Can a chemical reaction happen with only one substance? For example, if you select water as the first substance but you don't add another substance, will a chemical reaction happen or not?

Explanation:

Yes, a chemical reaction can happen with only one substance.

A typical example of such reaction is the radioactive decay.

Radioactive decay is the loss of elementary particles from an unstable nucleus, ultimately changing the unstable element into another more stable element. .

When radioactive elements are let alone, they decompose to form  stable harmless elements.

An example of radioactive decay is attached in the image below;

No

Explanation:

A chemical reaction cannot happen with water alone. This is because, a chemical reaction must involve collision of particles from different reactants in order to give a product. No chemical specie can react with itself without some kind of interaction with another chemical specie which may or may not enter into the rate equation for that reaction.

Only nuclear reactions are spontaneous and involve only one reactant for spontaneous radioactive decay.

1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%