Significant Figures: Count, Round and Calculate

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Science · Chemistry
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Page 2
Significant Figures: Count, Round and Calculate
Science · Chemistry · Grades 9-10 · 50 minutes
Time needed
50 minutes (one class period)
Prep
Copy the 2-page worksheet (one per student). Write the four counting rules from the
worksheet on the board.
Technology
None. A basic calculator is optional.
Student
independence
High. Students work alone, then check with a partner.
Copies to make
One 2-page worksheet per student
Materials
Pencils, Worksheet, Calculators (optional), Whiteboard or chalkboard
Goal for today: Students will count significant figures in measurements, round to a stated number of significant
figures, and report calculated answers with the correct precision.
Before the bell
☐ Copy the worksheet, one per student (2 pages).
☐ Write on the board: 1) Nonzero digits count. 2) Zeros between nonzero digits count. 3) Zeros at the start
never count. 4) Zeros at the end count only if there is a decimal point.
☐ Keep the answer key on your desk. You do not need any science background; every rule is printed on the
worksheet.
Say this to start
Good morning! Today is a measurement practice day in chemistry. Scientists report numbers so that they show how
carefully something was measured. The rules for that are called significant figures. All the rules are on the worksheet
and the board, so you will work on your own and then check with a partner.


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Science · Chemistry
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Page 3
Run of show
Minutes
What you do
What students do
0–5
Take attendance. Hand out the worksheet. Ask students to
read the rules box at the top silently.
Read the rules box and underline anything
that is new to them.
5–12
Read the worked examples aloud: 0.0072 has 2 significant
figures (the leading zeros do not count). 3.200 has 4 (zeros
at the end count because of the decimal point). 1005 has 4
(zeros between nonzero digits count).
Follow along and copy the three examples
into the margin.
12–24
Tell students to complete the table (counting) and
questions 1–5 (rounding). Walk the room and point stuck
students back to the four rules.
Count significant figures for each
measurement, then round each number as
directed.
24–40
Tell students to do questions 6–10. Remind them: for
multiply and divide, count significant figures; for add and
subtract, count decimal places. Calculators are fine.
Solve each calculation, then round the
answer using the correct rule.
40–47
Have partners compare answers to questions 11 and 12
and explain any differences to each other. Then read the
answers for the table and questions 1–10 aloud from the
key.
Discuss, check their work, and correct any
mistakes.
47–50
Have students answer question 13 (the exit question) and
collect all worksheets in a stack for the teacher.
Write a short answer and hand in the
worksheet.
If things go wrong
• If students say they have not learned this yet, tell them the rules box teaches everything needed. Have them
do the table first and check it with a neighbor.
• If students disagree about 600 (how many significant figures), tell them to use the rule as printed: trailing
zeros with no decimal point do not count. It is an agreed convention for this worksheet.
• If a calculator shows many digits, remind students that the calculator does not know about significant figures.
They must round the answer themselves.
• If the room gets loud, return to silent work for five minutes and let partners talk only in the last ten.
Early finishers
• Write three of your own measurements with different numbers of significant figures (for example 0.0050 or
120.0) and swap with a neighbor to count them.
• Explain in two sentences why a ruler marked in millimeters gives more significant figures than a ruler marked
in centimeters.
Support and challenge
• Support: Complete only the table and questions 1, 2, 3, 6 and 8. Use the rules on the board for every answer.
• Challenge: Answer this on the back: 12.5 × 0.0200 ÷ 3.1. Give the answer with the correct significant figures
and explain which measurement limited it.


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Science · Chemistry
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Page 4
Notes for the teacher (you fill this in)
Jot a few lines before you leave. A full Daily Sub Report is in the free Sub Survival Kit at SubstitutePlans.com.
How far the class got / what to pick up next:
Students who were especially helpful:
Anything the teacher should know: