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Category Archives: Chemistry
Oils
“Oil” is a general name for any kind of molecule which is
nonpolar
that just means that its electrons are evenly distributed
PHET Polar molecules app
liquid at room temperature
of course, it could become solid if cooled, or evaporate if heated
Molecule has one end which is hydrophobic and another end which is lipophilic
The hydrophobic end likes to stick to water molecules. But hates sticking to oils.
The lipophilic end likes to stick to oil molecules, but hates sticking to water,
Made with many C and H atoms
Oils are usually flammable. Here we see oils in an orange skin interacting with a candle.
So Petroleum is?
Petroleum is a mix of naturally forming oils, which we drill from the Earth, and use in a variety of ways. See our article on petroleum and producing power.
Natural gas power
Content objective:
What are we learning? Why are we learning this?
content, procedures, skills
Vocabulary objective
Tier II: High frequency words used across content areas. Key to understanding directions, understanding relationships, and for making inferences.
Tier III: Low frequency, domain specific terms
Building on what we already know
What vocabulary & concepts were learned in earlier grades?
Make connections to prior lessons from this year.
This is where we start building from.
We can burn natural to release heat, and use that heat to create heat (e.g. cooking) or electrical power.
Burning natural gas is a chemical reaction called combustion.
In physics, power has a very specific meaning. It is the rate that energy is transformed from one form into another form.
How was natural formed?
Molecules
It is a mixture of hydrocarbon molecules
Mostly methane (CH4)
And some other higher alkanes (acyclic saturated hydrocarbons)
And some small percent of CO2, N2, H2S (hydrogen sulfide,) or He
..
Rotary catalytic mechanism of mitochondrial ATP synthase
Introduction
(Text in this section adapted from “ATP synthase.” Wikipedia, The Free Encyclopedia. 27 Mar. 2019.)
ATP synthase is an enzyme that creates the energy storage molecule adenosine triphosphate (ATP).
ATP is the most commonly used “energy currency” of cells for all organisms. It is formed from adenosine diphosphate (ADP) and inorganic phosphate (Pi).
The overall reaction catalyzed by ATP synthase is:
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ADP + Pi + H+out ⇌ ATP + H2O + H+in
The formation of ATP from ADP and Pi is energetically unfavorable and would normally proceed in the reverse direction.
In order to drive this reaction forward, ATP synthase couples ATP synthesis during cellular respiration to an electrochemical gradient created by the difference in proton (H+) concentration across the mitochondrial membrane in eukaryotes or the plasma membrane in bacteria.
Molecular animation of ATP synthase
Here is a three dimensional animation of all the proteins working together in this complex. We see it situated in a lipid bilayer (organelle membrane.)
Here is another animation of a similar complex.
Video
Rotary catalytic mechanism of mitochondrial ATP synthase
Learning Standards
(TBA)
Biology, Chemistry, Simple machines
Elements necessary for life
Major elements – CHONSP
Carbon – Used as the major building unit of all organic molecules.
Hydrogen – major component of water. Major component of all organic molecules.
Oxygen – major component of water. Must be transported by our red blood cells.
Nitrogen – needed in all amino acids and proteins. Needed in chlorophyll, which is necessary for photosynthesis.
Sulphur – Used in in fats, body fluids, skeletal minerals, and most proteins.
Phosphorus – Necessary to make DNA and RNA. Also a component of bones and teeth.
Microbial Genomics and the Periodic Table, Lawrence P. Wackett, Anthony G. Dodge and Lynda B. M. Ellis
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Microbial Genomics and the Periodic Table, Lawrence P. Wackett, Anthony G. Dodge and Lynda B. M. Ellis
There are many essential trace elements in humans
Arsenic – “Despite its poisonous reputation, may be a necessary ultratrace element for humans. It is a necessary ultratrace element for red algae, chickens, rats, goats, and pigs. A deficiency results in inhibited growth (*)
Boron – essential for cell membrane characteristics and transmembrane signaling
Calcium ions are essential for muscle contractions and the clotting of blood. Necessary for cell walls, and bones.
Chlorine – Digestive juices in the stomach contain hydrochloric acid.
Chromium – essential trace element that potentiates insulin action and thus influences carbohydrate, lipid and protein metabolism.
“Chromium is an essential trace element and has a role in glucose metabolism. It seems to have an effect in the action of insulin. In anything other than trace amounts, chromium compounds should be regarded as highly toxic.” (*)
“Cobalt salts in small amounts are essential to many life forms, including humans. It is at the core of a vitamin called vitamin-B12. “ (*)
“Copper is essential for all life, but only in small quantities. It is the key component of redox enzymes and of haemocyanin.” (*)
Fluorine forms a salt with calcium. This salt makes the teeth and bones stronger.
“Iodine is an essential component of the human diet and in fact appears to be the heaviest required element in the diet. Iodine compounds are useful in medicine.” (*)
Iron – used in the hemoglobin molecules, allows your blood to hold oxygen. Iron is only about 0.004 percent of your body mass,
Magneisum “Chlorophylls (responsible for the green colour of plants) are based upon magnesium. Magnesium is required for the proper working of some enzymes.” (*)
Manganese – essential for the action of some enzymes
“Molybdenum is a necessary element, apparently for all species. … plays a role in nitrogen fixation, enzymes, and nitrate reduction enzymes.” (*)
Nickel is an essential trace element for many species. Unknown if so in humans.
“Potassium salts are essential for both animals and plants. The potassium cation (K+) is the major cation in intracellular (inside cells) fluids (sodium is the main extracellular cation). It is essential for nerve and heart function.” (*)
Selenium – essential component of one of the antioxidant defense systems of the body
“essential to mammals and higher plants, but only in small amounts…. may help protest against free radical oxidants and against some heavy metals.” (*)
Silicium – probably essential for healthy connective tissue and bone
Sodium (Na+) and potassium (K+) ions – transmission of nerve impulses between your brain and all parts of the body.
Tin – expected to have a function in the tertiary structure of proteins
Tungsten is needed in very tiny amounts in some enzymes (oxidoreductases)
Vanadium – possible role as an enzyme cofactor and in hormone, glucose, lipid, bone and tooth metabolism.
“Zinc is the key component of many enzymes. The protein hormone insulin contains zinc.” (*)
(*) WebElements: THE periodic table on the WWW
https://www.webelements.com/arsenic/biology.html
Basic chemistry rules are actually magic number approximations
The basic rules of chemistry are magic number approximations
What is Lewis Theory?
This lesson is from from Mark R. Leach, meta-synthesis.com, Lewis_theory
Lewis theory is the study of the patterns that atoms display when they bond and react with each other.
The Lewis approach is to look at many chemical systems, study patterns, count the electrons in the patterns. After that, we devise simple rules to explain what is happening.
Lewis theory makes no attempt to explain how or why these empirically derived numbers of electrons – these magic numbers – arise.
Although, it is striking that the magic numbers are generally (but not exclusively) positive integers of even parity: 0, 2, 4, 6, 8
For example:
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Atoms and atomic ions show particular stability when they have a full outer or valence shell of electrons and are isoelectronic with He, Ne, Ar, Kr & Xe: Magic numbers 2, 10, 18, 36, 54.
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Atoms have a shell electronic structure: Magic numbers 2, 8, 8, 18, 18.
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Sodium metal reacts to give the sodium ion, Na+, a species that has a full octet of electrons in its valence shell. Magic number 8.
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A covalent bond consist of a shared pair electrons: Magic number 2.
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Atoms have valency, the number of chemical bonds formed by an element, which is the number of electrons in the valence shell divided by 2: Magic numbers 0 to 8.
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Ammonia, H3N:, has a lone pair of electrons in its valence shell: Magic number 2.
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Ethene, H2C=CH2, has a double covalent bond: Magic numbers (2 + 2)/2 = 2.
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Nitrogen, N2, N≡N, has a triple covalent bond: Magic numbers (2 + 2 + 2)/2 = 3.
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The methyl radical, H3C•, has a single unpaired electron in its valence shell: Magic number 1.
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Lewis bases (proton abstractors & nucleophiles) react via an electron pair: Magic number 2.
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Electrophiles, Lewis acids, accept a a pair of electron in order to fill their octet: Magic numbers 2 + 6 = 8.
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Oxidation involves loss of electrons, reduction involves gain of electrons. Every redox reaction involves concurrent oxidation and reduction: Magic number 0 (overall).
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Curly arrows represent the movement of an electron pair: Magic number 2.
