The Density-Specific Gravity Connection

Abstract

Recently a student asked, "Why do we use both density and specific gravity? They have about the same value". The answer can be found in our inability to measure volumes as effectively as we can measure mass. A milliliter sample of liquid can be weighed to four decimals on an ordinary analytical balance but to only one decimal for the volume using a small graduated cylinder. This disparity between mass and volume can be eliminated by a pycnometer, which allows very small samples to be analyzed and requires only weighings. In essence a pycnometer provides density comparisons between a standard liquid, usually distilled water, and an unknown sample.Mass of X Specific gravity = Volume Mass of Water Volume Since the volume is the same for both density measurements, it will cancel out leaving only the more valuable mass data. To convert specific gravity to density, simply multiply the specific gravity of the sample by the density of the standard at the temperature of use. A compromise is often encountered by the use specific gravity with exponential notation, which indicates the temperatures at which the two weighings were made, for example, 0.901 25/25.

Proposal for a Didactic Tool on Teaching Practices Related to the Selective Sorting of Plastic Waste According to Relative Density in High Schools: Case Study in Burkina Faso

“…The term consecrated in French to name this “relative value”, d species , experimentally measurable and dimensionless, is the term “Densité”. The term “density”, which could have been its translation in English, would designate in English speaking countries the “voluminal mass” ,, or “absolute density”. The term “density” is used in this report and corresponds to the “relative density” or “specific gravity” that is measured by the densimeter.…”

“…In practice, the teaching of relative density in middle school focuses on the concepts of flotation and sinking or heaviness without paying attention to the mass and volume that could have extended reflection on the concept of relative density even without involving calculations. In high school, on the other hand, the intuitive aspects are ignored, and the emphasis is placed on two aspects, memorizing a definition and solving problems using mathematical formulas, which students perform even if they do not have a conceptual understanding of relative density. , …”

“…At these usual units, the relative density seems to be confused with the voluminal mass. The omission of the quantity in the denominator of the quotient representing the voluminal mass of water, and thus of its units, constitutes implicit forms of breach of the didactic contract 12 or of change of the rules of the didactic game according to Sensevy's theory. 23 The didactic contract is the set of explicit or implicit mutual expectations of each of the two actors in the didactic situation.…”

“…In high school, on the other hand, the intuitive aspects are ignored, and the emphasis is placed on two aspects, memorizing a definition and solving problems using mathematical formulas, which students perform even if they do not have a conceptual understanding of relative density. 12,13 Teaching/Learning Theories Constructivism is a theory of learning founded by Jean Piaget from those of psychology. 24−27 This theory holds that the learner is an actor of her/his intellectual development by adaptation to her/his environment.…”