- How to recognize chemical changes?
- characteristics
- Change in temperature or release of light
- Gas formation
- Formation of precipitate
- Irreversibility
- Types
- Examples
- Yellowing of papers
- Fireworks
- Cavities
- Algae growth
- Dissolution of the Alka-Seltzer
- Bodily pestilences
- Putrefaction
- Coke and mint
- Explosions
- Wood burning
- Sun tanning
- Red cabbage juice
- Cook an egg
- Leather fading
- Baked
- Corrosion
- Batteries
- Mosquito blood
- Iodine Clock
- Elephant paste
- Burn paper
- Apple oxidation
- Food rot
- Yogurt production
- Open bottle of champagne
- Wine production
- References
The chemical changes are the changes that occur in substances or matter from chemical reactions. They are characterized by being irreversible, since they involve 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 a myriad 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.
The change in color of the leaves during the fall is an example of a chemical change. Source: Jake Colvin via Pexels.
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 degrade them and take advantage of the energy and chemical material that they extract from them.
How to recognize chemical changes?
Example of chemical change
One of the main characteristics to recognize them in nature is to observe if a change in color occurs, or if a particular smell is produced. Thus, 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 leaves, they can reverse color changes through other reactions that "deactivate" the camouflage.
However, the different colors of the sky and clouds are not the product of chemical but physical changes: the Raleigh scattering. Meanwhile, the whitening of clothes and the removal of their stains, as well as the staining of clothes with dyes, are chemical changes.
characteristics
It was mentioned that the change in color and odor 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 addressed. The more characteristics that can be perceived, the more likely it is that we are facing a chemical change and not a physical one.
Change in temperature or release of light
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 the fact that there are molecules with high vapor pressures that are easily carried by the air, or the release of small molecules, such as CO 2, H 2 S, CH 4, O 2, etc., which are end products of certain reactions.
Formation of precipitate
If the reactions take place in a liquid medium and the formation of a precipitate begins to be observed, whatever its color or texture, a chemical change is said to have occurred. Likewise, it could be the case where two gases mix and form ashes or salts.
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 yet, in many cases it is practically impossible.
This irreversibility is observed in a change of the composition, or in the deterioration of the matter. For example, a tree that burns in flames and ends 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
Types
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 we speak of inorganic "changes", it is said that no carbon skeleton compound is involved, but rather all transition metal complexes; oxides, sulfides, hydrides, nitrides, halogens, among other compounds.
Meanwhile, carbon dioxide compounds participate in organic and biochemical changes, with the difference that the former usually occur outside organisms (except for the reactions carried out by drugs), and the latter within them (with regard to enzymes)., metabolism, biomolecules).
Examples
Yellowing of papers
Paper folds turn yellow over time. Source: makamuki0 via Pixabay.
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 incorporate oxygen atoms that end up weakening their intermolecular interactions.
Fireworks
Fireworks represent a combustion reaction, in which a mixture of propellants, potassium nitrate, carbon, and sulfur burns while electronically exciting the metal salts that color the resulting explosions.
Cavities
Caries generate a color change in the teeth due to their demineralization, caused by the acids generated when microorganisms digest sucrose from food. Dental enamel, which consists of a mineralogical matrix of calcium phosphates, loses its ions as acidity increases and cavities formation progresses.
Algae growth
The layer of algae and microorganisms that grows in lakes is sometimes such that it can become sticky. Source: Michael Meiters (https://www.flickr.com/photos/psychofreakx3/5963134909)
The eutrophication of lakes encourages the growth of algae, which cover their surface in 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.
Dissolution of the Alka-Seltzer
The Alka-Seltzer when dissolved in water begin to release bubbles that demonstrate their effervescence. The gases are due to the CO 2 generated by the dissolution of NaHCO 3, 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 indicate 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 rot of fruits like bananas and avocados shows all these characteristics.
Coke and mint
A mixture of coke and mint erupts like a soda volcano. Source: Michael Murphy
Although it is formally a physical reaction, without breaking chemical bonds but with the consecutive participation of an equilibrium between the species H 2 CO 3 and CO 2, the eruption of coca-cola-mint can be taken as a reference to detect other explosions that do may be purely chemical.
Peppermint provides nucleation sites for tiny bubbles of CO 2 to form, which absorb some of the dissolved CO 2 by interfering with its interactions with water molecules. Thus, the dissolved CO 2 concentrates in multiple small bubbles, which oppose the increasing surface tension of water caused by sugar and other compounds.
The result is that the CO 2 bubbles escape dragging the water that does not want to "release" them. This causes the characteristic foam of these peppermint rashes to appear, as seen in the image above.
Explosions
Explosions are usually very vigorous combustion reactions, from which smoke, light and heat are released. Wherever an explosion occurs, you are facing a chemical change.
Wood burning
Wood is essentially made of cellulose and lignin, both organic polymers. Its carbon atoms react vigorously with the oxygen in the air, causing CO and CO 2, 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.
Sun tanning
A chemical change is reflected in the tans of those who enjoy a day at the beach. Source: Pxhere.
The redness or tanning of our skin, when under 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.
