Which is a better oxidising agent : dilute nitric acid or concentrated?
The simple answer to your question is concentrated. Nitric acid is a very powerful oxidizing agent. Therefore, simply stated, concentrated nitric acid would be a better oxidizing agent. However, it does depend on your definition of "better." Here is what I mean. Concentrated nitric acid is a very, very strong oxidizing agent. It might be too strong for the situation you need. Therefore, in your case, dilute nitric acid might be the "better" choice. There is the saying..."Too much of a good thing can be bad." This is sometimes the case with concentrated nitric acid. Good luck with your debate in class.
What does sodium hydroxide dissolve?
Sodium hydroxide is notorious for dissolving things like grease and fats as well as human skin! That is why it is commonly called CAUSTIC SODA. It is therefore widely used to clean ovens, unblock drains and dissolve the hair that can build up and block showers. Hot concentrated solutions are therefore very dangerous and should be handled with great care in the lab. NaOH soln will also dissolve metals showing amphoteric properties eg aluminium and zinc and tin If sodium hydroxide is kept in glass bottles it will slowly dissolve the glass making the bottle glass becoming marked over time. It will also dissolve some organic acids that are not very soluble in water by forming their soluble sodium salt. [acid + alkali = salt. + water] As another answer points out, you need NaOH in aqueous solution, and because of the water present it will dissolve many things such as sugar (glucose) but there is no reaction. On the other hand, many water soluble salts eg copper sulfate will not dissolve in the water present as the insoluble metal hydroxide forms as a protective coating on the surface by double displacement reactions. Perhaps the attack on normally inert metals in boilers, known as caustic embrittlement should be mentioned. Caustic embrittlement is the phenomenon in which the material of a boiler becomes brittle due to the accumulation of caustic substances. In summary: Will Dissolve: Skin, fats, hair, fur and other animal tissues, amphoteric metals, soda glass, It Won't Dissolve: Plastics,Rubber, Iron and steel, copper