Answer:
Concentrated sulfuric acid is a nonvolatile and hygroscopic liquid, capable of oxidization and dehydration, whereas dilute sulfuric acid is not. If you drop concentrated sulfuric acid on white sugar, for example, the color of sugar will turn from white to black (picture). This is because sugar is dehydrated by concentrated sulfuric acid and as a result becomes carbon. Such a reaction does not occur with dilute sulfuric acid. So why is there such a difference between the two of them?
 
This might be the easiest answer:  The reactivity of any compound will be decreased when it is diluted with a non-reactive solvent. So it would be natural that the reactivity of concentrated sulfuric acid is decreased when it is diluted with water.
 
This is true, however, this is too general. Let’s focus on a further discussion of sulfuric acid.
 
Since the bond between H⁺ and HSO^4- in sulfuric acid (H₂SO₄) is weak, the bond is easily broken, and H₃O⁺ is formed when water is added to sulfuric acid. This reaction is highly exothermic.  The hydration of HSO^4- is also highly exothermic.
 
In other words, H₂SO₄ diluted with water is too unstable to exist as “H₂SO₄” any more. So it changes to H⁺ ion and HSO^4- ion, both of which are surrounded by water and become much more stable.  Both ions are less reactive than the pure H₂SO₄ molecule. Therefore, “reactive” H₂SO₄ turns to “non-reactive” ions when it is diluted with water.
 
Therefore, the ability to dehydrate is a property of the undiluted H₂SO₄ molecule. The ability to oxidize is also a property of H₂SO₄ molecule.
 
By the way, 100% pure sulfuric acid is not a compound we commonly use. SO₃ gas is adsorbed into concentrated sulfuric acid to make 100% sulfuric acid. This means that toxic SO₃ gas can evaporate and could damage your eyes and skin when you open the bottle. 100% pure sulfuric acid is a very dangerous chemical.
 
 
Acknowledgement
We would like to thank an anonymous professor for his careful teaching and kind support for this answer.

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