Apple cider vinegar has a reputation for helping to balance pH and thus, has been used to help the scalp regain its balance after a lot of product use, according to Cosmopolitan. Byrdie, meanwhile, notes that it's this pH-balancing effect that theoretically makes apple cider vinegar a candidate for skin care as well, especially when it comes to oily skin and breakouts.

As for using apple cider vinegar to treat a sunburn, Healthline admits that applying an acidic solution to a burn seems counterintuitive, but also says that if the ACV is diluted enough, it's one to try as a home remedy. 

To safely apply apple cider vinegar to a sunburn, one option — especially for a large sunburn — is to mix the apple cider vinegar with water and put it in a spray bottle. Then you can spray it on the affected area and let it dry. According to Healthline, there is no recommended ACV-to-water ratio, but it seems wise to start with a heavily diluted mix. 

Another application method is to put the diluted mixture on a washcloth and wipe it onto the burn. However, if your sunburn is sensitive to the touch, another choice is to put some of the apple cider vinegar in a cool bath and bathe in the solution. The cool water will provide relief to the burn while the vinegar works on the skin (and a tub full of water will certainly dilute the vinegar, via Healthline).