Unfinished Flat

Explain to me what is a V7 chord in piano.?

My teacher calls V7 chord the engine of music and he keeps telling me V7 is how the music moves. I understand that V7 chord sounds unresolved/unfinished. Can you help me understand how to build a V7 chord for instance in the key of B flat and how to resolve it so that it sounds finished.

Sure, no problem.

The V7 chord is built on the 5th degree of the scale (also called the dominant). So let’s look at the notes of the B-flat major scale:
Bb, C, D, Eb, F, G, A

The fifth degree is F, so we will start there. In other words, F will become the root of the V7 chord. This kind of seventh chord is made with a major chord with an added minor seventh. So, let’s start with the F major chord:
F A C

Then add the minor seventh. If you aren’t sure what that is, it will be two half-steps down from F on the piano: F -> E -> Eb

So your F7 (V7) chord is: F A C Eb.

There are two main reasons why it sounds unresolved. In tonal music you want to resolve to the tonic, or first note of the scale (Bb in this case). The third of the V7 chord is the leading tone of the scale, in other words it leads to the tonic (A leads to Bb in this case).

The other reason is that it contains a tritone between the 3rd and 7th (between A and Eb in the case of F7). The tritone is a dissonant interval. Dissonance creates tension in the music which demands to be resolved by returning to the I chord (Bb major), for example.

Beethoven, Symphony No.10 in E Flat Major, “Unfinished”