You fill a tank with gas at 60 C to 100 kpa and seal it you decrease the temperature to 10

The given question is incomplete. The complete question is :

You fill a tank with gas at 60C and 100 kPa and seal it. You decrease the temperature to 10C but keep the volume constant. What happens to the pressure?

Thus the pressure will decrease to 85.0 kPa

Explanation:

Gay-Lussac's Law : It is defined as the pressure is directly proportional to the temperature of the gas at constant volume and number of moles.

(Volume, moles of gas are constant)

Mathematically,

where,

are the initial pressure and temperature of the gas.

are the final pressure and temperature of the gas.

We are given:

Putting values in above equation, we get:

Thus the pressure will decrease to 85.0 kPa

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.