Chemistry · Some Basic Concepts Of Chemistry · NEET
It is the small amount of doubt in the last digit of any measured value. When you write 11.2 mL, the '11' part is certain (you are sure of it) and the last '2' is estimated. NCERT says an uncertainty of ±1 in the last digit is always understood unless told otherwise. So 11.2 mL really means somewhere between 11.1 and 11.3 mL.
No, we can't. NCERT gives two reasons: (1) the limitation of the measuring instrument, and (2) the skill of the person making the measurement. Every instrument can only read up to a certain fineness. A better instrument reduces uncertainty but never removes it. Example: a platform balance gives 9.4 g, but a more precise analytical balance gives 9.4213 g for the same object. So the '4' in 9.4 g was the uncertain digit.
Always the LAST digit you write. NCERT rule: significant figures are the digits known with certainty PLUS one last digit that is estimated (uncertain). In 9.4 g, the 9 is certain and the 4 is uncertain. In 0.200 g, the last 0 is the uncertain digit. Only the final digit wobbles; all digits before it are trusted.
It means the true value could be one unit higher or lower in that last place. If a burette reading is 11.2 mL, the ±1 is in the last (tenths) place, so the real volume lies between 11.1 mL and 11.3 mL. This is why we do not add extra digits we are not sure of — writing 11.234 mL would falsely claim we know the reading much better than we do.
No, they are related but different. Error is how far your value is from the true value. Uncertainty is the built-in doubt in the last digit of ANY reading, even a correct one — it exists because instruments and people have limits. You can have very small error but still have uncertainty. In NEET's chapter, uncertainty is handled by writing the correct number of significant figures.
They are two sides of the same idea. We SHOW the uncertainty of a measurement by choosing how many significant figures to write. More significant figures = a more precise (less uncertain) instrument. That is why 9.4 g (2 sig figs, platform balance) and 9.4213 g (5 sig figs, analytical balance) describe the same object but with different uncertainty. Reporting the right number of significant figures is how chemists communicate uncertainty honestly.
NEET directly tests significant figures and rounding rules, which come straight from this uncertainty topic. If you know the last digit is uncertain, you know why answers must be rounded to the correct number of significant figures and why you cannot write extra fake digits. Questions on rounding (like the 2026 re-NEET one) reward students who understand this.
The numbers 17.0145 and 21.0235 were rounded to three figures after the decimal point. The resulting numbers, respectively, are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. A better instrument only reduces uncertainty and pushes it to a later digit. Every measurement, however fine, still has an uncertain last digit. Exactness is impossible in real measurement.
NCERT says unless otherwise stated, an uncertainty of ±1 in the last digit is always understood. So 11.2 mL means 11.2 ± 0.1 mL by default.
No. Counting exact objects, like 2 balls or 20 eggs, gives exact numbers with infinite significant figures and no uncertainty. Uncertainty applies only to measured quantities read off an instrument.
You show it by writing the correct number of significant figures — the certain digits plus one uncertain last digit. That is the standard chemistry way to communicate how precise a measurement is.
