… from Science Analogies.Info

Atomic Size is like the Growth Rings of a Tree

As you go down a vertical group in the periodic table, the trend is that the atomic radius increases as more energy levels are added on. This is similar to the observation that as a tree adds on growth rings, the stem becomes larger in radius.
Source: Original
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Atomic Size is like the Spots on a Giraffe’s Neck

The increasing atomic size as you go down a vertical group or family in the periodic table is similar to the pattern of increasing size shown by the spots on a giraffe’s neck as you go from its head down to the body.
Source: Original
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Energy Levels are like Rows in a Parking Lot

Parking lot rows are analogous to the energy levels in an atom in the following ways:
Different numbers of electrons are found in different energy levels. Similarly, not all rows will hold the same number of cars, depending on the location and shape of the rows.
The energy levels, generally speaking, fill from the nucleus outwards. Similarly, the arena in which the basketball or hockey game is being played, represents the nucleus of the atom. The parking lot rows that are closest to the arena are always the ones which get filled up first, and the farther away ones later.
The ionization energy of an electron in an outside energy level is smaller than for an electron in an inner level. Similarly, a car parked on an outer row of the parking lot can generally get out of the lot easier after the event, than a car located in one of the congested inner rows, closer to the arena (nucleus).
Source: Original
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Energy Levels are like Standing on a Ladder

One of the main postulates of the Bohr Model of the atom is that electrons can only exist in certain stable energy levels. It is analogous to saying that when you stand on a ladder you have a certain amount of potential energy at each rung position, and you can not stand at any in betwen position or have any in between amount of energy. If you do try to stand at a position in between two rungs, you will always automatically slide down to a lower rung and a correspondingly lower energy position.
Source: Original
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Energy Levels are like a Stick Shift

According to the Bohr Model, electrons can only exist in particular energy levels around the nucleus. This is like a stick shift on an automobile … it only works when it is in 1st gear, 2nd gear, etc … and not at any in between position.
Source: Unknown
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Photoelectric Effect is like Shooting with Different Guns

When a beam of low frequency, low energy light is directed onto a metal surface, it will reflect off with no effect upon the metal. If many light sources of the same frequency are used, there is still no effect on the metal. An analogy would be that if you shoot at a person in a suit of armor using either a single BB gun or many BB guns at once, the person inside the armor will not be affected.
If light of higher frequency and energy from only a single source is used, then this greater amount of energy may be absorbed and cause electrons to be promoted and ejected. Similarly, if the BB gun is replaced by a more powerful rifle, then it would only take one shot to pierce the armor and hurt the person inside.
Source: McCullough, Thomas Simple Analogies in General Chemistry Journal of Chemical Education July 1992, 69(7), 543.
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Photons are like Backpacks

According to the Quantum Theory, an electromagnetic wave guides a flow of energy which is transported along the wave in bundles (photons) of size hf. A simplified view of the situation would be to say that if a wave was represented by a group of people marching in a line, the photons would be analogous to the backpack each person was wearing, and each of which contained a particular amount of material. This analogy could be extended to say that older, more energetic adults would carry a greater load in their backpacks than would children.
Source: Original
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Wave Mechanical Model is like a Power Boat

When a power boat is cruising on a lake, the wave which it produces has its greatest amplitude right at the boat, and the amplitude decreases as the distance from the boat increases. Thus you could locate the most probable location of the boat by analyzing the amplitude and energy of its associated water wave.
This is analogous to the wave mechanical model which visualizes the atom as a positive nucleus surrounded by vibrating electron waves. The Schroedinger Wave Equation describes the amplitude and other characteristocs of the waves which are associated with the moving electrons, and thus it also is able to describe the energy and location of the orbiting electrons.
Source: Original
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Wave Particle Duality is like Watching a TV Screen

Sometimes electrons are described as being small particles of matter. At other times when we describe the organization and behavior of electrons, we treat them as wave patterns.
Someone who had never seen a television would report seeing recognizable images of people, buildings, etc. when watching the screen from across the room. However, a similar observer, if placed with their eye right up against the screen, would report seeing dots of varying colors and brightness. Both descriptions of the screen are correct; the perspective of the viewer determines which properties are observed at that moment.
Source: Licata, Kenneth P. Chemistry Is Like a … Science Teacher 1988, 55(8), p.42


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Coordinate Covalent Bonding is like Borrowing Library Books

Coordinate covalent or dative bonding is often described in a simple fashion by saying that it involves one atom donating or giving a pair of electrons to another, so that this bonding partner can have a full outer shell. When electrons are counted up in an electronic dot diagram, this coordinate covalent pair is counted with each of the atoms. It is similar to borrowing a pair of books from the public library … the books are given to you and you treat them essentially as if they belonged to you; yet at the same time the books are counted as being part of the library collection.
Source: Original
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Covalent Bond Types are like Eating in a Restaurant

