What type of liquid sugar is 100 proof?

I don't understand the question. The term "100 proof" is generally
used in describing an alcoholic beverage that is 50% alcohol. I don't
think liquid sugar has any alcohol in it, unless it has been fermented
in some way.

You can make sugar syrup by adding some water ot dry sugar and heating
and stirring it until all the sugar dissolves.

acording to wikipedia at http://en.wikipedia.org/wiki/Glucose, the
melting point of sugar is 146-150 degrees C.  It should be possible
(under careful laboratory conditions) to take pure Glucose crystals
and heat them up to the required temperature.  You will have a 100%
pure liquid sugar, no contamination of any kind.  Although, you may
have to heat it up in a vacuum/non-oxygen environment; I don't know
what temperature that sugar burns at, but if you take away the O2, the
sugar will be unable to burn.

Unless you wanted to know about a sugar solution (involving water),
then you're on your own.

You can melt sugar quite easily in a dry pan in your kitchen. If you
heat it slowly you'll see it melting - like ice melts to water, only,
of course at a higher temperature.
When/if some of it browns, you get caramel.

If you try it at home, be careful not to add water during your
'experiment': melted sugar has a much higher temperature than the
boiling point of water, and small amounts of water will instantly &
explosively vapourise. Larger amounts of water, well, then we're
talking cooking :-)

http://www.deliaonline.com/cookery-school/how-to/how-to-make-caramel,53,AR.html