If you want to change the mass of an atom what do you do

The mass of an individual atom will not change unless the atom undergoes a nuclear change. The average mass of an element remains essentially staple as an average of the common isotopes. However the mass number of a sample of an element can and will change over time as the radioactive isotopes undergo nuclear changes.

Explanation:

Is it right

The atomic mass is the sum of the protons and neutrons. As the atomic number increases it causes an increase in the atomic mass.

Explanation:

The number of neutrons in a nucleus affects the mass of the atom but not its chemical properties. Thus, a nucleus with six protons and six neutrons will have the same chemical properties as a nucleus with six protons and eight neutrons, although the two masses will be different.

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

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

that tool would be a compass