b) The reactivity of alkali metals increases as atomic mass increases.
Answer:
The standard enthalpy of formation of HgO is -90.7 kJ/mol.
Explanation:
The reaction between Hg and oxygen is as follows.

From the given,
Molar mass of HgO = 216.59 g/mol
Mass of HgO decomposed = 18.5 g
Amount of heat absorbed = 7.75 kJ
From the reaction,
The standard enthalpy of formation = 
During the decomposition of 1 mol of HgO , 90.7 kJ of energy absorbed.
For the formation of 1 mol of HgO , 90.7 kJ of energy is release
Therefore, the enthalpy of formation of mercury(II)Oxide is -90.7 kJ/mol
Different forms of matter have different melting/boiling points. For example, at 100 degrees Celsius, H2O (water) will turn from lliquid to gas. But NaOH (table salt) doesn't even go from solid to liquid until some 800 degrees Celsius. So, in order to figure out which state matter is at 35 Celsius, you'd have to be more specific about what kind of matter...
Pure substance element I think
Slow chemical change
It is a chemical change because the erosion is due to the chemical reaction between the acid and the in the rain and the calcium carbonate.
It is slow due to the concentration of acid is low.