It does depend on the amount of oxygen and carbon dioxide.  If you put a candle inside a tightly sealed jar it will only burn for a little while because combustion (burning) takes up oxygen hydrocarbons (something that contains hydrogens and carbons) and produces carbon dioxide and water vapour, surprisingly enough.  The candle would eventually burn through … Read more

The acid denatures ("unties") the protein's complex structure.  There is a protein in the apples called polyphenol oxidase that reacts with a particle in the cells of an apple.  When you break the cells (by cutting the apple or dropping it) these two things react together and create a substance called melanin, which is a brown or … Read more