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Ammonia, NH3, and phosphine, PH3, are isoelectronic in that they have the same Lewis structure. Both have three covalent bonds and a lone pair of electrons: Magic numbers 2 & 8.
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Aromaticity in benzene is associated with the species having 4n+2 π-electrons. Magic number 6.Naphthalene is also aromatic: Magic number 10.
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Etc.
Lewis theory is numerology.
Lewis theory is electron accountancy: look for the patterns and count the electrons.
Lewis theory is also highly eclectic in that it greedily begs/borrows/steals/assimilates numbers from deeper, predictive theories and incorporates them into itself, as we shall see.
Ernest Rutherford famously said
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Patterns
Consider the pattern shown in Diagram-1:
Now expand the view slightly and look at Diagram-2
You may feel that the right hand side “does not fit the pattern” of Diagram-1 and so is an anomaly.
So, is it an anomaly?
Zoom out a bit and look at the pattern in Diagram-3, the anomaly disappears
But then look at Diagram-4. The purple patch on the upper right hand side does not seem to fit the pattern and so it may represent anomaly
But zooming right out to Diagram-5 we see that everything is part of a larger regular pattern.
Image from dryicons.com, digital-flowers-pattern
When viewing the larger scale the overall pattern emerges and everything becomes clear. Of course, the Digital Flowers pattern is trivial, whereas the interactions of electrons and positive nuclei are astonishingly subtle.
This situation is exactly like learning about chemical structure and reactivity using Lewis theory. First we learn about the ‘Lewis octet’, and we come to believe that the pattern of chemistry can be explained in terms of the very useful Lewis octet model.
Then we encounter phosphorous pentachloride, PCl5, and discover that it has 10 electrons in its valence shell. Is PCl5 an anomaly? No! The fact is that the pattern generated through the Lewis octet model is just too simple.
As we zoom out and look at more chemical structure and reactivity examples we see that the pattern is more complicated that indicated by the Lewis octet magic number 8.
Our problem is that although the patterns of electrons in chemical systems are in principle predictable, new patterns always come as a surprise when they are first discovered:
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The periodicity of the chemical elements
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The 4n + 2 rule of aromaticity
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The observation that sulfur exists in S8 rings
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The discovery of neodymium magnets in the 1990s
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The serendipitous discovery of how to make the fullerene C60 in large amounts
While these observations can be explained after the fact, they were not predicted beforehand. We do not have the mathematical tools to do predict the nature of the quantum patterns with absolute precision.
The chemist’s approach to understanding structure and reactivity is to count the electrons and take note of the patterns. This is Lewis theory.
As chemists we attempt to ‘explain’ many of these patterns in terms of electron accountancy and magic numbers.
Caught In The Act: Theoretical Theft & Magic Number Creation
The crucial time for our understand chemical structure & bonding occurred in the busy chemistry laboratories at UC Berkeley under the leadership of G. N. Lewis in the early years of the 20th century.
Lewis and colleagues were actively debating the new ideas about atomic structure, particularly the Rutherford & Bohr atoms and postulated how they might give rise to models of chemical structure, bonding & reactivity.
Indeed, the Lewis model uses ideas directly from the Bohr atom. The Rutherford atom shows electrons whizzing about the nucleus, but to the trained eye, there is no structure to the whizzing. Introduced by Niels Bohr in 1913, the Bohr model is a quantum physics modification of the Rutherford model and is sometimes referred to the Rutherford–Bohr model. (Bohr was Rutherford’s student at the time.) The model’s key success lay in explaining (correlating with) the Rydberg formula for the spectral emission lines of atomic hydrogen.
[Greatly simplifying both the history & the science:]
In 1916 atomic theory forked or bifurcated into physics and chemistry streams:
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The physics fork was initiated and developed by Bohr, Pauli, Sommerfield and others. Research involved studying atomic spectroscopy and this lead to the discovery of the four quantum numbers – principal, azimuthal, magnetic & spin – and their selection rules. More advanced models of chemical structure, bonding & reactivity are based upon the Schrödinger equation in which the electron is treated as a resonant standing wave. This has developed into molecular orbital theory and the discipline ofcomputational chemistry.
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Note: quantum numbers and their selection rules are not ‘magic’ numbers. The quantum numbers represent deep symmetries that are entirely self consistent across all quantum mechanics.
