Which of the following processes are spontaneous and which are non-spontaneous ?
(i) Flow of air from high pressure to low pressure.
(ii) Formation of ice in a refrigerator.
(iii) Spreading of a drop of ink in water kept in a beaker.
(iv) Reverse osmosis.
(v) Burning of coal in air.
(vi) Dissolution of Cu in `ZnSO_4` solution.

Explanation

To determine which processes are spontaneous and which are non-spontaneous, we need to analyze each process based on the definition of spontaneous processes. A spontaneous process is one that occurs naturally without the need for external energy input, while a non-spontaneous process requires external energy to occur.

Let's evaluate each process one by one:

1. **Flow of air from high pressure to low pressure**:
- This process occurs naturally as air moves from areas of high pressure to low pressure without any external energy input.
- **Conclusion**: Spontaneous.

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