A nonpolar covalent bond forms when electrons are shared equally between atoms. A polar covalent bond forms when electrons are not shared equally … the more electronegative atom “gets” more electrons than he gives away by sharing. A coordinate covalent bond is imagined to form when one atom donates the entire pair of electrons which are shared with the other atom.
A restaurant analogy for these situations could be as follows:
A nonpolar covalent bond is formed if you give your friend half of your cheeseburger in exchange for half of his chicken burger. A polar covalent bond would be like your friend taking all of your cheeseburger and in exchange giving you just a small bite of his chicken burger. A coordinate covalent bond forms if you notice a homeless person outside, bring them into the restaurant, and give them your whole dinner to eat.
Source: Original
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Resonance Hybrid is like a Mule

The actual electronic structure of a molecule or ion which involves resonance is often explained by saying that it is the average of several contributing electronic structures which are drawn so as to show the double bond in several different locations in the species. In order to remind students that the actual resonance hybrid structure doesn’t alternate from one contributing structure to another from time to time, but rather has its own special structure all the time, it is convenient to use the analogy that a resonance hybrid is like a mule . When you look at a mule ( which is a cross between a horse and a donkey) you don’t see a donkey at one time and a horse at another… you see a mule at all times.
Source: Sienko, M. J. and Plane, R. A. Chemistry Toronto: McGraw-Hill, 1964 p.94
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Activated Complex is like a Window Sill

When enough activation energy is added, the reacting molecules interact very closely with each other to produce the activated complex. It is an aggregate of particles which has a higher potential energy than either reactants or products, and represents an intermediate stage or situation that must be reached before the reaction will proceed to produce the products.
A fire fighting analogy can be used to illustrate this idea. If firemen wish to access a fire on the second floor of a building, they can’t enter if they just climb up the ladder so their feet are at the level of the second floor. They must climb a little higher, to a higher potential energy location, so that their feet are at the level of the window sill; then they can climb in and drop down to floor level to complete their entry.
In reverse, the firemen can’t go from the second floor directly to the ground; they must first climb up to the window sill then go down their ladder to the ground. In both cases the window sill, just like the activated complex, represents a specific higher energy stage that must be reached in order to move from the original to the final situation.
Source: Licata, Kenneth P. Chemistry Is Like a … Science Teacher 1988, 55(8), 41
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Catalyst Behaviour is like a High Jump Competition

In a high jump competition … when the bar is very high, only a small per cent of athletes will be able to successfully get over. This is analogous to a chemical reaction which has a very large activation energy so that only a small percent of molecules have sufficient energy to reach the activated complex stage and react.
When the bar is set quite low, it takes less energy for the jumper to get up to the necessary height to clear the bar, resulting in a greater per cent of successful jumps. This is analogous to the action of a catalyst …. it provides a reaction mechanism which decreases the activation energy required to reach the activated complex stage. This produces a greater fraction of successful collisions and a faster reaction rate.
Source: Original
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Catalyst is like a Minister

A catalyst is a substance which stimulates or causes a chemical reaction to take place, but is not itself permanently changed as a result of the reaction. A catalyst is like a minister at a wedding ceremony … the minister causes the ceremony to take place, plays a role in determining how fast the ceremony takes place, and is not himself permanently changed as a result of the ceremony … unlike the couple getting married, who are permanently changed as a result of the ceremony!
Source: Original
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Collision Geometry is like a Piece of a Jigsaw Puzzle

Just because two molecules collide does not necessarily mean that they will react with each other. A successful collision may require that the two molecules or species must collide with the correct collision geometry, that is, be oriented in just the correct fashion so that certain atoms will encounter each other during the collision. It is similar to saying that when putting a piece into a jigsaw puzzle, you can’t just put it in any way you want …. the piece will only fit successfully if it is the right side up and is turned with the correct orientation so that the projections and indentations match up …. that is, it must have the correct “collision geometry”.
Source: Original
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Endothermic and Exothermic Reactions are like a Sponge and Water

An endothermic reaction is one which absorbs heat energy, so the products have a higher enthalpy or heat content than the reactants. This is like taking a sponge and pouring 50 ml of water into it … the sponge ends up with a greater water content than before the “endothermic” reaction. Furthermore, the amount of water added (50 ml) corresponds to the heat of reaction.
In an exothermic reaction, heat energy is given off to the surroundings so the product molecules have a lower heat content than the reactants. This is like taking a wet sponge and squeezing it into a funnel … the sponge ends up with a lower water content than before the “exothermic” reaction. The amount of water which is squeezed into the funnel and collected, is a good concrete way to visualize the heat of reaction.
Source: Original
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Limiting Reactant is like Meals at a Cafeteria

The action of a limiting reactant to determine the extent to which a reaction takes place is like producing meals at a cafeteria. When you run out of chicken pieces, no more chicken dinners can be produced, even though there might be a good supply of mashed potatoes and vegetables … the other “reactants”.
Source: Original
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Rate of Reaction is like Rate of Production at a Factory

A chemical reaction can be imagined as analogous to an automobile manufacturing plant. Raw materials like iron, plastic and glass are the reactants and cars are the finished product which roll off the assembly line at the other end of the factory. To measure the rate of this reaction, you could measure the rate at which a product is produced (eg. #cars/day) or measure the rate at which a single reactant is used up ( eg. #tons of iron used up/day). This is the same idea as measuring the rate of a chemical reaction.
Source: Original
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Wrestling Analogy for the Collision Theory

The most basic statement of the Collision Theory is that molecules must collide with each other in order to react. This is similar to saying that wrestlers must actually contact one another in order to wrestle.
Source: Original
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Analogies in this section deal with the properties of solutions and electrolytes of all types, and related concepts.

