Give one example in each case
Change which occurs on mixing two substances, but cannot be reversed.

Explanation

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks for an example of a change that occurs when two substances are mixed together, and this change cannot be reversed. This indicates we are looking for an irreversible change.

2. **Define Irreversible Change**: Irreversible changes are those that cannot be undone. Once the change has occurred, the original substances cannot be recovered in their initial form.

3. **Identify an Example**: One common example of an irreversible change when mixing two substances is the combination of cement and water.

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