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The chemistry fork started when Lewis published his first ideas about the patterns he saw in chemical bonding and reactivity in 1916, and later in a more advanced form in 1923. Lewis realised that electrons could be counted and that there were patterns associated with structure, bonding and reactivity behaviour.These early ideas have been extensively developed and are now taught to chemistry students the world over. This is Lewis theory.
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Lewis Theory and Quantum Mechanics
Quantum mechanics and Lewis theory are both concerned with patterns. However, quantum mechanics actively causes the patterns whereas Lewis theory is passive and it only reports on patterns that are observed through experiment.
We observe patterns of structure & reactivity behaviour through experiment.
Lewis theory looks down on the empirical evidence, identifies patterns in behaviour and classifies the patterns in terms of electron accountancy& magic numbers. Lewis theory gives no explanation for the patterns.
In large part, chemistry is about the behaviour of electrons and electrons are quantum mechanical entities. Quantum mechanics causes chemistry to be the way it is. The quantum mechanical patterns are can be:
- Observed using spectroscopy.
- Echoes of the underlying quantum mechanics can be seen in the chemical structure & reactivity behaviour patterns.
- The patterns can be calculated, although the mathematics is not trivial.
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Metabolism
What are we learning about?
* Hundreds of chemical reactions occur simultaneously in every living cell.
* The entire set of them are collectively known as metabolism .
* in some reactions, complex molecules are broken down to produce energy
* in other reactions, energy is used to build up complex molecules.
Anabolism
From Greek ἁνά, “upward” and βάλλειν, “to throw”
All the chemical pathways in which cells bond smaller molecules together to make macromolecules (larger ones.)
The energy source is another set of processes, catabolism (see below)
Anabolism is used to
create news cells
build muscles and tissues.
grow and mineralize bone
Catabolism
From the Greek κάτω kato, “downward” and βάλλειν ballein, “to throw”
All the chemical pathways in which cells break down large molecules into smaller ones.
Cells gain energy from the breakdown, or create smaller pieces, which become building materials in anabolism.
Catablism is used to:
break proteins down into amino acids
break DNA molecules down into individual nucelotides
convert sugar into ATP and other small organic molecules
Another way to show these metabolic pathways:
BBC Bitezie revisions
Endocrine hormones regulate our metabolism
Hormones can be classified as anabolic or catabolic.
Anabolic hormones are the anabolic steroids, which stimulate protein synthesis and muscle growth, and insulin.
Catabolic hormones include
cortisol (breaks down large molecules into simple sugars, for quick energy)
glucagon (breaks down large molecules into glucose and fatty acids)
adrenaline – increases blood flow to muscles, output of the heart, blood sugar level.
Our article on the endocrine hormone system.
What does metabolism look like inside a cell? Here’s a simplified view:
Image from An Introduction to Nutrition, v. 1.0. 2012books.lardbucket.org/books/an-introduction-to-nutrition
Metabolic map
This is a metro-style map of the metabolism of most life on Earth.
Image by Bert Chan, Hong Kong, via Wikimedia. https://www.behance.net/bertchan
Interactive Metabolic Pathways Map – New Edition | Sigma-Aldrich
Related articles
Scaling-and-biophysics: As animals get larger and larger, how would their metabolism need to change?
Cellular respiration: An introduction
Interactive metabolism maps or apps
Metabolic pathways from Learn.Genetics
Clickable metabolic map from metabolicpathways.teithe.gr
Wiley college textbook step-by-step animations
Roche biochemical pathway online map
Learning Standards
MS-LS1-3. Use argument supported by evidence for how the body is a system of interacting subsystems composed of groups of cells. Emphasis is on the conceptual understanding that cells form tissues and tissues form organs specialized for particular body functions. Examples could include the interaction of subsystems within a system and the normal functioning of those systems.
MS-LS1-7. Develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism. Emphasis is on describing that molecules are broken apart and put back together and that in this process, energy is released.
TBA
Chlorine and Bleach terminology
Terminology alert: Sometimes people use the same word to describe different things. For example, “chlorine” can refer to:
Chlorine
a single, neutral, chlorine atom. These are unstable.
We normally never encounter one.
They almost instantly bind to each other to form chlorine molecules.
