difference between colon and dash

Question

I'm trying to explain to students the difference between colon and dash, but I'm having trouble explaining why dash is better than colon in the following sentence:

1. In 2016, Leonardo DiCaprio finally won an Oscar: on the sixth time he was nominated.

2. In 2016, Leonardo DiCaprio finally won an Oscar--on the sixth time he was nominated.

So far I've been telling students that both define, explain, or add details, but a dash adds a dramatic effect like a drum roll. However, in this case, I believe what makes the second sentence better is not the drum roll. What makes a dash better in this case, and when is a colon better than dash after "Leonardo DiCaprio finally won an Oscar"?

Answer 1

In addition to its other functions, I remember learning at some point that the (em) dash can represent a pause that is longer than the pause of a comma or colon. And I feel like that is what you mean by adding the "dramatic effect". The longer pause creates a sort of suspension.

I doubt that this "longer pause" that I learned is universally held. However, I think it complements the information given in this source. Although this sources is comparing parentheses and dashes, it still highlights the point nicely:

Think of dashes as the opposite of parentheses. Where parentheses indicate that the reader should put less emphasis on the enclosed material, dashes indicate that the reader should pay more attention to the material between the dashes. Dashes add drama—parentheses whisper.

So I would tell them something like a dash is supposed to be a stronger, or longer, break. This helps make the emphasis. A colon is a weaker, or shorter, break. Its effect is not as strong.

I think a colon is preferable if you are trying to include information that is not meant to have a dramatic effect. For example,

In 2016, Leonardo DiCaprio finally won an Oscar: his greatest accomplishment yet.