Question.
What gas caused the change to the appearance of
limewater?

Carbon dioxide

Explanation: I don't know if this is right but this is what I found on Google

conduction is the transfer of thermal energy (heat in transfer) due to collisions between the molecules in the object. collisions between adjacent atoms and molecules transfer kinetic energy from the warmer to the cooler object. the objects must be in physical contact.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.