Step 1: Understand the task.
We have four sentence parts and must arrange them in the right order to make one clear, correct sentence. The parts are (i) When a chemical substance, (ii) the food poisoning took place, (iii) in the food preparation, (iv) was mistaken for salt and used.
Step 2: Find the main statement.
Part (ii), 'the food poisoning took place', is a complete idea and works well as the main clause to start with.
Step 3: Add the condition.
Part (i), 'When a chemical substance', begins a 'when' clause that tells us the cause. So it follows the main clause.
Step 4: Complete the action.
Part (iv), 'was mistaken for salt and used', tells what happened to that chemical substance. So it comes right after part (i).
Step 5: Finish with the place.
Part (iii), 'in the food preparation', tells where it was used, so it ends the sentence. The full order is (ii), (i), (iv), (iii), giving 'The food poisoning took place when a chemical substance was mistaken for salt and used in the food preparation.'
Step 6: State the conclusion.
The correct order is (ii), (i), (iv), (iii).
\[ \boxed{(ii),\ (i),\ (iv),\ (iii)} \]