Electrolytes are like Nuts and Bolts

If a bolt represents a positive ion, a nut analogous to a negative ion and assembled units represent undissociated molecules of the electrolyte, these units can be used to represent various types of electrolytes:
A strong electrolyte would be represented by an equal no. of separate nuts and bolts in a box (complete dissociation).
A weak electrolyte would be represented by assemblying most nuts and bolts into pairs, and having only a few disconnected (small per cent ionization).
A non-electrolyte would be represented by assemblying all the nuts and bolts into pairs so that none were disconnected (no ionization).
A more concentrated solution of a weak electrolyte would be represented by showing a greater number of assembled nuts and bolts, but a smaller per cent of them dissociated into separate pieces.
Source: Fortman, John J. Pictorial Analogies X: Solutions of Electrolytes Journal of Chemical Education January 1994, 71(1), 27
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Hydronium Ion Concentration is like a Seesaw

Imagine a seesaw or teeter totter with hydronium ion concentration at one end and hydroxide ion concentration located at the other end. As the seesaw operates, it will show the required inverse relationship between these factors …. as the hydronium ion concentration goes up, the hydroxide ion concentration goes down, and vice versa.
Similarly, a seesaw with pH at one end and hydronium ion concentration at the other, shows the relationship these factors have …. as hydronium ion concentration goes up, the pH goes down, and vice versa.
Source: Fortman, John S. Pictorial Analogies XI: Concentrations and Acidity of Solutions Journal of Chemical Education May 1994, 71(5), 431
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Entropy is like Marbles in a Box

Entropy is an important factor in discussing equilibrium. Either the forward or reverse reaction will be favoured by the entropy factor … that is, allowing atoms and molecules to move from a state of lesser to greater disorder.
Imagine taking a small box and into it packing marbles so that all the marbles in each horizontal layer are the same color, or show some other definite pattern. When the box is shaken for a while then the contents examined, you will see that the marbles are now quite randomly arranged … they have spontaneously moved to a state of greater disorder (higher entropy).
Source: Smoot, R.C., Price, J. and Smith, R.G. Chemistry A Modern Course Don Mills: Maxwell Macmillan Canada, 1987, p.386
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Equilibrium is like Substituting Players in a Sports Game

Equilibrium is a situation attained when the rates of forward and reverse reaction are equal, so that there is no change in the concentrations of the reactants and products. Equilibrium can be reached at various points; the concentrations of reactants and products need not be equal, only their rate of exchange. The equilibrium point also shifts in response to stresses placed upon the system.
In a sports game like soccer or basketball, for every new player substituted onto the field, an old player must leave – thus the rate of these opposing reactions are equal. There is no change in the number of players on the field, even though their identities are different. There is no requirement that the number of players on the field and on the bench be equal (and usually they are not equal); the only thing that must be equal is the rate of exchange between these two groups.
In a game like hockey, a penalty would be like a stress … it increases the number of players leaving the ice, compared to the number of players going onto the ice. At this new equilibrium point the rate of player exchange is again equal for the duration of the penalty. When the penalty is over, the equilibrium point shifts back to the original position.
Source: Licata, Kenneth P. Chemistry Is Like A … The Science Teacher 1988, 55(8), 43.
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Graham’s Law of Diffusion is like Running a Race

According to Graham’s Law, the velocity or rate of diffusion of a gas is inversely related to the square root of the molecular mass. Thus, molecules of small mass travel more rapidly and molecules of larger mass travel more slowly. This idea can be easily remembered by considering the following analogy:
Consider 10 runners who were so closely matched that they had essentially equal times for running a 1000 metre race. Now suppose half of the runners were required to carry an extra 10 kg mass that was attached to them by a belt or backpack. Likely the 5 who had to carry the extra mass would now lag behind the rest of the runners.
Source: Based on Toon, E.R. and Ellis, G.L. Foundations of Chemistry New York: Holt, Rinehart and Winston, 1973 p. 136
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Mean Free Path is like Bumper Cars

The mean free path is the average distance which a molecule travels between collisions with neighboring molecules. This is like riding in the bumper cars at a carnival … you only can travel a short distance, on the average, before being involved in a collision with another bumper car.
Source: Original
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Phases of Matter are like Students at School

Students in a classroom are analogous to the particles of a solid, since they have a regular arrangement and a limited freedom of movement. Students shifting or turning in their seats represent the vibrational motion of solid particles.
During breaks between classes, students have a wider range of motion. They now also have limited translational motion which allows them to move among one another to the doorway and through the halls, but are still confined to the volume of the school. This is analogous to the behaviour of liquid particles.
At the end of the day, students are like gas particles since they have unrestricted and primarily translational motion which causes them to escape from their school building and diffuse throughout the community.
Source: Licata, Kenneth P. Chemistry Is Like A … The Science Teacher 1988, 55(8), 42.

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