Learn Russian: Серебро более тяжёлое, чем железо, но менее ценное, чем золото. - Silver is heavier than iron, but less valuable than gold.

In Russian, you can compare things in two ways: by putting the second item in the genitive case, or by using чем (than) followed by the nominative case. Since this sentence uses чем, the words железо (iron) and золото (gold) stay in their dictionary form (nominative).

Why do the adjectives тяжёлое and ценное end in -ое?

They end in -ое because they are describing серебро (silver), which is a neuter noun. When using более (more) or менее (less), the main adjective acts like a normal adjective and must match the subject's gender, number, and case.

Could I use the short word тяжелее instead of более тяжёлое?

Yes! Тяжелее is the one-word comparative form of "heavier". Both серебро тяжелее, чем железо and серебро более тяжёлое, чем железо are correct and mean the same thing. The two-word form with более is just a bit more formal.

Do I always need a comma before чем in this type of sentence?

Yes, Russian punctuation rules strictly require a comma before чем whenever it is used to mean "than" in a comparison.