Chemistry · Some Basic Concepts Of Chemistry · NEET
You count DECIMAL PLACES, not total significant figures. This is the most common mistake. For addition and subtraction, find the number with the least digits AFTER the decimal point. Your answer must be rounded to that many decimal places. (The 'count significant figures' idea is only for multiplication and division.)
Step 1: Add or subtract the numbers normally and get the raw answer. Step 2: Look at each starting number and count how many digits come after its decimal point. Step 3: Find the smallest of those counts. Step 4: Round your raw answer to that many decimal places. Example: 12.11 + 18.0 + 1.012 = 31.122 raw. The number 18.0 has only 1 decimal place (the least), so the answer is reported as 31.1.
Because 18.0 is only known to one decimal place. The digit after that is uncertain for 18.0, so we do not know the second and third decimals of the total. The answer cannot be more precise than the weakest measurement in the sum. So we round 31.122 to one decimal place, giving 31.1. This is the exact example given in NCERT.
No. Only decimal places matter for addition and subtraction. The final answer might end up with more or fewer significant figures than the starting numbers, and that is fine. For example, 100.5 - 0.5 rules are based on decimal places, not on how many significant figures the result shows.
The same rule applies. Round to the least decimal places among the numbers you subtracted. Subtraction can sometimes leave very few significant figures (this is called loss of significance), but the decimal-place rule still decides how you write the answer. Example: 2.5 - 0.24 = 2.26 raw, but 2.5 has 1 decimal place, so the answer is 2.3.
Two different rules. ADDITION and SUBTRACTION: match the least DECIMAL PLACES. MULTIPLICATION and DIVISION: match the least SIGNIFICANT FIGURES. Mixing these two up is the number-one error students make in the NEET measurement section. Remember: plus/minus -> decimals; times/divide -> sig figs.
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.
Round the final answer to the same number of decimal places as the number that has the least decimal places. Do not count total significant figures for addition or subtraction.
Because when you line up numbers to add them, the uncertainty sits in the decimal columns. The least precise decimal place limits how precisely you can know the total, so the answer cannot show more decimals than that.
Yes. ReNEET 2026 asked a direct rounding question. Learn the 'digit-before-5 even stays, odd rises' rule, because rounding is the last step whenever you apply the decimal-places rule.
No. Exact numbers (like 20 eggs) have infinite significant figures and infinite decimal places, so they never limit the answer. Only measured numbers with real uncertainty decide the decimal places.
Next learn multiplication and division in scientific notation, where you match the LEAST significant figures instead of decimal places. Keeping the two rules separate is key to scoring in the measurement section.
