The real meaning of の when asking a question

Question

It seems to be the season for asking questions about questions, and with every question and answer I read I get more confused.

What is the difference between:

1) どこに行く
2) どこに行くの

I'm assuming 1) is a simple, informal, friendly question with no hidden meanings and nuances.

For 2) I read all sorts of things in different places. Some say that:

• 2) and 1) are entirely equivalent.
• The の in 2) has the nature of the explanatory の. e.g. I'm seeking clarification on something.
• 2) can have an accusatory or rude tone to it and should be used with care.

Maybe I'm confused because people are giving different explanations in different contexts. Can somebody clear up my confusion? Are all these different interpretations correct? If so what context determines each meaning?

Nice to have: If someone would be so kind as to try and compare 1) and 2) with のか and の/んだ as well it would be great to see it all in one place.

Answer 1

This の is still explanatory-no used to seek clarification.

A「いってきます。夜には戻るね。」
B「どこに行くの？」

In this case, の is almost mandatory because B is seeking the reason why A is leaving. You definitely have to learn how to use this の properly. It's still possible to drop this の, but it's very blunt and unrealistic in ordinary speech. A stereotyped tough guy like Terminator or Kratos (in God of War) would say "どこに行く?" or "どこに行くんだ?" instead, but no one around me speak like this in reality.

A「日曜日にどこかに遊びに行こうよ。」
B「いいね！ どこに行く？」

In this case, の is usually not used because B is not seeking an explanation for A's statement. If B said の, it would sound like B is being passive and expecting A's suggestion, i.e., it would sound a bit more like "Where do you have in mind?" rather than simple "Where?"

A「それ、おいしそうだね。」
B「食べたい？」

This is a natural and friendly invitation.

A「それ、おいしそうだね。」
B「食べたいの？」

This is when の can be unfriendly or dubious, if not necessarily rude; this sounds like "Does that mean you wanna eat it?". In general, you should use の with care in "Do you want to" or "Would you like to"-type questions.

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You can read more about this の here: Question Markers: か and の

Answer 2

When you persuade the listener to decide the destination, only どこに行く？works and you can't use the の version.

On the other hand, どこに行くの？ asks one who is seemingly going somewhere about the destination. In this case, you can use どこに行く？ too.

Answer 3

Or can using か before the question mark help?

Example: あなたはピカチュウを[捕]{つか}まえましたか？ Anata-wa Pikachū-o tsukamaemashita-ka? It means Have you caught Pikachu?

まだこれを読めないか？ Mada kore-o yomenai-ka? You still can't read this?