
When candle is lit, the heat of the flame melts the solid wax to liquid wax. This signifies a physical change from solid state to liquid state. Moreover, there is also a change in the shape and size of the candle, which again is physical change.
b) Chemical Change
The wax acts as a fuel when we burn the candle and is basically carbon. On burning it produces flame. The flame shows different colours. In the yellow part, carbon combines with oxygen to form carbon dioxide and in the blue region Hydrogen combines with oxygen to form water. The unburnt carbon is deposited as a black substance called soot. This signifies a chemical change in which carbon is getting converted to carbon dioxide and hydrogen is getting converted to water.