por The chemical changes They are the modifications that occur in substances or matter from chemical reactions. They are characterized by being irreversible, since they imply the formation of new bonds that need energy or other reactions to reverse their effects.
Life and its biochemistry, for example, are made up of constant chemical changes that reveal an endless number of reactions that take place within living beings. The result is the growth, development and aging of plant and animal tissues; that as is well known, scientifically and thermodynamically, it is an irreversible process.
That is why in living creatures, from invertebrates to vertebrates, we find chemical changes even while they consume their food, since they use enzymatic reactions to break them down and take advantage of the energy and chemical material they extract from them.
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How to recognize chemical changes?
One of the main characteristics to recognize them in nature is to observe if a color change occurs, or if a particular odor is produced. It is thus that in the seasons of the year it is appreciated how the trees gradually begin to change the color of their leaves; in them chlorophyll and its natural pigments undergo chemical reactions.
The reddening of the leaves in autumn is a very clear example of a chemical change. Connecting this with life, octopuses and chameleons camouflage themselves thanks to a series of chemical reactions that involve the cells of their skins; but unlike the leaves, they can reverse the color changes through other reactions that «deactivate» the camouflage.
However, the different colors of the sky and clouds are not the product of chemical changes but physical ones: Raleigh scattering. Meanwhile, the bleaching of clothes and the removal of their stains, like the staining of clothes with dyes, are chemical changes.
Characteristics
Mention was made that the change in color and smell are two of the main characteristics that a chemical change has occurred and that, therefore, a phenomenon involving one or multiple chemical reactions has occurred.
Next, other important characteristics that allow us to detect it directly with our senses will be discussed. The more features that can be perceived, the more likely it is that we are dealing with a chemical change and not a physical one.
Temperature change or light release
A chemical change is accompanied by an increase (exothermic reaction) or decrease (endothermic reaction) in temperature; that is, if heat is produced, or if, on the contrary, the sensation of cold is obtained, respectively. Also, sometimes the release of heat occurs parallel to the appearance of light.
gas formation
The formation of gases is directly related to changes in odors. If gas forms in a liquid medium, bubbling will be observed, indicative of a chemical change.
This is due to molecules with high vapor pressures that are easily swept away by air, or to the release of small molecules, such as CO2, H2S, CH4, O2, etc., which are end products of certain reactions.
precipitate formation
If the reactions take place in a liquid medium and the formation of a precipitate begins to be observed, whatever its color or texture, it is said that a chemical change has occurred. Likewise, it could be the case where two gases mix and ash or salts are formed.
irreversibility
Although it is true that there are reversible reactions capable of establishing an equilibrium, the changes generated with the characteristics described so far by themselves are irreversible; they need other reactions or energy of any kind to intervene for their effects to be reversed. And even then, in many cases it is practically impossible.
This irreversibility is observed in a change in the composition, or in the deterioration of matter. For example, a tree that burns in flames and ends up in ashes, whose final composition is different from that of the tree, will not be able to reverse the chemical change that has occurred by any means; Unless time could be turned back.
Guys
The chemical changes themselves are not subject to any classification: they are perceived with our senses and it can be inferred or examined what types of chemical reactions are taking place. Thus, such changes are due to a group of inorganic, organic or biochemical reactions.
When speaking of inorganic «changes», it is said that no compound with a carbon skeleton is involved, but rather transition metal complexes; oxides, sulfides, hydrides, nitrides, halogens, among other compounds.
Meanwhile, carbon compounds participate in organic and biochemical changes, with the difference that the former tend to occur outside organisms (except for the reactions carried out by drugs), and the latter inside them (regarding enzymes). , metabolism, biomolecules).
examples
yellowing of papers
The papers over time lose their whiteness to turn yellowish or brown. This process is due to the oxidation of its polymers, especially lignin, which together with cellulose gradually incorporate oxygen atoms that end up weakening their intermolecular interactions.
Fireworks
The fireworks represent a combustion reaction, in which a mixture of propellants, potassium nitrate, carbon, and sulfur burns while electronically exciting metallic salts that color the resulting explosions.
