RE: Tough Chicken Breasts
If nothing burned, you are simply over-cooking the chicken breasts. 8 minutes per side sounds like a long time for an average chicken breast (but will also depend on the thickness). I would recommend cooking it for about half of the time next time (8 minutes in total) and then test the chicken by making a small incision in the thickest part to see if the meat is no longer pink and cooked all the way through. Always make sure to let the meat rest as well after cooking it. Also, with pan frying, an oil with a higher smoking point (not extra-virgin olive oil) should be used so it does not burn/deteriorate during cooking (see attached drill-down in this lesson). Practice will make perfect. Cheers!