In the preparation of $\ce{H2SO4}$ through the Contact process, $\ce{H2SO4}$ is added to hot sulfur trioxide to form oleum. This oleum is then diluted by adding water to form $\ce{H2SO4}$.

$$\ce{H2SO4(l) + SO3(g) → H2S2O7(l)}$$

$$\ce{H2S2O7(l) + H2O(l) → 2 H2SO4(l)}$$

In a preparatory reaction, different reagents are used to obtain the desired product but here the product is itself a reactant and is used to make the product. Indirectly, $\ce{H2SO4}$ is being used to make $\ce{H2SO4}$.

Would this process be called as a preparation process?

  • $\begingroup$ Remembered the process of making big yogurt using a smaller yogurt! $\endgroup$ Commented Aug 17, 2023 at 20:33

2 Answers 2


You can call it whatever you like. Personally I would say you are preparing sulphuric acid because you end up with more than you started with - 1 mole is used to form 2 moles.


The term "preparation" generally refers to the synthesis of chemical species on a laboratory scale.

Since you can perform the contact process on a laboratory scale, it definitely qualifies as a "preparation" process.


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