Many chemicals do have a shelf life, some can be good very long periods of time,some have a very short shelf life, some chemicals like stannous chloride which has a short shelf life, its life can be extended somewhat by how it is stored,such as adding a piece of tin, keeping it acidic with free HCl, keeping air out...
Much of the shelf life depends on the chemical, and how it is stored.
Acids in general normally have very long shelf life, some can even get stronger on storage (percentage of the acid stored plays a factor here).
Much also depends on the acid and how it is stored.
Sunlight can decompose nitric acid for example, one reason it is often stored in amber glass, bottle caps can go bad and give a way for the NO2 and other NOx gases to escape.
Basically for each chemical the storage recommendations, and shelf life can be different, how each needs to be stored for safety also needs to be understood, storing different chemicals together can get very dangerous.
You should study each chemical you use, and have a good understanding of its properties, and how to use it, store it, and clean it up, understanding its dangers.
In the safety section you will find some good documents to help with this study, the internet is also a great resource for learning more about the chemicals you use.
It would be a good idea to read Hokes book all the way through the first time, then going back reading it a second time doing the getting acquainted experiments in the book, while studying the forum, a good place to start is the safety section (and dealing with waste), and the guide to the forum, as you study you will find materials to work with, and get an understanding of what you need.
Rushing out and buying things like chemicals and supplies or materials, before you know for sure what you need can be a mistake, you could end up spending a lot of money for some things you will not use, or overspend money on scrap that you could get free or at a more decent price...
Most people get excited by this when they first begin, they do not understand how much there is to learn, they try to jump right into the middle of the chemistry before they get any understanding or knowledge of it, without any understanding of what they are doing,or how to do it, they try to begin experimenting before they even understand the dangers, this is a fairly long slow process of gaining a knowledge and understanding, and that is what needs to be worked on first, the study and understanding is what gets that gold in your melting dish not the chemicals or the processes,
understanding of the material and where to find it, how to prepare the materials, understanding how to safely use the chemicals, and understanding the chemical reactions helps you get gold in that dish.
Dumping acids on metals, without that understanding is dangerous to yourself and others, and will get nothing but a dangerous mess to deal with.
Take your time studying, no need to hurry, the gold will come as you study, your knowledge is what you need to melt pure gold, everything else comes as your understanding grows.
