The most testable hypothesis from the options provided is:
If acid is added to a solution, then the pH of the solution will go down.
Reasoning:
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Testable: This hypothesis suggests a specific cause (adding acid) that will result in a measurable effect (the pH of the solution decreasing). It can be tested through experiments by measuring the pH level before and after the addition of acid to a solution.
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Specific and Measurable: The changes in pH can be quantitatively measured using a pH meter or pH indicator, allowing for clear observations about the effect of the acid.
The other statements are less clear in terms of testability for the following reasons:
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Plants produce glucose as the end product of photosynthesis: While this is known to be true, it is more of a scientific fact than a testable hypothesis because it doesn't set up a cause-and-effect relationship to be explored experimentally.
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Dropping a ball from a height of 10 m will cause it to bounce back up 5 m: This is a prediction that might not hold true; the actual bounce height can be influenced by many factors (like the type of ball, surface from which it is dropped, etc.). It does not establish a robust, reproducible outcome under controlled conditions.
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If certain foods are healthy for you, then they should also taste good: This is subjective and depends on individual taste preferences, making it difficult to measure or test consistently.
Therefore, the first statement about the effect of acid on pH is the most clearly testable.