Wednesday, Oct 23, 2008 - Block 4B

Students took out notebooks

Density
Density is a measure of how much mass (matter) is packed into how much space.
Went over ideas of previous density lab in which students measured masses and volumes of various sizes of wood blocks and various sizes of metal blocks, plotted mass vs volume, drew best fit straight lines through the origin, and then found the slopes. The slopes gave the densities.

Today we are only measuring one of each kind of object and using density = mass/volume to find the densities. Because we don't have the checks of several pieces and averaging out uncertainties, you have to take extra care to make the measurements as precisely as possible.

Went over procedure to measure mass. For small, wet, rolly substances - always use a cup and zero out the cup.

Finding volumes - since these objects are not regular shapes, we cannot measure the length, width, and height using a ruler and then multiply them together to find the volume as we did before. Instead we have to use a displacement method. There are two displacement methods, one for small objects that will fit into a small graduated cylinder, and the second for larger objects.

Displacement Method 1 Used for small objects that fit into the 10 ml graduated cylinder.
Put about 6 ml of water into the graduated cylinder. Carefully measure and record this volume of water.
Insert object(s). The water level will rise. Carefully measure and record the height of this larger volume of water + object.
Subtract to find the volume of the object.

With the gravel, I asked students to dump out most of the water from the grad cyl and then dump the gravel into a paper towel, dab off, and then put paper towel wrapped gravel back into the cup.

Displacement Method 2 Used for larger objects that won't fit inside a small graduated cylinder.
Place beaker underneath nozzle of displacement can. Fill displacement can with water until it overflows through nozzle. When it stops dripping, remove beaker, dump out water, and replace beaker.
Carefully drop object into displacement can. Water will overflow through the nozzle into the beaker. When the water stops dripping, pour the water from the beaker into the 50 ml graduated cylinder and record the volume.

Had procedures written on board with pictures as well as on lab sheet.

Explained density circle equation and how to use it to find the unknown. Cover up the unknown and the remainder tells you the equation. Showed an example.

Handed out lab sheets. Students first worked on filling out table using the density circle equation. When students were done, I checked work. If ok, they then could start lab. If not correct, students redid it.

Students did lab. When done, I checked results. If ok, they then got question sheet and worked on questions. Handed it in when done. If the lab results were not correct, then they redid that part of the lab.

Students handed in both lab and question sheet.

For homework, student were to type up material list and procedure for how to find the density of the gravel.