The study of the density of substances begins in the course of high school physics. This concept is considered fundamental in the further presentation of the foundations of molecular kinetic theory in the courses of physics and chemistry. The purpose of studying the structure of matter, research methods can be assumed to be the formation of scientific ideas about the world.
Initial ideas about a single picture of the world are given by physics. Grade 7 studies the density of matter based on the simplest ideas about research methods, the practical application of physical concepts and formulas.
Methods of physical research
As you know, observation and experiment are distinguished among the methods of studying natural phenomena. They teach to observe natural phenomena in elementary school: they carry out the simplest measurements, often they keep a “Calendar of Nature”. These forms of learning can lead the child to the need to study the world, compare observed phenomena, identify cause-and-effect relationships.
However, only a fully conducted experiment will give the young researcher the tools to uncover the secrets of nature. The development of experimental, research skills is carried out in practical classes and in the course of laboratory work.
An experiment in the course of physics begins with the definitions of such physical quantities as length, area, volume. At the same time, a connection is established between mathematical (quite abstract for a child) and physical knowledge. Appeal to the experience of the child, consideration of facts known to him for a long time from a scientific point of view contributes to the formation of the necessary competence in him. The purpose of learning in this case is the desire to independently comprehend the new.
Studying Density
In accordance with the problematic teaching method, at the beginning of the lesson, you can ask a well-known riddle: “Which is heavier: a kilogram of fluff or a kilogram of cast iron?” Of course, 11-12-year-olds can easily answer a question they know. But addressing the essence of the issue, the ability to reveal its peculiarity, leads to the concept of density.
The density of a substance is the mass of a unit of its volume. The density table of substances, usually given in textbooks or reference books, allows you to evaluate the differences between substances, as well as the aggregate states of a substance. An indication of the difference in the physical properties of solids, liquids and gases, discussed earlier, an explanation of this difference not only in the structure and mutual arrangement of particles, but also in the mathematical expression of the characteristics of a substance, takes the study of physics to a different level.
The table allows you to consolidate knowledge about the physical meaning of the concept being studiedsubstance density. The child, giving an answer to the question: “What does the value of the density of a certain substance mean?”, Understands that this is a mass of 1 cm3 (or 1 m3) substances.
The issue of density units can be raised already at this stage. It is necessary to consider ways of converting units of measurement in different reference systems. This makes it possible to get rid of static thinking, to accept other systems of calculus in other matters.
Determination of density
Naturally, the study of physics cannot be complete without solving problems. At this stage, calculation formulas are entered. The density formula in grade 7 physics is probably the first physical ratio of quantities for children. It is given special attention not only because of the study of the concepts of density, but also for the fact of teaching methods for solving problems.
It is at this stage that the algorithm for solving a physical computational problem is laid, the ideology of applying the basic formulas, definitions, patterns. The teacher is trying to teach the analysis of the problem, the way to search for the unknown, the peculiarities of using units of measurement by using such a ratio as the density formula in physics.
Example of problem solving
Example 1
Determine what substance a cube of mass 540 g and volume of 0.2 dm is made of3.
ρ -? m=540 g, V=0.2 dm3 =200 cm3
Analysis
Based on the question of the problem, we understand that it will help us to determine the material from which the cube is madedensity table for solids.
Consequently, let's determine the density of matter. In the tables this value is given in g/cm3, so the volume from dm3 translated to cm3.
Decision
By definition: ρ=m: V.
We are given: volume, mass. The density of matter can be calculated:
ρ=540g: 200cm3=2.7g/cm3, which corresponds to aluminum.
Answer: the cube is made of aluminum.
Determination of other quantities
Using the density calculation formula allows you to determine other physical quantities. Mass, volume, linear dimensions of bodies associated with volume are easily calculated in tasks. Knowledge of mathematical formulas for determining the area and volume of geometric shapes is used in tasks, which makes it possible to explain the need to study mathematics.
Example 2
Determine the thickness of the copper layer that covers a part with a surface area of 500 cm2 if it is known that 5 g of copper was used for the coating.
h - ? S=500cm2, m=5g, ρ=8.92g/cm3.
Analysis
The density table of substances allows you to determine the density of copper.
Let's use the density calculation formula. In this formula, there is a volume of a substance, based on which linear dimensions can be determined.
Decision
By definition: ρ=m: V, but there is no desired value in this formula, so we use:
V=S x h.
Substituting into the main formula, we get: ρ=m: Sh, whence:
h=m: S xρ.
Calculate: h=5 g: (500 cm2 x 8, 92 g/cm3)=0.0011 cm=11 microns.
Answer: The thickness of the copper layer is 11 microns.
Experimental determination of density
Experimental nature of physical science is demonstrated in the course of laboratory experiments. At this stage, the skills of conducting an experiment, explaining its results are acquired.
Practical task to determine the density of matter includes:
- Determining the density of a liquid. At this point, the guys who have already used a graduated cylinder can easily determine the density of a liquid using the formula.
- Determining the density of a solid body of regular shape. This task is also beyond doubt, since similar computational problems have already been considered and experience has been gained in measuring volumes by the linear dimensions of bodies.
- Determining the density of an irregularly shaped solid body. When performing this task, we use the method of determining the volume of an irregularly shaped body using a beaker. It is useful to recall once again the features of this method: the ability of a solid body to displace a liquid whose volume is equal to the volume of the body. Further, the task is resolved in the standard way.
Questions of increased complexity
You can complicate the task by inviting the guys to determine the substance from which the body is made. The density table of substances used in this case allows you to pay attention to the need to be able to work withbackground information.
When solving experimental problems, students must have the necessary amount of knowledge in the field of using physical instruments and converting units of measurement. Often this is what causes the greatest number of errors and shortcomings. Perhaps this stage of the study of physics should be given more time, it allows you to compare the knowledge and experience of research.
Bulk Density
The study of a pure substance is, of course, interesting, but how often are pure substances found? In everyday life, we encounter mixtures and alloys. How to be in this case? The concept of bulk density will prevent students from making the typical mistake of using average density values of substances.
It is extremely necessary to explain this issue, to give an opportunity to see, to feel the difference between the density of a substance and the bulk density is at an early stage. Understanding this difference is necessary in the further study of physics.
This difference is extremely interesting in the case of bulk materials. It is possible to allow the child to study bulk density depending on the compaction of the material, the size of individual particles (gravel, sand, etc.) during the initial research activity.
Relative density of substances
Comparison of the properties of various substances is quite interesting based on relative values. The relative density of matter is one of these quantities.
Usually, the relative density of a substance is determined bytowards distilled water. As the ratio of the density of a given substance to the density of a standard, this value is determined using a pycnometer. But this information is not used in the school course of natural science, it is interesting for deep study (most often optional).
The Olympiad level of studying physics and chemistry can also affect the concept of "relative density of a substance with respect to hydrogen". It is usually applied to gases. To determine the relative density of a gas, the ratio of the molar mass of the gas under study to the molar mass of hydrogen is found. The use of relative molecular weight is not excluded.
