The Physical changes They are those in which a change in matter is observed, without the need to modify its nature; that is, without breaking or forming chemical bonds. Therefore, assuming a substance A, it must have the same chemical properties before and after the physical change..
Without physical changes, the varieties of forms that certain objects can acquire would not exist; the world would be a static and standardized place. For them to occur, the action of energy on matter is necessary, whether in the mode of heat, radiation or pressure; pressure that can be exerted mechanically with our own hands.
For example, in a carpentry workshop you can observe the physical changes that wood undergoes. Saws, brushes, gouges and holes, nails, etc., are essential elements so that the wood, from a block, and by joinery techniques, can be transformed into a work of art; like a piece of furniture, a latticework or a carved box.
If wood is considered as substance A, it essentially does not undergo any chemical transformation after the furniture is finished (even if its surface receives a chemical treatment). If this piece of furniture is pulverized to a handful of sawdust, the molecules of the wood will remain unchanged.
Practically, the cellulose molecule of the tree from which the wood was carved does not modify its structure throughout this process..
If the furniture were on fire, then its molecules would react with oxygen in the air, breaking down into carbon and water. In this situation, there would be a chemical change, since after combustion the properties of the residue would be different from those of the furniture..
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The wood in the previous example can undergo physical changes in size. It can be laminated, cut, edged, etc., but never increased in volume. In this sense, wood can increase its area, but not its volume; which, on the contrary, is constantly reduced as it is worked in the workshop.
Once it is cut, it cannot be restored to its original shape, since wood is not an elastic material; in other words, it undergoes irreversible physical changes.
In this type of change, matter, although it does not experience any reaction, cannot return to its initial state..
Another more colorful example is playing with a yellow and a bluish clay. When kneading them together and after giving them the shape of a ball, their color turns green. Even if you had a mold to return them to their initial shape, you would have two green bars; blue and yellow could no longer be separated.
In addition to these two examples, blowing bubbles could also be considered. The more they are blown, the volume of them increases; but once free, no air can be extracted to reduce their size.
Although no emphasis is put on adequately describing them, all changes in the state of matter are reversible physical changes. They depend on pressure and temperature, as well as the forces that bind the particles.
For example, in a cooler an ice cube can melt if left to stand outside the freezer. After a while, the liquid water supplants the ice in the small compartment. If this same cooler is returned to the freezer, the liquid water will lose temperature until it freezes and becomes an ice cube again..
The phenomenon is reversible because the absorption and release of heat by the water occurs. This is so no matter where the liquid water or ice is stored..
The main characteristic and difference between a reversible and irreversible physical change is that in the first the substance (water) is considered in itself; while in the second, the physical appearance of the material is considered (wood, and not celluloses and other polymers). In both, however, the chemical nature remains constant..
Sometimes the difference between these types is not clear and it is advisable, in such cases, not to classify the physical changes and treat them as one.
Inside the kitchen innumerable physical changes take place. Making a salad is saturated with them. Tomatoes and vegetables are chopped at will, modifying their initial shapes irreversibly. If bread is added to this salad, it is cut into slices or pieces from a loaf of peasant bread, and spread with butter.
The anointing of bread and butter is a physical change, since its flavor changes, but molecularly it remains unchanged. If other bread is toasted, it will acquire more intense strength, flavor and colors. This time it is said that there was a chemical change, because it does not matter if this toast is cold or not: it will never regain its initial properties.
Foods that are homogenized in the blender also represent examples of physical changes.
On the sweet side, when chocolate is melted it is observed that it goes from a solid state to a liquid state. The preparation of syrups or sweets that do not involve the use of heat, also enter into this type of material changes..
In a playground in the early hours, you can see some canvases on the floor, inert. After a few hours, these are imposed like a castle of many colors where children jump inside.
This abrupt change in volume is due to the immense mass of air blown inside. Once the park is closed, the castle is deflated and saved; therefore, it is a reversible physical change.
Glass at high temperatures melts and can be freely deformed to give any design. In the image above, for example, you can see how a glass horse is being molded. Once the glassy paste cools, it will harden and the ornament will be finished.
This process is reversible, since by applying temperature again, it can be given new shapes. Many glass ornaments are created by this technique, which is known as glass blowing..
Also, in nature it can be seen how minerals adopt more crystalline structures; that is, they facet over the years.
This consists of a physical change product of a rearrangement of the ions that make up the crystals. Climbing a mountain, for example, you can find quartz stones that are more faceted than others..
When a soluble solid is dissolved in water, such as salt or sugar, a solution with a salty or sweet taste, respectively, is obtained. Although both solids "disappear" in the water, and the latter undergoes a change in its taste or conductivity, no reaction occurs between the solute and the solvent..
Salt (ordinarily sodium chloride), consists of Na ions+ and Cl-. In water, these ions are solvated by water molecules; but the ions undergo neither reduction nor oxidation.
The same happens with the sucrose and fructose molecules in sugar, which do not break any of their chemical bonds when they interact with water..
Here the term crystallization refers to the slow formation of a solid in a liquid medium. Returning to the example of sugar, when its saturated solution is heated to a boil, then left to rest, the sucrose and fructose molecules are being given enough time to arrange themselves properly and thus form larger crystals..
This process is reversible if heat is supplied again. In fact, it is a widely used technique to purify crystallized substances from impurities present in the medium..
In neon lights gases (including carbon dioxide, neon and other noble gases) are heated by means of an electrical discharge. Gas molecules become excited and undergo electronic transitions that absorb and emit radiation as the electrical current passes through the gas at low pressure..
Although the gases ionize, the reaction is reversible and they practically return to their initial state without the formation of products. Neon light is exclusively red, but in popular culture this gas is incorrectly designated for all lights produced by this method, regardless of color or intensity..
Here, the emission of light is slower after the absorption of high-energy radiation, such as ultraviolet. The colors are the product of this emission of light due to the electronic transitions within the molecules that make up the ornament (top image).
On the one hand, light chemically interacts with the molecule, exciting its electrons; and on the other, once the light is emitted in the dark, the molecule does not show any breaking of its bonds, which is expected of all physical interaction.
We speak then of a reversible physicochemical change, since if the ornament is placed in sunlight, it reabsorbs ultraviolet radiation, which it will then release in the dark slowly and with less energy..
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