There are two principle kinds of liqueurs - infusions and distillations. My site is all about infusions, so I'm not going to delve into the realm of distillation (not least because I have no experience, and no legal way of obtaining it). As you will have seen in the previous section, infusion liqueurs are made by soaking a flavoring substance in alcohol for a period of time until you have drawn the flavors out of it. Then you filter out your flavorings and are left with a tasty drink. But the question may occur to you, why does this work, and why is alcohol so much better at this than just soaking things in water?
The short answer is osmosis. Osmosis is the technical name for water passing through a membrane that is fine enough that water can pass through, but larger chemicals cannot. Water, of course, is a tiny molecule, smaller even than oxygen or nitrogen. So a very fine membrane can easily block the passage of large things, like sugar, salt, alcohol and children, but still allow water to pass through. That principle, in fact, is what a lot of our own bodily chemistry relies on to function properly.
So let's look at a raspberry. Inside you've got raspberry juice, which is water and sugar mostly. Outside you've got alcohol and water. Once you've got a liquid inside and outside, the skin will act like a membrane. The water inside is free to move across that membrane, but once it passes into the side that has less water, it is more likely to stay there. Your alcohol is concentrated higher than the sugar inside the berry, so over time, the water tends to move to the side that has the alcohol. The same process works with higher concentrations of sugar and salt, too. That's why salt shrivels things - the water passes out of the food and into the salt, and there it tends to stay. Alcohol, sugar and salt are each good at dehydrating things - and osmosis is why. That's also why your hands look the way they do after a bath - the water tends to move from the pure water on the outside into the skin of your hands. That wrinkled look is actually just too much water in the skin surface cells.
So what happens in liqueurs is that every exposed surface on your flavorings behaves like a membrane. Everywhere that your fruit touches alcohol, you have a membrane. And over time, the water moves out of the fruit and into the alcohol. That's one reason we tend to chop things up or poke holes in them - to give the alcohol a larger surface to work on.
But in the real world, no membrane is perfect. Occasionally, things much bigger than water find their way through. Whichever way the water is moving, the larger things move, too. So while your alcohol is stealing the water out of your raspberries, some large chemicals, like sugars, flavors and colors come along for the ride. So over the period of a few weeks to a few months, the alcohol has robbed your berries of all sorts of tasty morsels. Squeeze the remaining juice out of your berries and throw away the solids, and your alcohol has a nice raspberry flavor now.
However, you might wonder now why we go to all this trouble for things like raspberries, when we could just crush them to steal all the water out of them. The answer to that is that there's more to the skin than just a membrane. While we're stealing the water, sugar, color and flavor out of the inside of the berries, we're also stealing complex chemicals right out of the skin. For me, this was most dramatically illustrated by pomegranate liqueur. When we were developing this recipe, we tried just crushing the juice out of the seed-sacs adding just the juice to the alcohol. We also tried crushing the seed-sacs and adding both the juice and the sac pulp to the alcohol. The result was striking. Without the pulp, the liqueur was bland and even insipid. With the pulp, it was delicious, subtle and complex. The entire difference was in the chemicals that came out of the seed-sac membrane, and the seeds themselves. I expect the most important of those chemicals was tannin - the thing that gives tea and wine their special feel. It occurs naturally in many fruit skins, and adds so much to the flavor that you simply can't live without it. Wine-makers even add it to wines that just aren't quite up to par.
So that's why alcohol works at making liqueurs, and water doesn't. In fact, most all the essential oils and extracts you find in grocery stores are made the same way. Take your flavoring, soak it in pure alcohol until you've got all the good bits out of it, and then evaporate the alcohol, so all you have left are the good bits. If you're paying atention, you may realize another perfectly good way to make something like a liqueur - pack your flavorings in sugar instead of alcohol. That lets the sugar exert the force of osmosis on your berries instead of alcohol. The final result will be the same if you eventually add alcohol to the mix. In fact, I have a cherry liqueur recipe or two that does more or less precisely that.