What is a chemical change?
A chemical change is any change that results in the formation of new chemical substances with new properties.
For example, hydrogen reacts with oxygen to form water. This is a chemical change.
2H₂ + O₂ → H₂O
Hydrogen and oxygen are both colourless gases, but water is a liquid at ordinary temperatures.
Which of the following are chemical changes? (a) Sugar dissolves in warm water. (b) A nail rusts. (c) A glass breaks. (d) A piece of paper burns. (e) Iron and sulfur form a shiny nonmagnetic grey substance on heating.
(a) Not a chemical change. The sugar and water are still present.
(b) A chemical change. The reddish-brown rust is different from the iron.
(c) Not a chemical change. The glass is just in smaller pieces.
(d) A chemical change. The paper disappears. All that remains is a small amount of ash.
(e) A chemical change. Sulfur is yellow, and iron is magnetic. The product is neither yellow nor magnetic.
Here is video of a lab with a number of examples of both chemical and physical changes.
video from: Noel Pauller