Why is hydrogen considered the most efficient rocket fuel and how it relates to the concept of specific impulse (ISP)?

Question

Why is hydrogen considered the most efficient fuel? I mean I know it is very light and can be accelerated very fast, but can’t you use a denser fuel but throw it at a slower speed bunch, and it’ll still achieve the same force as it’s heavier?

Also, why do people say that a rocket engine with hydrogen fuel also generally has lower thrust compared to other denser fuels? Can’t that hydrogen engine just throw hydrogen out faster, since it’s lighter and easier to throw, to achieve the same thrust from an engine that uses a denser fuel?

And if you can answer this relating to the concept of ISP (specific impulse), which still confuses me, it’ll be even better!

Answer 1

If you get more bang for your buck, you get a better result for the amount of effort or money that you have put into something.

In this context and many others, eg specific heat, specific charge, etc, the word specific means per unit mass.

Hydrogen has the lowest molecular weight of any known substance and burns with extreme intensity ($3000^\circ\rm C$).
In combination with an oxidiser such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known rocket propellant.
Thus what hydrogen loses on mass in terms of momentum change it makes up with a much higher exhaust speed due to the very high temperature of the exhaust gases.