Cavities
Cavities generate a color change in the teeth due to their demineralization, caused by the acids generated when microorganisms digest the sucrose in food. Dental enamel, which consists of a mineralogical matrix of calcium phosphates, loses its ions as acidity increases and caries formation progresses.
algae growth
The eutrophication of the lakes encourages the growth of algae, which cover their surface with green as a result of a change in the chemical properties of the water; change that ends up being visible and negative for aquatic fauna.
Alka-Seltzer solution
Alka-Seltzer when dissolved in water begin to release bubbles that show their effervescence. The gases are due to the CO2 generated by the dissolution of NaHCO3, together with the citric and acetylsalicylic acids that come in the compacted tablet.
bodily pestilences
When we sweat it is inevitable, sooner or later, to perceive unpleasant odors located under the arms, on the skin, or on the feet. These bad odors are due to volatile organic molecules synthesized by microorganisms, and denote that a chemical change has occurred.
Putrefaction
The organic decomposition of living beings or food are chemical changes that involve a series of complex reactions. Bad odors are accompanied by a change in composition, as well as color. For example, the rotting of fruits like bananas and avocados show all of these characteristics.
coke and mint
Although it is formally a physical reaction, without breaking chemical bonds but with the consecutive participation of a balance between the H2CO3 and CO2 species, the Coca-Cola-Mint eruption can be taken as a reference to detect other explosions that could be purely chemical. .
Mint provides nucleation sites for small CO2 bubbles to form, which absorb some of the dissolved CO2 by interfering with its interactions with water molecules. Thus, the dissolved CO2 is concentrated in multiple small bubbles, which oppose the increasing surface tension of the water caused by sugar and other compounds.
The result is that the CO2 bubbles escape dragging the water that does not want to «release» them. This causes the characteristic foam of these mint eruptions to appear, as seen in the image above.
explosions
Explosions are usually very vigorous combustion reactions, giving off smoke, light, and heat. Where an explosion occurs, there is a chemical change.
wood burning
Wood is essentially composed of cellulose and lignin, both organic polymers. Its carbon atoms react vigorously with oxygen in the air, producing CO and CO2, depending on how complete the combustion is and the degree of oxygenation.
This is a chemical change because the polymers in the wood cannot return to their initial state, in addition to the release of heat, light and smoke.
Tanned
The redness or tan of our skin, when being under an intense sun, indicates that there was a change in its composition due to the reaction of the melanin contained in its cells, which oxidize and decompose.
purple cabbage juice
Starting from purple cabbage juice, an experiment can be set up to show color changes as a function of pH. It is easy to reproduce in any laboratory or kitchen.
If this juice is added to different test tubes, they are labeled, and different substances are added to them, from acid (vinegar) to alkaline (detergent), a range of colors will be obtained (image above).
This is because purple cabbage juice contains natural acid-base indicators, which respond to changes in pH. The more acidic the solution, the redder it will appear; and if, on the contrary, it is very basic, it will turn yellowish.
cook an egg
When an egg is fried or cooked, we allow the heat to denature its proteins, breaking its bonds and forming others while it loses its original structure.
faded leather
Leather colors can be faded by oxygen in the air and UV radiation, which breaks the chemical bonds between the dyes and the organic material.
Baked
The color change observed in breads and sweets, as well as their evident increase in volume compared to the raw mass, is indicative of various chemical changes.
First, by the rise caused by yeast and baking powder; and second, by the Maillard reaction, where proteins and sugars react with each other to turn the mixture golden.
Corrosion
Metals oxidize, so they lose their luster, darken and begin to corrode when the oxide layers formed can no longer adhere or cohere with the metallic interior. The most representative corrosion is that of iron, due to the characteristic brown color of its oxide.
batteries
Inside the batteries or cells, chemical reactions occur that generate electricity, electrical currents that run through an external circuit and power their devices. In essence, the anode loses electrons (oxidation), these electrons activate the device (remote control, phone, watch, prompter, etc.), and then end up at the cathode (reduction).
mosquito blood
If a mosquito bites us and we kill it instantly or after a few minutes, we will notice that the blood is light red. Meanwhile, if a lapse of a few…