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Fundamentals of chemistry

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Category: Fundamentals of chemistry
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atomChemistry is the science that studies matter, its structure, composition, properties, and the physical and chemical processes it undergoes, as well as the energy exchanges that accompany these processes.
 
Matter is defined as any substance that has mass and volume. It includes everything from the smallest objects to the vast stars of the Universe.
 
Chemistry relies on mathematics and physics to describe processes and, in turn, serves as the foundation for a multitude of sciences such as biology, geology, medicine...

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termite reaction Matter is anything that has mass and occupies space. Mass is the measure of the amount of matter that a body possesses. The force required to accelerate a body increases with its mass (Newton's second law).
 
Energy is the ability of a system to do work or transfer heat. Thus, a hot body has more energy than a cold one, and when in contact, heat flows from the cold body to the hot one. A gas inside a cylinder at high pressure pushes the piston outward, doing work.
 
In chemical processes, heat exchange is frequent. Many chemical reactions give off heat (any combustion), they are exothermic However, other processes absorb heat from the surroundings, they are endothermic. The vaporization process of liquid water is endothermic since it requires an input of heat.
 
 
 

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combustion magnesium In any physical or chemical process there is no change in the amount of matter. For a chemical reaction, the sum of the masses of the reactants must equal the sum of the masses of the products.
 
Consider the combustion of metallic magnesium. Magnesium burns with oxygen in the air to form magnesium oxide. The magnesium oxide formed has a greater mass than magnesium metal. The difference in mass coincides with the mass of oxygen used in the combustion.
 
Keep in mind that a nuclear reaction does not obey the law of conservation of matter, since there is a significant conversion of matter into energy.
 
 

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Statement of the principle of conservation of energy: “Energy cannot be created or destroyed in a chemical reaction or physical process. It can only turn from one form to another."
In chemical reactions energy is released, if they are exothermic, and absorbed in the case of being endothermic. The reactants of an endothermic reaction, plus a certain amount of heat (energy) give the products. It can be shown that the energy of the products is equal to the sum of the energy that the reactants had plus the heat input.
Experience indicates that in any physical or chemical process energy is conserved, although it can pass from one form to another. Chemical energy can be converted into heat, light, electrical energy, etc.

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In nuclear processes, matter can be converted into energy. The relationship between a certain mass and the energy it is equivalent to is given by Einstein's equation, E=mc 2 .
 
The energy released when matter is transformed into energy is equal to the product of the mass of matter transformed by the speed of light squared. On a microscopic scale, the transformation of energy into matter has also been observed.
 
We state the principle of conservation of matter and energy as follows: "The combined amount of matter and energy in the universe is fixed."

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states of matter solid liquid gas Matter can be classified into three clearly differentiated states.
In the solid state, substances are rigid, difficult to deform, have a significant hardness, and are not very compressible. In crystalline solids, the atoms are arranged in defined positions, generating an ordered structure that is repeated in space. An example of a crystalline solid is common salt (NaCl), the sodium and chlorine atoms are arranged to form small cubes that by repetition generate the crystal.
 
In the liquid state , the particles (atoms or molecules) are free to move and the substance takes the shape of the container that contains it. Liquids, like solids, are not very compressible, although they deform without any effort. A substance in the solid phase has a slightly higher density than in the liquid phase. There are exceptions such as the case of water, ice is less dense since it floats on the liquid.

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Each substance has properties that allow it to be distinguished from others. These properties can be classified into two types: physical and chemical.
 
Some important physical properties are: density, melting point, boiling point, electrical and thermal conductivity, hardness, color. These properties depend on the state of aggregation in which the substance is (solid, liquid or gas). The density of solid water and gas are very different.
 
The chemical properties are related to the reactivity of each substance. A chemical property of sodium is its reaction with water. It is a redox process in which protons in water are converted to hydrogen and sodium to sodium cation. Magnesium oxidizes in the presence of oxygen to give magnesium oxide, it is a chemical property of magnesium.

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A chemical change is a process in which one or more substances combine to generate new compounds, with different chemical properties from the starting reagents. Chemical processes are associated with energy exchanges.
An example of a chemical change is the combustion of magnesium. The reaction of magnesium and oxygen forms a new substance, magnesium oxide, whose physical and chemical properties do not resemble either magnesium or oxygen.
In physical changes does not change the chemical composition of substances. The change from solid to liquid water (melting) is a physical change. During this process the physical properties of the water change, but it is not transformed into another substance, it is still water.
Physical changes, like chemicals, involve exchanges of energy. The melting of water is an endothermic process and requires an input of energy.

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granite Mixtures are combinations of pure substances. Each substance retains its physical and chemical properties in the mixture. For example, if we put 50 ml of methanol and 50 ml of water in a glass, a mixture is obtained, in which both methanol and water retain their properties.
 
When the properties of a mixture do not vary from one point to another, it is called homogeneous . In heterogeneous mixtures the properties change as we move through the solution. The ethanol-water mixture is homogeneous, the air formed by nitrogen, oxygen, carbon dioxide and water vapor is another example of a homogeneous mixture.
The mixture of common salt and sucrose (table sugar) solids is heterogeneous. As we move through the mixture we find sucrose at some points and salt at others.
 
