Which is not a sign of a physical change? *
melting
boiling
bubbles given off
condensing

The difference between a physical and chemical reaction is composition.
In a reaction, the composition of the  change, whereas in a  reaction, one may find change in the smell, appearance or type of matter without actual change in the composition. Despite the name, during a physical "reaction", no reaction is actually taking place. 

For something to be labelled a reaction, a change in the elemental composition of the substance has to experience a change.

ヽ Physical properties include many other aspects of a substance. Luster, malleability, ability to be drawn into a thin wire, density, viscosity, solubility, mass and volume are some physical properties. Any change in these physical properties is referred to as a physical change.

ヽ Physical changes are changes that result in a difference in display without changing the composition. Texture, color, temperature, shape or change of state (Boiling Point and Melting Point are significant factors in determining this change) are a few examples.

ヽ Chemical changes, on the other hand, are different. A chemical change occurs when the substance's composition is changed. When bonds are broken and new ones are formed, a chemical change occurs. Change in temperature, color, smell (after reaction has occurred) and the formation of a precipitate or bubbles are indicators of a chemical change.

Tracing back to your question;

～ Melting
Let's use the example of an ice cube melting.

When you melt an ice cube, you have a physical change because you're adding energy. Evidence of this is the phase change of solid to liquid; it signifies that you added enough energy for this phase change to occur.

～ Boiling
Let's use boiling water as an example.

When you boil water, it doesn't lose any of its properties. Rather, it simply undergoes a change in state during the process (the change of state being from liquid to gas). This tells us that boiling water is a physical change and not a chemical change.

～ Bubbles
When bubbles form, a change in temperature may also be taking place, as temperature change and the formation of bubbles often go hand-in-hand. An example of this may be the reaction below, in which the formation of gas bubbles (Carbon Dioxide) form.

The formation of bubbles (gas being given off) is an indicator of a chemical reaction taking place.

～ Condensation
Condensation is the physical change of a substance in a gaseous state being changed into its liquid state as a result of energy being lost due to heat loss or applied pressure.

From the above explanations and examples, we can see that the formation of bubbles is the option which is  a sign of a physical change!

Your answer is the third option.

Hope this helps! ッ

answer: c. the experiment shows that the red substance experienced a chemical change.

explanation: chemical change is the change in which chemical composition of the substance changes. physical change is the change in which there is no change in chemical composition.

chemical changes are accompanied by following changes:

a) precipitate formation

b) energy is released or absorbed

c) formation of bubbles

d) change in color

as it is given that the red color has disappeared and there is production of gas, it means the red substance has undergone a chemical change.

e