Chlorine
molecule (Cl2) – deadly gas
Chlorine
A chlorine ion is chlorine atom that has picked up an extra electron.
In small quantities these ions are essential for life.
“Chlorine”
There are many chemicals used to bleach laundry, or disinfect swimming pools.
The most common is “chlorine bleach”, sodium hypochlorite.
Chemical formula is NaOCl
In water this breaks down into a sodium cation (Na+) and a hypochlorite anion (OCl−
or ClO− ).
Visualizing the electron distribution in sodium hypochlorite a little more accurately.
How to make sodium hypochlorite
Add chlorine gas (Cl2) to caustic soda (NaOH).
Then sodium hypochlorite, water (H2O) and salt (NaCl) are produced according to the following reaction:
Cl2 + 2NaOH + → NaOCl + NaCl + H2O
How does sodium hypochlorite disinfection work?
By adding hypochlorite to water, hypochlorous acid (HOCl) is formed:
NaOCl + H2O → HOCl + NaOH–
Hypochlorous acid is divided into hydrochloric acid (HCl) and oxygen (O).
Sodium hypochlorite is effective against bacteria, viruses and fungi.
Cell phone chemistry
Chemistry is everywhere – even in your phones
Article 1: “Digging for rare earths: The mines where iPhones are born. How are these unusual minerals extracted from the ground and why is that process an environmental risk? CNET’s Jay Greene explains.” – from CNet 9/26/12
Digging for rare earths: The mines where iPhones are born
Article 2: Pay dirt: Why rare-earth metals matter to tech (FAQ) It was once an obscure topic only for geologists. But China’s control over rare earth elements used in green- and high-tech equipment is causing alarm as the nation cuts exports.
Pay dirt: Why rare-earth metals matter to tech
Here is the full PDf handout: Periodic table of iPhones (Full PDF handout)
Article 3:
Does cell phone use cause cancer?
Article 4:
Measuring data with smartphone apps
Learning Standards
Massachusetts
ETS3. Technological Systems
7.MS-ETS3-2(MA). Compare the benefits and drawbacks of different communication systems.
7.MS-ETS3-4(MA). Show how the components of a structural system work together to serve a structural function. Provide examples of physical structures and relate their design to their intended use.
College Board Standards for College Success: Science
Objective C.2.1 Periodic Table
Students understand that the periodic table is an organizational tool that can be used for the prediction and classification of the trends and properties of elements.
C-PE.2.1.1 Predict, based on its position in the periodic table, the properties of a given main group element. Properties include appearance, electronegativity, type of bond formed, and ionic charge. Make a claim about the type (metal, nonmetal, metalloid) of the given element. Give examples of other elements that would have similar properties, and explain why they would have similar properties.
Students apply, as well as engage and reason with, the following concepts in the performance expectations:
Properties of an element can be predicted based on its placement in the periodic table. Groups of elements exhibit similar properties with predictable variations; rows of elements have predictable trends.
Elements are often classified as metals, nonmetals and metalloids
AAAS Benchmarks
All matter is made up of atoms, which are far too small to see directly through a microscope. 4D/M1a
The atoms of any element are like other atoms of the same element, but are different from the atoms of other elements. 4D/M1b*
There are groups of elements that have similar properties, including highly reactive metals, less-reactive metals, highly reactive nonmetals (such as chlorine, fluorine, and oxygen), and some almost completely nonreactive gases (such as helium and neon). 4D/M6a
CSTA K-12 Computer Science Standards
CD.L2-07 Describe what distinguishes humans from machines, focusing on human intelligence
versus machine intelligence and ways we can communicate.
CD.L2-08 Describe ways in which computers use models of intelligent behavior (e.g., robot motion,
speech and language understanding, and computer vision).
CD.L3A-01 Describe the unique features of computers embedded in mobile devices and vehicles
(e.g., cell phones, automobiles, airplanes).
CD.L3A-10 Describe the major applications of artificial intelligence and robotics.
Common Core ELA. WHST.6-8.1 Write arguments focused on discipline-specific content.
Galvanic cell
A galvanic cell is a device in which chemical energy is converted into electric energy through the transfer of electrons. This is accomplished through a redox reaction.
The reduction half-reaction of the redox reaction occurs at the cathode (RED CAT)
The oxidation half reaction occurs at the anode (AN OX).
