As with MANY of the puzzles in this book, do not overthink this one--as I have done over and over, only to be saved by my brilliant 14 year old son! I went down a "proper" or "accurate" path here, but then backed it up to a simpler solution.
Spoiler!
So... don't worry about a "proper" chemistry answer as it's written in the book. But, for those of you who may be of a similar mindset as mine, it should read something along the lines of this for the intended answer:
Below are some graded hints leading to the solution. It assumes you know keys 49, 53, 54,and 55. If you don't know them, then don't continue.
Spoiler!
This puzzle is basically a chemistry themed rebus, where you add/subtract words and parts of words to come up with the final answer, which is ultimately a single common word that is one of the elements of the periodic table. All you need to focus on is the equation: {key.53}+{key.49} - ({key.55}{key.54}) . All else on the page can be ignored.
Spoiler!
Placing the keys into the equation yields the rebus: ice + heat - (hydro gen). Work to solve each half of the equation on either side of the minus sign.
Spoiler!
when you mix ice+heat, you get water. When you combine the two word stems on the other side, you get 'hydrogen'. Now the rebus becomes 'water - hydrogen'
Spoiler!
The intuitive leap you need to make is to see 'water' as the chemical formula 'H2O', a compound of hydrogen and oxygen. The rebus is now 'H2O - hydrogen'
Spoiler!
While removing one hydrogen from water leaves you with a hydroxy ion (OH-), that makes for an unwieldy answer. So, remove all the hydrogen instead. What is left?
And below is the answer. DO NOT CLICK IF YOU DO NOT WANT TO KNOW THE ANSWER!