L-ascorbic acid is one of the most vexing ingredients in the skincare world. It’s been heavily researched and is wildly effective on a number of skin concerns. On the other hand, it’s also incredibly fragile, and once it’s been dissolved in water, it breaks down within hours of exposure to light, air, or heat. (If you’ve ever had a vitamin C serum turn brown, that’s the ascorbic acid breaking down.)

First, let’s cover the benefits of L-ascorbic acid in a skincare routine. It’s one of the most powerful antioxidants we have, meaning that it can scavenge and neutralize free radicals. Those free radicals, often caused by sun exposure, pollution, or smoke, will wreak havoc on your skin by damaging collagen, elastin, even DNA. Ascorbic acid can stop those free radicals in their tracks.

L-ascorbic acid has also been shown to stimulate collagen production in the dermis, the middle layer of the skin. That’s especially important because you can’t put collagen on your skin and have it reach your dermis (the molecule is too big) and you can’t eat collagen and have it reach your skin (your digestive system breaks it down first). Stimulating collagen production with vitamin C is one of the best ways to increase skin firmness.

Vitamin C is also a great brightening ingredient, meaning it can help even your skin tone by fading dark spots. Those dark spots, also called hyperpigmentation, are deposits of excess melanin caused by irritation, inflammation, or hormones. Ascorbic acid inhibits an enzyme called tyrosinase, which means those spots of melanin overproduction can’t overproduce anymore.

Now, onto why it’s tricky: as mentioned above, it’s an incredibly difficult ingredient to formulate with. It’s only soluble in water, not any other liquids; and once it’s dissolved in water, it becomes incredibly fragile to light, heat, and air. That’s why most water-based ascorbic acid products turn orange or brown within weeks. (If it doesn’t, check the ingredients list: it may be using a less-powerful vitamin C derivative, not ascorbic acid.)

C+C Serum solves this problem by suspending superfine L-ascorbic acid in a lightweight silicone base. The base evaporates once you apply it, leaving behind just the vitamin C and other actives. C+C Serum is also stored in an opaque airless container, protecting that fragile star ingredient from the elements.