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### Chemistry

#### Significant Figures

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Rebekah Case 220 Points

I am teaching significant figures soon and I am curious..how do you introduce sig figs to your students? I want to do something interactive to show them the importance of the topic besides just starting out with the rules. Any great inquiry lessons, cool demos? Let me know. Thanks!

Paul Molitor 20 Points

If you have a good periodic table you will noticed that atomic masses are listed with sig figs ranging from 5 to 8 or 9. If you have already discussed atomic masses and natural abundaces of isotopes this can lead to interesting discussions.

Pamela Auburn 68625 Points

Rebekah, Significant figures are important because measurement involves estimation. I typically have lessons on measurement and introduce sigfigs in this context. You might have students measure the dimensions of an object pointing out what is known and what is estimated. The last digit the guess is 1/10 the finest calibration. When they think the measurement falls "exactly" on for example 6 they are in fact estimating the 6.0. If you point this out, it can make the "rules" easier to understand. Zeros are significant when they call between to sig figs and when they are the measurement estimate. To me this is a bit more real than using the "unnecessary for holding the decimal place" That rule is too abstract. Trailing zeros are significant when they derive from measurement estimates.

Rebekah Case 220 Points

Thank you for the ideas. I had two students come to the front. One of the students measured the height of the other student using three different meter sticks (each one with a different calibration). This turned out to be a great way to introduce the topic, since they had to estimate the last digit each time.

Florence Hulihee 495 Points

I do this activity where all the students are given cards with a number, a zero, or a decimal point on it. They then leave the room except for 2 guardians. I give the guardians them a rule to let students back into the room - no zeros allowed. After everyone but the zeros get in, individually, they have to come up with the rule. Next, they have to form groups of 2 or 3 and I tell the guardians that only the zeros in the middle of 2 numbers can get in. Then they have to come up with the rule of captured zeros. Then we add in decimal points and so on until we cover all the rules of sig figs. The kids like it because it's a lot of in and out and they figure out the rules for themselves and it really seems to stick with them.

Gwendolyn Huggins 1060 Points

This is a great way to get students involved. Thanks for sharing. The resouces were great.

Chris Leverington 4035 Points

Florence...that seems brilliant!

Rochelle Tamiya 4095 Points

Thank you for posting this topic....and thank you Adah for your resource help! I really do like the worksheet with the rationale. I don't really teach rationale since it is addressed by our math teacher the first quarter but it is such a great resource to have, especially when we get to the data analysis portion of our science fair projects.

Thanks for posting to this topic. The high school wants us (8th grade middle school teachers) to teach sig figs. Now, I'm not exactly sold on the idea that they're appropriate for middle school. But, I really do like your 2 activities. I think a combining the 2 activities would give me the most bang for my buck with my kids. I can use the ruler idea to first teach my kids that there's always imprecision in measurement, then I can use the in-and-out activity to have my students develop the rule. Afterwards, we can go back to the rulers, and they can practice. Then, they can use the worksheets Adah provided so they can have EXTRA practice at home, and then I can post the ppt on my class site (or make my own, since I don't want to plagiarize) for kids who need the extra help or miss class! Super-keen ideas! Thanks!

Carolyn Mohr 92326 Points

Hi Everyone!
I enjoyed reading each of your posts on this topic. I checked to see what was in the Learning Center related to significant figures. I was delighted to find one article that I thought was a worthwhile 'read'. It is Science From the Pond Up: Using Measurement to Introduce Inquiry
The way the instructor helped students think about their use of significant figures was masterful. I think this particular lesson could be adapted for use with middle school through college level students.
Carolyn

That's such a great resource! Thank you for sharing! I think I can adapt this to my aquaponics class easily! I can probably use it as a model for my earth/space science course, too. Thanks so much! These are all great resources!

I really like Florence's idea. I do an activity where they use different amounts of water in graduated cylinders (using different sizes) and using balances to measure the mass of pennies. The cylinders force them to predict the next digit and the balance teaches them that even the balances predict the last digit. But I really like Florence's idea.

