Final answer:
When ascorbic acid is heated on paper, it oxidizes more rapidly, turning brown due to the heat providing the activation energy. Oxidation can occur without heat, but heat accelerates the process. Ascorbic acid also plays a role in preventing diseases like tyrosinosis by preventing unwanted oxidation.
Step-by-step explanation:
Ascorbic acid, also known as vitamin C, acts as an antioxidant by being a reducing agent that can be oxidized to dehydroascorbic acid in the process of preventing cell damage. When ascorbic acid is used as invisible ink on paper and heated, the heat accelerates the oxidation process, causing the substance to change color and become visible as it turns brown. This is partly because the heat provides the activation energy needed for the reactions to take place more rapidly, similar to how paper is combustible but requires a flame to initiate burning.
Oxidation does occur in the absence of heat, with examples including the rusting of iron or the bleaching action of hydrogen peroxide which releases oxygen as it decomposes. However, heating ascorbic acid provides the necessary energy to speed up this oxidation process on paper. The resulting color change is due to complex reactions leading to the formation of various compounds with different colors.
As for whether ascorbic acid turns brown when not on paper, it can still undergo oxidation in the presence of air, but factors such as the environment, concentration, and presence of other compounds can influence the reaction and the visible outcome. Preventing oxidation is a key role of ascorbic acid in the body, such as in the case of tyrosinosis, where ascorbic acid can prevent oxidation of certain compounds that would otherwise lead to the disease.