In a beaker, sugar is dissolved in water, and then the water is heated and evaporates. The sugar is recovered, and heat is again applied. Vapor is released, and the material in the beaker changes...

In a beaker, sugar is dissolved in water, and then the water is heated and evaporates. The sugar is recovered, and heat is again applied. Vapor is released, and the material in the beaker changes from white to black. What must you know to determine if a chemical change occurred?

 

 
 
Expert Answers
ncchemist eNotes educator| Certified Educator

There are two types of basic changes in chemistry, physical and chemical.  A physical change is simply a change in the form of a substance without any actual chemical bonds being broken or formed.  A chemical change requires a chemical reaction whereby chemical bonds are either broken or new bonds are formed.

When the sugar and water mixture is heated to boiling, the vapor that is released is water vapor.  We know that this is a physical change to the mixture because we added water to the system, so the removal of water requires no chemical reaction.  Instead, it merely requires heat to change the water from liquid to gas.  When the sugar is recovered as a white solid when all of the water is gone, the heat is increased and the sugar turns black and more vapor is released.  Since there is only one chemical entity present in the beaker (sugar), a change like this is most likely chemical in nature since you have a chemical solid producing another chemical solid and a gas.  In order to verify this, we must know the chemical content of the vapor released or the chemical content of the black solid remaining in the beaker.  If we can show that one or both is chemically different than sugar, then a chemical change has occurred.

In reality, the cooking of sugars with high heat is a process called caramelization.  Simple sugars like table sugar will break down to form carbon dioxide and water vapor (the gasses being released) and also  carbon residue (the black solid remaining in the beaker).