Which substance found in fossil fuels can form a harmful acid when the fuel is burned?

Answer is: sulfur.

Fossil fuel combustion increases the acidity of rain because the sulfur dioxide is produced.  

Because of fuel combustion, sulfur dioxide goes up into the atmosphere as the hot gases rise, than it reacts with water and oxygen in the air and form sulfuric acid:  

Balanced chemical reaction: SO₂(g) + 2O₂(g) + 2H₂O → 2H₂SO₄.  

pH (potential of hydrogen) is a numeric scale used to specify the acidity or basicity an aqueous solution.

When pH is less than seven (pH<7), solution is acidic.

answer: the rate of diffusion increases with increase in temperature.

increase in temperature, increases the molecular/particles speed (kinetic energy. diffusion means movement of particles through the material.

generally, the heavier the particle the more it interacts with the environment and more the temperature matters. so higher temperature results in higher diffusion.