To maintain the flow of electrons something is needed to transfer positive charge. This can be accomplished in two ways:
(1) A salt bridge. This allows the transfer of positive charge through the movement of positive ions. In the example below
Copper is the cathode in the cathode half-cell.
Here is where the reduction of Cu2+ ion to Cu metal occurs.
Zn metal is the anode in the anode half-cell.
Here is where the oxidation of Zn to Zn2+ ion occurs.
Other ions are present for charge neutralization, ionic conduction, and completion of the circuit.
This is the basis for most batteries.
You can usually see the + marked on the battery’s cathode, while the other end is the anode.
As we said above, to maintain the flow of electrons something is needed to transfer positive charge.
Another way to do this is to use a porous disk:
Electrodes are not stable
Electrodes slowly corrode.
Here is an example from a Zinc and Copper Galvanic cell
Here, electrons flow in the wire (above the solution) from Zn to Cu.
because Zn is a more active metal than Cu , it tends to lose e-
So the Zn electrode is oxidized: a Zinc ion and 2 free e- are made per original Zn atom
This Zn ion breaks apart from the electrode and floats off into the solution
tba
Pure metals corrode because they aren’t stable
Why do the electrodes corrode? Well the real question is “Why don’t all metals corrode”?
Look around you – what metals don’t corrode (rust)? Only gold, platinum and a few others. Every other metal does.
Look for pure metals… good luck – you won’t find any. They’re all already chemically bound to other substances. Instead of finding copper, we find copper ore. Same for iron, or anything else.
How do we get pure metals, then? We need to expend a lot of energy to separate the metal that we want from the other atoms.
Here’s the physics explanation of why this is so. It has been excerpted and adapted from Corrosion of metals (author unknown.)
Pure metals contain more bound energy, representing a higher energy state than that found in the nature as sulphides or oxides.
All material in the universe strives to return to its lowest energy state.
Same for metals. They tend to revert to their lowest energy state which they had as sulphides or oxides. They revert to a low energy level by corrosion.
For batteries, we see electrochemical corrosion. Takes place in an aqueous environment.
All metals in dry air are covered by a very thin layer of oxide, about 100Å (10-2µm) thick. This layer is built up by chemical corrosion with the oxygen in the air. At very high temperatures, the reaction with the oxygen in the air can continue without restraint and the metal will rapidly be transformed into an oxide.
At room temperature the reaction stops when the layer is thin. These thin layers of oxide can protect the metal against continued attack, e.g. in a water solution. In actual fact, it is these layers of oxide and/or products of corrosion formed on the surface of the metal that protect the metal from continued attack to a far greater extent that the corrosion resistance of the metal itself.
These layers of oxide may be more or less durable in water, for instance. We know that plain carbon steel corrodes faster in water than stainless steel. The difference depends on the composition and the penetrability of their respectively oxide layers. The following description of the corrosion phenomenon will only deal with electrochemical corrosion, i.e. wet corrosion.
Corrosion cells
How do metals corrode in liquids? Let us illustrate this, using a corrosion phenomenon called bimetal corrosion or galvanic corrosion. The bimetal corrosion cell can e.g. consist of a steel plate and a copper plate in electrical contact with one another and immersed in an aqueous solution (electrolyte).
The electrolyte contains dissolved oxygen from the air and dissolved salt. If a lamp is connected between the steel plate and the copper plate, it will light up. This indicates that current is flowing between the metal plates. The copper will be the positive electrode and the steel will be the negative electrode.
The driving force of the current is the difference in electrical potential between the copper and the steel. The circuit must be closed and current will consequently flow in the liquid (electrolyte) from the steel plate to the copper plate. The flow of current takes place by the positively charged iron atoms (iron ions) leaving the steel plate and the steel plate corrodes.
The corroding metal surface is called the anode. Oxygen and water are consumed at the surface of the copper plate and hydroxyl ions (OH-), which are negatively charged, are formed. The negative hydroxyl ions “neutralize” the positively charged iron atoms. The iron and hydroxyl ions form ferrous hydroxide (rust).
In the corrosion cell described above, the copper metal is called the cathode. Both metal plates are referred to as electrodes and the definition of the anode and the cathode are given below.
Anode: Electrode from which positive current flows into an electrolyte.
