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Title of the Experience

Density: freshwater vs. seawater

Name of the teacher

Roberto Checchi

Country where it took place

Italy

School typology

Vocational School

Thematic Area

Physics, Science

Experience typology

Teaching in class, Laboratory, Online Learning

Type of contact

Direct

Description of the Experience

Students’ proactive approach is fundamental for the laboratorial didactics. The teacher proposes a problem to be solved and at the same time ask the students to propose some experimental hypothesis.
To discover the differences between freshwater and seawater we measured both their densities taking the same quantity and volume of both of them. The greater density of the seawater is immediately visible. Then, indirectly, we measured the density of an egg and studied the Archimede’s principle. This nice experience transfers key concepts of Chemistry and Physics.
Taking pictures of each phase allow the students to provide documentary evidence of the studied phenomenon.
Students’ online research and the following deepening of the issues carried out guarantee customarily the reaching of the objectives.
The internal part of the egg is composed by a thick liquid (a mixture of water and organic molecules): is it thicker or less thick than water? The egg, put in the freshwater bowl, sinks because it is thicker than the freshwater. Instead, if we add NaCl (in the proportion of 35:1000) to the freshwater, reproducing the seawater solution, the egg keeps afloat. This demonstrates that the seawater is thicker than the egg.
This experience explains the reason why it is easier to swim in the sea than in a pool: in the seawater Archimede’s force buoys up our body mass more than elsewhere.

Comments on this Teachers Experience

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Date: 2014.11.06

Posted by Raul V. Lerma (Spain)

I like this experience because is very simple, it is easy to understand and implement and it doesn't requiere specific materials.
Using this practice students understand completely the concept of density in an experimental way and this will increase the motivation of students through learning.

Date: 2014.10.16

Posted by Milena Gosheva (Bulgaria)

The demonstration illustrates the floating of bodies according to the densities of different kinds of water. Very easy and useful way to visualize in physics classes in 8th grade. Students remember it and can quickly bring their knowledge to similar situations.They remember the law of Archimedes and conditions for floating of the bodies.

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