Solubility is a pretty important part of making bath & body products, but we don't talk about it much. In essence, solubility is the ability of a solid, liquid, or gas (the solute) to dissolve in a liquid solvent to create a homogeneous solution (homogenous meaning a mixture that shows no variation in properties, like Kool-Aid dissolved in water - you don't see big particles in there).
The solubility of the solute depends on the type of solvent (water, oil, alcohol, and so on), the temperature, and the pressure. (There is a concept called the STP or SATP, which stands for standard temperature and pressure. I was taught it was 25˚C and 1 atm or or the pressure at sea level, but it can vary depending upon the textbook and instructor.)
If something is soluble in water, the solubility generally goes up when you increase the temperature and generally goes down when you reduce the temperature. Something like salt dissolves readily in cold water, but dissolves better and faster in warm or hot water. This is one of the reasons we dissolve extracts or allantoin or other solid things in warm water or add it to the heated water phase - it will dissolve or mix easier and create a nice homogeneous mixture.
If two things are immiscible, it means they will not mix or dissolve well (like oil and water without an emulsifier). Some things are very soluble - salts, for instance - and some things are kinda soluble, meaning they aren't completely soluble or need some help to be soluble, like an increase in heat or some serious stirring. If we add too much of a very soluble thing or take something that isn't completely soluble and add more of it or fail to raise the temperature, we get a precipitate, which is a solid of the solute that can fall to the bottom of our creation. We see this kind of thing when adding too much salt to water (the stuff on the bottom is the precipitate).
In this toner, I dissolved honeysuckle and green tea extract at 0.5% each. If I put too much of this extract in or used the wrong kind of solvent (oil, for instance), the extracts would fall to the bottom and form a gooey mess, which would be the precipitate. As you can see, there is no gooey mess! And yes "kinda soluble" is a science-y term!
If you've made a toner and found a precipitate at the bottom that requires you to shake it before using, or if you've made a lotion in which your allantoin or MSM crystallized, you've either added too much or added it at the wrong temperature or used a solvent that won't dissolve the compounds. (Try mixing shea butter in alcohol - won't work! Now dissolve it in oil! Success!)
A rule of thumb is "like dissolves like". Water dissolves water soluble things; oil dissolves oil soluble things. Water is polar; oil is non-polar. (If you want to learn more about this topic, I suggest this wiki page for a great place to start!)
So why am I sharing all of this chemistry fun with you on such a lovely March day? Because there are good reasons for suggesting 0.5% of an extract other than the effect it might have on your skin or bank account. When we're formulating, we need to take into account the possibility that our lovely extracts or additions might precipitate out and cause an icky mess on the bottom of our bottles, which is not very pretty and a waste! Many manufacturer's data sheets or bulletins on ingredients include the solubility - generally expressed by moles of the ingredient - so you can see if how much you should add.
There's a cute java application showing the solubility of salt here. It's fun to keep increasing the moles until you see the precipitate! (Or you can try this at home with a glass of water and some salt, or in your bath with some Epsom salts!)