Yes, you can mix vitamin C powder with water. This is a common way to consume vitamin C supplements, as it is a water-soluble vitamin. You can also mix it with juice or smoothies, as per instructions on the product label.
However, it is important to note that vitamin C is sensitive to heat and light. To preserve its potency, it's best to mix it with cold or room temperature water and avoid prolonged exposure to sunlight.
For topical use, you can mix vitamin C powder with water-based products like serums, essences, and lotions. However, it is essential to follow the recommended usage instructions and consult with a skincare professional if you have any concerns.