Pamela Auburn 68625 Points

This is old story about the importance of minding sigfigs The science teachers at a Baltimore County middle school wished to acquire a steel cube, one cubic centimeter in size to use as a visual aid to teach the metric system. The machine shop they contacted sent them a work order with instructions to draw the cube and specify its dimensions. On the work order, the science supervisor drew a cube and specified each side to be 1.000 cm. When the machine shop received this job request, they contacted the supervisor to double check that each side was to be one centimeter to four significant figures. The science supervisor, not thinking about the "logistics", verified four significant figures. When the finished cube arrived approximately one month later, it appeared to be a work of art. The sides were mirror smooth and the edges razor sharp. When they looked at the "bottom line", they were shocked to see the cost of the cube to be \$500! Thinking an error was made in billing, they contacted the machine shop to ask if the bill was really \$5.00, and not \$500. At this time, the machine shop verified that the cube was to be made to four significant figure specifications. It was explained to the school, that in order to make a cube of such a high degree of certainty, in addition to using an expensive alloy with a low coefficient of expansion, many man-hours were needed to make the cube. The cube had to be ground down, and measured with calipers to within a certain tolerance. This process was repeated until three sides of the cube were successfully completed. So, "parts and labor" to prepare the cube cost \$500. The science budget for the school was wiped out for the entire year. This school now has a steel cube worth its weight in gold, because it is a very certain cubic centimeter in size.

[size=100]Sig figs is something is establish at the beginning of the year. I like this one activity where students are grouped into teams. Then each group is given a set of index cards with various numbers like zero, whole numbers, decimals, positive and negative, and fractions. I've already set up a clothes line across the room, students objective is to organize the cards in the right placement and right spacing between the numbers. At first students don't know where to put numbers on the line. When they see zero, positive and negative numbers,they place zero directly in the middle of the line. I just try to build in good habits when solving math related problems, especially when there are decimals in the problem. Students don't know to what decimal number to round to, and also found out that many students aren't sure or confidence on how to round numbers. [/size]

Chris Leverington 4035 Points

Rounding has been my big annoyance lately. A lot of kids just seem to drop the numbers off the end without considering whether they should round up or not. Ex..16.599 becomes 16.5 or just 16 I almost slapped a kid the other day...we were doing a limiting reagent problem and it came out that they needed .2 moles of 1 chemical and they had .15. I asked the kid if they had enough of the chemical and she responded, "well, I could round .15 up to .2...so ya I do!"

Mitchell Miho 3090 Points

I am so glad that someone has took the time to post on this topic. My department has been debating on whether or not we should spend time to go over sig figs in our curriculum at the beginning of the year as well. We came to the conclusion that it is necessary, especially in science because not being able to round or even know what the sig figs stand for is really a disservice to the students in any science course. I'm not sure if any of you folks have this problem, but a bunch of the students in today's classrooms forget to put the decimal point in the correct place or don't understand how to read exponents. Thanks for all of your input and great resources, next year I am definitely taking the time to refresh their memories and run a lab that can possibly show them the hazards of writing or forgetting to write the correct sig figs especially in math and science where there are times that we do need to be precise.

Chris Leverington 4035 Points

How much do you focus on sig figs throughout the year. Yesterday I gave my honors students their stoichiometry test. I told them that sig figs would be a portion of the grade. There were 2 5 point questions and 4 10 point questions. I counted 1 point per problem for sig figs. I had a kid who did every problem correctly, but had sig figs wrong in every problem. I realized that if i took off 1 point for sig figs, that he would end up with a B on the test...which I didn't feel was fair. I don't think its too much to ask my honors kids to remember the sig fig rules(they're posted on the wall)...but some just refuse to do it.

Pamela Auburn 68625 Points

The frustration I have in teaching significant figures is that students memorize a set of rules without really understanding what is doing on. I recently adapted the lab here www.olemiss.edu/projects/.../project133.doc? and they seem to get a bit better understanding that sig figs relate to measurement error. It also helps to explain the error here means variability and mistakes

Chris Leverington 4035 Points

Pam...the link doesn't work :( Can you repost..I'd love to see the activity.

Pamela Auburn 68625 Points

Sorry Chris Don't you hate when that happens Pls find attached

#### Attachments

PopcornSigFig.doc (0.04 Mb)

Rebecca Falin 71530 Points

Neat activity. Thanks for posting!

Tori Popoloski 1500 Points

:) awesome ideas

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