I can both sing and speak C#3 . There would be a difference between using the note for singing and speaking. What is the difference between singing and speaking? If it depends on what kind of vocal technique one is using them please tell me so. I mean, some singing techniques are closer to speech than others. My question is not so much about the theory but more on the practical side.
I think user45266's answer is spot-on - I would just add that singing also typically involves aligning the syllables in time with a musical rhythm.
Loosely, perhaps one could say:
Speaking is the utterance of words with pitch, pitch inflections and timings as expected for normal communication in the language in question. Typically, variations in pitch and timing are interpreted as adding nuance to the meaning - the study of these features is called Prosody.
Rapping is the utterance of words with pitch and pitch inflections expected for normal communication in the language in question, but in a musical rhythm
Singing is the utterance of words with pitch tending towards a musical scale, and usually in a musical rhythm.
(Responding to OscarCunningham's comment) - The use of musical pitches with a normal speaking rhythm isn't so common as far as I know - but an example of something that is close is Recitative in opera.
If I were pressed to answer, I'd say that singing is vocalising at defined and intentional frequencies. Speaking does not require any conscious effort to control the pitch of one's voice, and it doesn't need to have a clear pitch (whispering, vocal fry).
In some music, of course, the line gets blurred. Often, singers will deliver a line in a spoken way, giving a rough approximation of a melody but emphasizing the delivery style of conversation. Rap can sound like it has a melody, and sometimes it does (live performances of certain rappers sound almost nothing like the song one hears on the radio). There are plenty of styles (opera?) that intentionally mix the two, and though it's not a popular opinion, it could probably be argued that singing originated as a certain way of speaking.
Physically, there's no real difference between speaking and singing. Speaking and singing are just two human societal constructs for variances in the same phenomenon.
In my opinion you speak with a 'random' pitch. You just go with what your body is the most comfortable with and on some words this might me slightly higher and sometimes lower. If you ask a question, you raise the pitch in many languages etc. but you don't go like, when I ask a question I always go up a major third above the pitch I was speaking before:P So even tho you're using multiple pitches, it doesn't seem organized to a certain scale for example. Also you don't speak in a certain rhythm to a specific beat/bpm. Just do me a favor. Can you read this out loud please? -> E E E E E A A A A A
-Thanks! ;) But now... did it sound anything like this? Click me
Probably not, because you had no specific pitch and no specific rhythm in mind. You probably just read them monotonous and if there would've been a metronome in the background it probably wasn't a steady beat either.
Most rapping is for me where singing begins, because you speak with specific pitches to a specific rhythm. Just watch the first example here: Click me This for me would be singing or at least musical speaking :D
Other than that... here are just some definitions which also may help you to make a choice where singing begins for you :P
singing: to utter words or sounds in succession with musical modulations of the voice; vocalize melodically.
Singing is the act of producing musical sounds with the voice and augments regular speech by the use of sustained tonality, rhythm, and a variety of vocal techniques.