NCERT line-by-line
1. TITLE
NCERT says "A choice of change of different units does not change the number of significant digits" — Here's how NEET tests it
2. THE NCERT STATEMENT
NCERT EXACT LINE:
"A choice of change of different units does not change the number of significant digits or figures in a measurement."
— NCERT Class 11, Chapter 1, Page 3
3. WHY THIS LINE MATTERS
- How many times has NEET tested this? Indirectly tested almost every year in calculations and explicitly tested multiple times as conceptual unit conversion questions.
- What makes this statement tricky? When students convert units, the number gains or loses zeroes (e.g., $2.308 \text{ cm}$ becomes $0.02308 \text{ m}$ or $23080 \text{ \mu m}$). Visually, the number changes shape, causing students to mistakenly count the new placeholder zeroes as significant figures.
- What do students typically misremember? Students often assume that because a decimal point moves or disappears during conversion, the precision or the significant figure count also changes.
4. MCQ
According to NCERT, changing the units of a measurement is characterized by:
A) It does not change the number of significant figures in the measurement.
B) It changes the number of significant figures based on the location of the decimal point.
C) It increases the precision and least count of the measurement.
D) It alters the total number of significant and reliable digits.
5. ANSWER + LINE BREAKDOWN
Correct Answer: A
Break down the NCERT line:
- "A choice of change of different units" means converting a physical quantity from one system or scale to another (like $cm$ to $m$, or $g$ to $kg$).
- "does not change the number of significant digits" means the count of reliable digits plus the first uncertain digit remains exactly the same, no matter how many placeholder zeros are added.
- The key phrase is "location of decimal point" because its shift during unit conversion is of absolutely no consequence in determining the number of significant figures.
6. HOW NEET TWISTS THIS
Show 2-3 ways NEET can ask the same fact:
- As direct question: "A substance has a mass of $5.74 \text{ g}$. If this mass is expressed in kilograms ($0.00574 \text{ kg}$), how many significant figures does the mass have in kilograms?" (Answer: Three. The unit change doesn't alter the SF count).
- As assertion-reason:
Assertion: The measurement $2.308 \text{ cm}$ has the same number of significant figures as $0.02308 \text{ m}$.
Reason: A choice of change of different units does not change the number of significant figures in a measurement. (Answer: Both A and R are true, and R is the correct explanation of A).
- As an except question: "All of the following statements about significant figures are true EXCEPT: (a) Trailing zeros without a decimal are not significant (b) Change of units alters the significant figure count (c) Non-zero digits are always significant (d) Intervening zeros are significant." (Answer: b).
7. MEMORY LOCK
MEMORIZE:
Units Change, Sig Figs Stay! (Location of the decimal point is just an illusion).
Mnemonic: C-U-N-S (Change Units, No Shift in SF). To avoid confusion altogether, always use scientific notation—the power of 10 is irrelevant to significant figures!
8. RELATED NCERT LINES
Also remember these from the same page:
- "This shows that the location of decimal point is of no consequence in determining the number of significant figures."
- "To remove such ambiguities in determining the number of significant figures, the best way is to report every measurement in scientific notation (in the power of 10)."
9. NCERT REFERENCE
NCERT Class 11, Chapter 1, Page 3-4, Section 1.3
10. CTA
All important NCERT lines from this chapter → Physics and Measurement Guide