The substances that form a mixture can be separated by physical methods, such as: distillation, extraction, crystallization, methods that are based on magnetic properties of the substances, ect.
Thus, a heterogeneous mixture of iron and sulfur can be separated with a magnet, based on the magnetic properties of iron. The result of separating a mixture is obtaining pure substances, pure iron and pure sulfur.
The homogeneous mixture of water and methanol can be separated based on the different boiling points of both components. This separation technique is known as distillation. The result is obtaining both substances that form the separate mixture.

Read more: Elements, compounds, substances and mixtures.

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dough pattern Chemistry is based on observation and experimentation, trying to obtain a series of results that in many cases can be expressed numerically, allowing their comparison with those obtained in other experiences.
 
In all observation and experimentation we look at those properties that are susceptible to comparison and that, therefore, we can measure. These properties are called magnitudes.
 
There is a small group of magnitudes, called fundamentals, from which all the others can be obtained. These magnitudes are: length, mass, time, current intensity, temperature, amount of substance and light intensity.
 
The international system of units assigns a unit to each magnitude. The meter is the unit of length. If when measuring a distance, 20m is obtained, it means that the length is 20 times the unit of the international system.
 
The definitions of the fundamental units in the international system, in accordance with the corresponding resolutions of the General Conference on Weights and Measures (CGMP), are the following:
 
1.- Unit of length. The meter is the distance that light travels in a vacuum in 1/299,792,486 seconds. Approximately 39.37 inches. A meter is divided into 100 centimeters and 1000 millimeters.

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Scientific notation is used to express very large or very small numbers. For example, 18 grams of water contain 602,300,000,000,000,000,000,000 water molecules. In scientific notation it can be expressed as 6,023 10 23 .

 

In scientific notation the number 0.000005 is expressed as 5 10 -6,

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The results obtained in a measurement are not exact. Every measurement implies an estimate. For example, suppose we need to measure an object with a ruler graduated in millimeters. When measuring we obtain a result between 38 and 39 millimeters, we estimate that the object measures 38.5 millimeters. This result has an exact part 38 and a part that is estimated (approximate) which is the last digit 5. The number 38.5 mm contains three significant figures. The last digit is doubtful, but is considered a significant figure. When giving the result of a measurement we include an approximate digit, but only one.

Next we will use a measuring cylinder to measure volumes of liquids. To the right of the specimen the calibration lines are enlarged. On the right scale we move from 10 ml to 10 ml. The left scale is graduated so that it varies from milliliter to milliliter.

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When determining the number of significant figures, the zeros used to position the comma are not taken into account. The number 0.0023 has only two significant figures. The number 0.0000002354 has four significant figures.
 
Writing the number in scientific notation shows the significant figures more clearly: 2.3 10 -3 and 2.354 10 -7 .
How many significant figures does the number 2,300 10 5 have? The answer is four significant figures; And 2.0300 10 -20 ? The answer is five significant figures.

Read more: Calculations Using Significant Figures

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Suppose that 8.56 is the result of an operation carried out with the calculator. If the number of significant figures is only two, we should return 8.5. But since the third digit is greater than 5, the second digit is rounded to 6. The final result is 8.6.

When the number to be removed is less than 5, the preceding digit does not change. If 5 is the number to be eliminated, the preceding digit is replaced by the nearest even number.

Let's see examples:
8.48 rounds to 8.5; 2.43 rounds to 2.4; 2.45 rounds to 2.4; 2.35 rounds to 2.4.

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The accuracy gives us the degree of agreement between the measured value and the true one.

The precision is related to the reproducibility of the measurements. Indicates the degree of agreement of several individual measurements.

A scale can be very precise, if when making several measurements it always gives the same result. But it is inaccurate, if that result does not agree with reality. Therefore, measuring instruments must be required to be exact and precise at the same time.


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The conversion factors are based on multiplying by fractions that have the same quantity in the numerator and denominator but in different units. Some examples of unit factors are the following:



In the conversion factors, the units guide us in the calculations. All the units are canceled until reaching the desired result.

Let's see an application of the conversion factors: Knowing that one erg is equal to 1 10 -7 joules. Convert 3.74 10 -2 ergs to joules.

The solution can be obtained by applying conversion factors:

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Density is defined as mass per unit volume. It is obtained by dividing the mass of a sample by its volume.



The most used units are g/cm 3 , g/ml and also g/l. Since each substance has a unique density, this information can be used to identify substances.

Relative density relates the density of the substance to that of water, both at the same temperature.



Up to 25ºC we can take 1g/ml as the density of water. Therefore, at room temperature the density of a substance coincides with its relative density. At higher temperatures, the density of water differs from 1g/ml and both magnitudes cease to coincide.

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Heat is a form of energy transfer, which occurs by virtue of a temperature difference. The flow of heat always occurs from the hot body to the cold.

The temperature of a body can be measured with mercury thermometers, which consist of a reservoir for mercury attached to a capillary. When the tank is heated, the mercury expands through the capillary. The higher the temperature, the higher the mercury rise is observed.

In order to measure temperatures, it is necessary to have a temperature scale. Swedish astronomer Anders Celsius developed the so-called Celsius temperature scale. He takes as points of reference the melting of water, to which he assigns 0º Celsius, and its boiling point at atmospheric pressure, to which he assigns 100º Celsius. Between these points there are one hundred divisions, each representing a degree Celsius.

A widely used temperature scale in the United States is Fahrenheit. On this scale the freezing and boiling points of water are taken to be 32ºF and 212ºF.

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