In a laboratory experiment, a fermenting aqueous solution of glucose and yeast produces carbon dioxide gas and ethanol. The solution was heated by burning natural gas in a Bunsen burner to distill the ethanol that formed in the flask. During the distillation, the ethanol evaporated and then condensed in the receiving flask. The flame of the burner was kept too close to the bottom of the flask and some of the glucose decomposed into a black carbon deposit on the inside of the flask. During this experiment the following changes occurred. Which of these changes involved a physical change and not a chemical change?
1. evaporation of ethanol2. condensation of ethanol3. formation of a carbon deposit inside the flask4. formation of carbon dioxide gas from glucose5. burning of natural gas6. formation of ethanol from glucose by yeast

A physical change is the conversion of one form into another. Like water is in natural state is liquid but at applying negative temperature like in a freezer, it will convert into ice which is just another form of water. At high temperature, water will convert into vapors and form steam. In all three states, water is just water in a different form. That's what a physical change is. It is a reversible process.

A chemical change is the conversion of one molecule into another form which cannot be reversed. Like burning of coal. You won't get coal after you have burnt it.

In the same experiment, there have been many physical and chemical changes.

1. Evaporation of Ethanol is a physical change as external heat is applied which separates ethanol from water in form of vapors.

2. Condensation of Ethanol is also a physical change where vapors of ethanol are being converted into liquid form.

3. Formation of carbon deposit inside the flask: This is a chemical change. As the experiment contained glucose, upon higher temperatures, glucose loses water molecules and forms carbon. Now, since we cannot convert carbon back to glucose in a simple step, therefore, it is a chemical change which cannot be reversed.

4. Formation of Carbon dioxide gas from Glucose: This is a chemical change. Carbon dioxide cannot be fixed into glucose without the help of a plant or certain chemical methods therefore it is a chemical change and cannot be reversed by normal means.

5. Burning of natural gas: It is a chemical process. Natural gas contains methane as the primary source of burning. It reacts with oxygen present in air and forms carbon dioxide and water and releases heat which is used to elevate the temperature of ethanol-water mixture to obtain purified ethanol.

6. Formation of ethanol from glucose by yeast: It is also a chemical process, biochemical to be specific. Under the absence of air (Fermentation), yeast converts glucose to ethanol which is a chemical process because glucose has been used up and is not glucose anymore. It has been converted to ethanol.