Science and Engineering Practice 3 - Planning and Carrying Out Investigations
Cross Cutting Concept: Scale, Proportion, and Quantity
This lab is my student’s first opportunity to handle equipment and perform measurements. They will be carrying out an investigation (S&E Practice 3 - Planning and Carrying Out Investigations) to determine the quantities (CCC: Scale, Proportion, and Quantity) of distance, mass, and volume using metric units. If students are expected to plan investigations and collect data to prove a finding they must be familiar with the tools required and the relevance of the numbers they are collecting. In other words, practice makes perfect. This lab is intended to be a precurser to other labs where accurate measurements define the learning process.
Most of my students have been exposed to metric measurements, but don’t have an innate understanding of this base-10 system. The more practice they receive, the more comfortable they become with the metric system.
Demonstrating the proper techniques for using a metric ruler, triple beam balance scale, and a graduated cylinder are new and critical tools that are employed in my science lab on a regular basis, so familiarity is critical. I start out by demonstrating how a metric ruler is divided into centimeters and millimeters and how to record a measurement.
I actually teach my kids in a separate lesson how a triple beam balance scale works [see Triple Beam Balance Activity], but this is their first time being able to manipulate the three weights in order to create a measurement.
Finally, I demonstrate how a graduated cylinder is able to make very precise volume measurements of liquids. This is a major emphasis of this lab, since they have never seem this tool before. I try to dispel any anxiety by letting students know that graduated cylinders are nothing more than fancy measure cups, similar to those used in kitchens. I repeat a familiar mantra that chemistry is a lot like cooking; just don’t lick any spoons. The two skills they desperately need are 1) how to fill a graduated cylinder under a sink without making a mess, and 2) how to read a measurement.
1) Not making a mess – I explain that in order to place water in a graduated cylinder using a sink, the sink water must first be turned on gently. The graduated cylinder is then placed under the flow of water and removed when the amount is close to their target amount. They can make minute adjustments using this technique. I explain to them that if they put the graduated cylinder under the water first, then turn on the water they will most likely turn the water on too forcefully and cause a geyser to hit the ceiling.
TIP: I have found that if I make students responsible for cleaning any messes they make, they are more careful with equipment and scientific techniques.
2) Reading a graduated cylinder is easy once you understand the technique. Water, or any liquid, will form a curved surface in a small cylinder. This curve makes measuring volume challenging. The accepted technique is to use the lowest point of the curve as your basis of measurement. The highest point of the curve can be as much as 3 mL higher.
Since this is the first student lab of the year, I review expected norms for the kids. Specifically eye safety. The kids are responsible for wearing eye protection during the entire lab. They can only remove eye protection once I have checked them out. The students are placed in a group based on their assigned seats. I let them know that if there is a personal conflict within the group, that they should discuss it with me in a private setting. Each group is given a tray of needed equipment, a set of directions, a data chart, and an equipment sign-out sheet. I've included a list of materials needed for this lab.
It is their responsibility to check the condition of the equipment at the beginning of the period and report and damage immediately or they may be responsible for replacing broken materials. After they have reviewed the material list each student signs their name to the equipment list.
TIP: Over many years I have never had a student willfully damage any equipment. I understand that accidents do happen and anticipate a few broken pieces of equipment – they're only eighth graders.
Pass out one copy of the Equipment Measurement Lab sheet to each student.
1) Using your ruler (metric side only) measure to the nearest mm the length of this sheet of paper, record in Box A.
2) Using your ruler (metric side only) measure to the nearest mm the height (opposite side) of this sheet of paper, record in Box B.
3) Use deductive reasoning to measure the width of a single sheet of paper, record in Box C.
4) Using your ruler (metric side only) measure to the nearest mm an object in this classroom (you may get up and walk around), record in Box D.
5) Using the triple beam balance scale weigh to the nearest tenth of a gram the small plastic cup provided, record in Box E.
6) Fill the plastic cup half way with water, weigh the water filled cup, record in Box F.
7) Use deductive reasoning to figure out how much the water (only the water) weighs, record in Box G.
8) Fold the sheet of paper in half and weigh the paper, record in Box H.
9) You now want to accurately remove 5g of salt (NaCL) from the container. Since you know what the paper weighs add that number to the weight of the salt (NaCl) (paper weight + 5g) and set the scale to this number. Using the handle of the spoon, carefully add salt (NaCl) onto the paper until the scale balances itself out, record in Box I.
10) Using the folded paper, carefully add the salt (NaCl) back to it’s container.
11) Using the graduated cylinder, carefully add water until it is half full.
12) Remember that water forms a meniscus in small cylinders, measure the water to the nearest mL, record in Box J.
Since this is their first student lab of the year, the only publishing result I require is for them to complete the lab questions in complete sentences and turn them in before the period is over.
Student Work Example
I remind the kids know that they should learn these measurement procedures, as we will be using them for the rest of the year. Early success now will produce lab success for the rest of the year.
I point out that in later labs, such as 'Chemical and Physical Changes' and 'Determining Freezing Points', they will use this equipment. By advertising future labs I can get the students buy-in early.
1) Chemical and Physical Changes lab - students will perform five experiments to determine if a reaction is physical or chemical.
2) Determining Freezing Points lab - students observe cancel wax hardening into a solid (freezing) and figure out the freezing point of candle wax.
If the students are still not buying into the need for these skills I tell that that in high school they will probably have a lab partner that they fall in love with. What better way to impress this special someone then to explain how to use laboratory equipment. Sounds silly, but middle-schoolers get this.