Density Straw (Mara)
Mara Desso
Principle(s) Illustrated
Density
Concentration
Solution chemistry
Standards
HS-PS2 Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
HS-ESS2-3 Develop a model based on evidence of Earth's interior to describe the cycling of matter by thermal convection
HS-ESS2-5 Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.
Questioning Script
Prior knowledge & experience:
Students have most likely seen the demonstration of various liquids of different densities in a graduated cylinder. This demonstration is different in that all four liquids are salt water with food dye, not different liquids.
Root question:
Why do the liquids separate when drawn into the straw in one order, but mix when drawn into the straw in a different order?
Target response:
Each liquid is a different concentration of sodium chloride dissolved in water (5%, 10%, 15%, and 20%). When drawn up in order from least dense to most dense, the liquids will remain separated because the least dense is on top. However, if a lower density liquid is drawn into the straw after a higher density liquid, the higher density liquid will sink to the bottom of the straw, mixing the two colors together.
Common Misconceptions:
Students will think that the food dye is causing the liquids to have different densities. To disprove this, mix the same food dyes in regular water, and then perform the experiment again. The liquids will not separate because they all have the same density.
Students will also think that all the liquids are different- be sure to tell them that they are in fact the same type of liquid (just different concentrations- but don't tell them that until the end!).
Photographs and Movies
Put all photos in the class Picasa Album and reference here. Do not upload the photos directly to this wiki as there is not enough memory in the website.
Put all movies in your own Youtube account. Make sure that the account is set for public viewing. Insert the Youtube videos here.