Water boils-----you heat it to 100 Degree Celsius. ?

The statement "Water boils-----you heat it to 100 Degree Celsius" requires a conjunction that establishes a conditional relationship. Out of the provided options, "if" is the most appropriate choice.

When we say that water boils "if" you heat it to 100 degrees Celsius, we are indicating that this temperature is the specific condition under which water reaches its boiling point. This relationship is foundational in understanding physical properties of substances, as it establishes a clear cause-and-effect scenario: the action of heating water to a specific temperature results in the boiling of the water.

Let’s look at the other options to clarify why they are not suitable:
- "Unless" would suggest that water will boil except when heated to 100 degrees Celsius, which does not make sense in the context.
- "Until" implies a duration, suggesting that boiling occurs before reaching 100 degrees, which is incorrect.
- "Although" introduces a contrast, which doesn't fit in this context.

Therefore, the best option is "if," as it illustrates the direct condition necessary for the process of boiling to occur.

To help you remember the answer, focus on these terms: Conditional, Cause-Effect, Boiling Point.