By : Aamarpali Puri
In Chemistry, there are often many ideas that are frequently misinterpreted. Sometimes because of students coping with making sense of abstract concepts. Since science is constantly changing to adapt to new discoveries and methods, some misconceptions may be due to old ideas or legends. Following is meant to generate an awareness of some of the misconceptions found in Class IX Chemistry, particularly that of the atomic and molecular models.
|“Proper” concept (to date, of course!)|
|Atoms can be seen with a microscope.||Atoms cannot be seen with a microscope. The extent of an atom’s small size is often not well understood. For example, there are about one million atoms across the width of human hair, but many students guess a number in the hundreds or thousands.|
|Atoms are alive (because they move)||
Atoms vibrate because they all possess some thermal energy. They do not possess the characteristics of living things (i.e. needing energy to survive, producing wastes, reproduction, adaptability, etc.). The nuclei of cells and atoms are not synonymous.
|Atoms are like cells with a membrane and nucleus|
|Atoms can reproduce after the nuclei divide|
|Atoms have electrons circling them like planets around a star||
Electrons do not follow a simple circular pattern around the nucleus.
|An electron shell is like an eggshell or clamshell, thin and hard||Shells are not physical shells like eggshells. They are not thin or hard. They are regions around the nucleus where electrons can be found.|
|The electron shell is there to protect the nucleus, like an eggshell and a yolk|
|The electron cloud is like a rain cloud, with electrons suspended in it like droplets of water.
The cloud contains the electrons but is made of something else
|Electrons are not suspended motionless in the “electron cloud”. Instead, they are constantly moving throughout the “cloud”, which is not made of any other kind of matter.|
|The electron shell is a matrix of some kind of stuff with electrons embedded in it|
|Atoms “own” their electrons||There are not different kinds of electrons for different atoms. Atoms do not “possess” their specific electrons. Electrons are the same and can be transferred from one atom to another.|
|Molecules are basic, simple, indivisible entities||Molecules are made of smaller entities (atoms) which reorganized into different molecules. Therefore molecules are divisible.|
|Molecules of solids are hard, molecules of gases are soft||
Molecule shape, size and mass do not change between solid and gas phases. Just because the phase as a whole appears different, e.g. often the gas is less visible than when in the solid form, doesn’t mean that the molecules themselves have changed, only the forces between them. Changes of state are physical changes.
|Molecules of solids are biggest, molecules of gases are smallest|
|Molecules of solids are cubes, molecules of gases are round|
|Vapour molecules weigh less than solid molecules (e.g. water vapour vs. ice)|
|Molecules expand when heated||Molecules themselves do not expand. The substance heated may appear to expand because heat causes molecules to move faster (and further apart).|
|Molecules are glued together||Forces of attraction hold molecules together, not glue.|
|Bonds store energy,
Breaking chemical bonds releases energy,
Bond making requires energy
|Not all bonds release energy when broken or require energy to form. Exothermic reactions can form new molecules in which the products possess less energy than the reactants; hence, release energy when the bonds form and require energy to be broken.|
|Ionic pairs, such as Na+ and Cl–, are molecules||Ions are not considered molecules, which contain covalent bonds. A better word to use for ionic pairs in ionic compounds may be formula unit.|
|The chemical bond is a physical thing made of matter||Chemical bonds are not made of a separate form of matter, but the electrons that are shared and forces of attraction.|
|Chemical Bonds –Ionic|
|Ionic compounds form neutral molecules, such as Na+Cl– molecules, in water||In water, ionic compounds dissociate into their ions, which are not neutral molecules because they possess a charge and the solution can act as an electrolyte.|
|Bonds within “ionic molecules” are stronger than inter-molecular forces||Ionic compounds are not composed of “molecules”, but of ions which are attracted to one another. For example, an Na+ ion that is surrounded by Cl– ions is attracted to all of the Cl– ions, even though they are not all considered part of the “formula unit”. It is these bonds that are broken when the ionic compound is dissolved in water, resulting in Na+ and Cl– ions.|
|Na+Cl– bonds are not broken in dissolving; only inter-molecular bonds are broken|
|Chemical Bonds –Covalent|
|Electrons know which atom they came from||There are not different kinds of electrons for different atoms. Atoms do not “possess” their specific electrons. Electrons are the same and can be transferred from one atom to another.|
|Atoms know who owes them an electron|
|Electron pairs are equally shared in all covalent bonds||Electrons pairs are not shared equally in all covalent bonds. In some, one atom attracts the electron pair more than the other atom (i.e. a difference in electronegativity), and causes the electron pair to be closer to it than to the other atom.|