Red cabbage juice
Test tubes with indicator of purple cabbage. Source: Indikator-Blaukraut.JPG: Supermartlderivative work: Haltopub
Starting from the juice of purple cabbage, 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 acidic (vinegar) to alkaline (detergent), there will be a range of colors (upper image).
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 losing its original structure.
Leather fading
Leather colors can fade due to oxygen in the air and UV radiation, both of which cause the chemical bonds between the dyes and the organic material to break down.
Baked
The change in color observed in breads and sweets, as well as their evident increase in volume compared to raw dough, is indicative of various chemical changes.
First, because of the rising 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 rust, so they lose their luster, darken and begin to corrode when the oxide layers formed can no longer adhere or bond with the metal interior. The most representative corrosion is that of iron, due to the characteristic brown color of its oxide.
Batteries
Within batteries or cells, chemical reactions occur that generate electricity, electrical currents that run through an external circuit and activate their devices. In essence, the anode loses electrons (oxidation), these electrons activate the device (remote control, phone, watch, pointer, etc.), and then end up at the cathode (reduction).
Mosquito blood
If a mosquito bites us and we kill them instantly or within a few minutes, we will notice that the blood is light red. Meanwhile, if a few hours elapse and we kill this same mosquito, we will see that the blood is dark, it will even present brown tones.
This change in color is indicative that the blood has undergone chemical reactions within the mosquito.
Iodine Clock
The iodine clock reaction is one of the most representative of a chemical change. Source: Daniel J. Lulu
One of the most impressive chemical changes is seen in the famous iodine clock reaction. It is so called because its speed can be controlled by modifying the concentration of the reactants. The reaction comes to an end when an intense, very dark blue color appears, in great contrast to the initial transparency observed.
This color (upper image) is due to interactions between starch and the anionic complex I 3 -. One of its versions starts from iodate, IO 3 -, and bisulfite, HSO 3 -:
IO 3 - + 3HSO 3 - → I - + 3HSO 4 -
The I - reacts with the IO 3 - from the medium to generate iodine:
IO 3 - + 5I - + 6H + → 3I 2 + 3H 2 O
And this in turn reacts with more bisulfite:
I 2 + HSO 3 - + H 2 O → 2I - + HSO 4 - + 2H +
Once the HSO 3 - is finished, the second reaction will predominate until there is an excess of I 2, which will bind to the I - to form the I 3 -. And finally, the I 3 - will interact with the starch molecules darkening the solution.
Elephant paste
Elephant paste experiment. Source: Ferbr1 via Wikipedia.
Again, and finally, a specific chemical reaction is named but with changes that are too visible to be left out: elephant toothpaste (top image). In certain videos, the amount of foam is such that you could literally brush an elephant's mouth.
This reaction is based on the catalytic decomposition of hydrogen peroxide, H 2 O 2 (hydrogen peroxide), with iodide ions, I -, from soluble salts such as NaI or KI. The first reaction that occurs is, according to the following chemical equation:
H 2 O 2 + I - → H 2 O + IO -
The IO - species subsequently reacts with H 2 O 2 in a second reaction:
IO - + H 2 O 2 → H 2 O + O 2 + I -
Where catalyst I is regenerated - (not consumed).
Note that the final products are H 2 O and O 2. If detergent and colorants are added to the reaction mixture, the water together with the oxygen will foam vigorously, which will rise through the container and shoot upwards, against gravity.
Burn paper
When paper is burned, carbon dioxide, water vapor, and ash are produced. These three substances are chemically different from the first substance, so it is a chemical change.
Apple oxidation
When an apple is chopped and left in the open, it changes from being an ivory color to being brown or ocher. This is called oxidation.
Food rot
When food spoils, a chemical change occurs. For example, rotten eggs go through a decomposition process that causes them to change color and smell.
Yogurt production
Yogurt is the result of a chemical change involving milk and certain bacteria, such as Streptococcus thermophilus and Lactobacilli bulgaricus.
Open bottle of champagne
When you open a bottle of champagne or a soft drink, you notice a bubbling. This bubbling means that the carbonic acid in the drink has broken down, releasing the carbon dioxide.
Wine production
The creation of wine from grapes is a fermentation process. This is an example of an anaerobic chemical reaction in which sugar is transformed into ethyl alcohol and carbon dioxide.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
- Helmenstine, Anne Marie, Ph.D. (October 08, 2019). Chemical Change Definition in Chemistry. Recovered from: thoughtco.com
- Wikipedia. (2019). Chemical change. Recovered from: en.wikipedia.org
- Mandeep Sohal. (September 29, 2019). Chemical Change vs. Physical Change. Chemistry LibreTexts. Recovered from: chem.libretexts.org
- Nathan Crawford. (2019). What is a Chemical Change? - Properties, Types & Examples Video. Study. Recovered from: study.com
- HowStuffWorks. (2019). Why do newspapers turn yellow over time? Recovered from: science.howstuffworks.com
- Science Buddies. (June 14, 2012). Spurting Science: Erupting Diet Coke with Mentos. Recovered from: scientificamerican.com
- Quimitube. (2014). Laboratory experiences: The iodine clock. Recovered from: quimitube.com