Cathode: Electrode through which positive electric current leaves an electrolyte.
When positive iron atoms go into solution from the steel plate, electrons remain in the metal and are transported in the opposite direction, towards the positive current.
Videos
https://www.youtube.com/watch?v=C26pH8kC_Wk
Learning Standards
HS-PS1-10(MA). Use an oxidation-reduction reaction model to predict products of reactions given the reactants, and to communicate the reaction models using a representation that shows electron transfer (redox). Use oxidation numbers to account for how electrons are redistributed in redox processes used in devices that generate electricity or systems that prevent corrosion.*
Bond lengths in molecules
Some people say not to use ice cubes in soda or wine because “it makes flavor molecules contract”, which supposedly makes a drink taste worse. Is this correct?
First note that flavor comes from individual molecules dissolved in solution. These are monomers, not polymers. That’s going to be important.
Let’s break the question down into two parts: Does cooling a drink make flavor molecules contract? Does cooling a drink change how our tongue perceives flavor from such molecules?
(A) Does cooling a drink make flavor molecules contract?
Let’s start with the claim that ice makes flavor molecules contract. Sounds reasonable, after all, in everyday life we see that coldness can shrink materials. For instance, a bimetallic strip consists of two different materials. Each has a different expansion coefficient (way it responds to temperature changes.) When heated, one metal expands more than the other, which forces the metal to bend. These strips are used as switches in some thermostats.
image from hyperphysics.phy-astr.gsu.edu
More commonly, we see large scale materials contract when cooled, like highways. That is why roads over bridges, and in parking lots, need thermal expansion joints. Otherwise the shrinking and expansions would otherwise break the surface.
image from Ontario Ministry of Transportation, Bridge Repairs
Characterization of Typical Potent Odorants in Cola-Flavored Carbonated Beverages
Molecules in Coca Cola (Compound Interest)
Molecules in wine (Compound Interest)
Molecules in whiskey (Compound Interest)
But why do these materials contract when cooled? Has each individual molecule shrunk? No. To understand why we need to think about intermolecular vs intramolecular forces.
Intermolecular forces hold one molecule to another.
image from dynamicscience.com.au
Intramolecular forces hold atoms together within a single molecule. (e.g. chemical bonds, covalent bonds)
The naming comes from Latin roots
inter meaning between or among
intra meaning inside
When we use ice to cool a drink from 75 F down to 45 F, does the size of an individual molecule change in any appreciable way? Well, as we see here, a heated molecule can vibrate faster or slower – but the distance between atoms doesn’t appreciably change: the size of any single molecule is constant.
Nope. First, notice that the temperature in either F or C is misleading. Those temperatures are only measured relative to the freezing point of water. A better comparison is comparing their absolute temperature using the Kelvin scale
75 F = 24 C = 297 K
45 F = 7 C = 280 K
Wow, on an absolute scale this doesn’t change the temperature of the molecules very much.
Now sure, cooling does reduce molecular vibration: this allows intermolecular forces to pull the molecules closer together, which causes shrinking of the material as a whole. Still, this doesn’t change the bond lengths within a molecule.
(B) Does cooling a drink change how our tongue perceives flavor from such molecules?
If flavor change exists then I would think it is because the tongue tastes certain molecules better at certain temperatures.
image from blogs.unimelb.edu.au, The University of Melbourne, Scientific Scribbles, Let’s talk about Taste
images
https://maxfacts.uk/help/oral-food/ttt
discussion
https://www.ncbi.nlm.nih.gov/books/NBK236241/
https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072592/
https://www.livescience.com/20286-foods-taste-hot-cold.html
https://academic.oup.com/chemse/article/42/2/153/2547704
https://www.beveragedaily.com/Article/2005/12/19/Food-temperature-affects-taste-reveal-scientists
https://www.quora.com/How-exactly-does-temperature-affect-the-taste-of-food-and-beverages
Learning Standards
Next Generation Science Standards
HS-PS1-3. Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.
Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS1-3. Cite evidence to relate physical properties of substances at the bulk scale to spatial arrangements, movement, and strength of electrostatic forces among ions, small
molecules, or regions of large molecules in the substances. Make arguments to
account for how compositional and structural differences in molecules result in
different types of intermolecular or intramolecular interactions.
Common Core State Standards Connections:
KaiserScience 
