$5.$ The process of $\_\_\_\_\_\_\_\_\_\_$ cracking involves breaking down heavier crude oil molecules into lighter fractions using heat.

$6.\_\_\_\_\_\_\_\_\_\_$ cracking improves the yield of gasoline by using a catalyst to aid the cracking process.

$7.$ Hydrocracking is a refining process that uses $\_\_\_\_\_\_\_\_\_\_$ to convert heavy hydrocarbons into lighter, more valuable products.

$8.$ In petroleum refining, $\_\_\_\_\_\_\_\_\_\_$ reforming is used to improve the octane number of gasoline.