In the measurement 5.678 grams, which is the uncertain digit?

Explanation:

Every measurement has an uncertainty associated with the instrument used to measure.

The measurement represented by 5.678 grams has 4 significant digits: all the non-zero digits in a measurement are significant (non-zero digits and zeros between non-zero digits are always significant).

When you use a scale to measure the mass of an object, such scale has an appreciation and the last number that you can read is between two possible numbers, i.e. the digit 8 in the given measurement could have been a little bigger or a little smaller, so the real measurement could have been something between 5.677 . . . or 5.678 . . .

The last digit in any measurement is always uncertain.

8 is the uncertain digit.

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a corroding iron, if that's one of your choices