|The strengths of covalent bonds and intermolecular forces are similar||The strength of a covalent bond, an intramolecular force (within the molecule, i.e. between atoms), is much greater than that of intermolecular forces (between molecules). Hence, molecules can be pulled apart more easily than breaking apart the molecules themselves.|
|Freezing and boiling are examples of chemical reactions||Freezing and boiling are examples of changes of state, which are physical reactions, not chemical. Other changes of state include melting, condensation, and sublimation. One characteristic that changes of state do share with chemical changes: energy is either added or removed from the system, unlike other physical changes.|
|Physical changes are reversible while chemical changes are not||A very common misconception. Chemical changes are also reversible. Consider equilibrium reactions in which forward and backward reactions are both occurring at the same time, as well as Le Chatalier’s Principle. Some physical changes are also hard to reverse, for example, crushing a rock.|
|The original substance vanishes “completely and forever” in a chemical reaction||The original substance can be produced if the reaction can be reversed under the necessary conditions.|
|Mass is conserved, but not the number or species of atoms||Atoms are not created or destroyed in standard chemical reactions. Therefore, the number and species of atoms do not change, and hence mass is also conserved.|
|Reactions that proceed more rapidly also proceed further (more completely.)||This shows a discrepancy between the concepts of speed and completeness. A reaction can reach equilibrium before it has been “completed”, regardless of how fast the reaction proceeds.|
|Chemical reactions will continue until all the reactants are exhausted||Reactions can reach equilibrium before the reactants are exhausted. Equilibrium constants and Le Chatalier’s Principle.|
|Chemical equilibrium is a static condition||Students may believe that no reactions are occurring at equilibrium because the net reaction is zero. However, this means that reactions are still occurring—both forward and backward reactions are occurring at the same rate, and no net change is observed. Chemical equilibrium is dynamic.|
|A candle burning is endothermic, since heat is needed to initiate the reaction||Heat is needed at the beginning to initiate, or activate, the reaction. Once activated, the reaction proceeds without further energy input, and releases energy in the form of light. Therefore, it is an exothermic reaction. Another example is heating a piece of magnesium metal in a Bunsen burner, which causes it to combine with the oxygen in the air, releasing a bright light and forming magnesium oxide.|
|Energy is used up in chemical reactions.
Energy is created in chemical reactions.
|Energy is not “used up” or “created” in chemical reactions. Instead, they are released or stored in the form of chemical bonds between atoms.|
|Oil doesn’t mix with water because oil and water molecules repel each other||Oil molecules are actually attracted to water molecules more than to their own molecules. This can be shown when a drop of oil, which is originally spherical in shape which minimizes the number of molecules which are not surrounded by its own molecules, is dropped onto the surface of water. When it hits the water, the oil droplet spreads out instead of staying spherical, showing that the attraction between oil and water is greater than between oil and oil. Oil and water remain in separate phases, however, because the water-water attractive forces are still much greater than oil-water attraction. It would require an input of energy for the oil molecules to come between water molecules.|
|Adding salt to water decreases the amount of time cooking||Adding salt to water does increase the boiling point. However, it takes longer for the water to reach this higher temperature (with a constant supply of heat from the stove), and the once the water has reached the higher temperature, the change is so small that it is not significant.|
|Strength (of acids and bases) and concentration mean the same thing||Concentration is the number of moles of solute that are dissolved in one liter of solvent. Strength is the percentage of those molecules that dissociate into ions.
An interesting fact: some weak acids (e.g. acetic) actually increase in strength as their concentration decreases.
(most misconceptions taken from here)
Arizona State University. 2001. Students Preconceptions and Misconceptions in Chemistry. Visited April 2002. <http://www.daisley.net/hellevator/misconceptions/misconceptions.pdf>
Kevin Lehmann, 1996. Bad Chemistry. Dept of Chemistry, Princeton University, NJ. Visited April 2002. <http://www.princeton.edu/~lehmann/BadChemistry.html>
O’Connell, Joe. 2001. Salt Myths and Urban Legends. Visited April 2002. <http://www.scbbqa.com/myths/Salt.html>
Oklahoma State. Common Student Misconceptions. Visited April 2002. <http://www.okstate.edu/jgelder/acidPage25.html#Com>
Werwa, Eric. 2000. Everything you’ve always wanted to know about what your students think they know but were afraid to ask. Visited April 2002. <http://www.mrs.org/microworld/werwa.pdf>