Rowland, Solomon, and Garcia's work suggested a modification to the original hypothesis: CFCs cause significant ozone depletion - and they do it much more rapidly with the help of polar…
Researchers needed to incorporate nitrogen dioxide into the hypothesis - but they weren't sure how it would affect the expectations generated by the hypothesis. Would we still expect to see…
A plot of chlorine monoxide and ozone concentrations from data collected by an aircraft passing through the Antarctic ozone hole. Outside the hole (left side of graph), ozone levels are…
General relativity seems up for a change. For example, it doesn't mesh with what we know about the interactions between extremely tiny particles. Will physicists develop a new theory that…
Science is always a work in progress and even theories change. For example, in the 1600s classical mechanics was the accepted explanation of the movement of objects both in space…
Molina and Rowland's over-arching hypothesis (that releasing CFCs into the atmosphere would cause significant ozone depletion) was based on many supporting hypotheses, backed by their own lines of evidence.
Students are given six knowledge statements and asked to rank them according to how scientific they feel the statements are. A group discussion ensues. This activity is adapted from Scharmann…
According to nuclear theory, deuterium atoms fuse and forming helium-4 which has a lot of energy - so much energy that it is unstable, so the atom discharges some of…
Ozone levels fluctuate so widely that it is difficult to detect subtle trends over a short-term period, as shown by these ozone measurements for the atmosphere over Switzerland taken between…
A graph showing world production of three major CFC types between 1970 and 1988. The dot marks the year (1977) that the US phaseout of CFCs was announced.
Unlike nuclear DNA (left), mitochondrial DNA is only inherited from the maternal lineage (right) - a quirk which allowed Margulis to determine whether mitochondrial DNA encoded unique traits.
Change in the amount of ozone in the Northern Hemisphere over a 17-year period. Note the large decline in ozone during the winter months and how the winter decline is…
The evidence supports the idea that a giant asteroid struck Earth at the end of the Cretaceous, but additional explanations (volcanic activity, global climate change, sea level change, and tectonic…
In the presence of another molecule to serve as a catalyst (not shown), nitrogen dioxide (NO2) and chlorine monoxide (ClO), a byproduct of the breakup of ozone molecules by CFCs,…
As new seafloor forms, the igneous rock records the current state of the Earth's magnetic field. Sedimentary rock layers forming at the bottom of the ocean may also record these…
Watson and Crick's model erroneously placed the bases on the outside of the DNA molecule with the phosphates, bound by magnesium or calcium ions, inside.
Media representations of science and science-related policy are essential for quickly communicating scientific messages to the broad public; however, some important parts of the scientific message can easily get lost…
Alvarez posed two hypotheses: either the clay was deposited over a few years and he would find no iridium, or over a few thousand years and the iridium would definitely…
The visiting American chemist, Jerry Donohue, provided a key piece of evidence when he revealed that the forms given for thymine and guanine in most textbooks were wrong. Note the…
An existing hypothesis proposed that a supernova at the end of the Cretaceous had caused the extinction of dinosaurs. Supernovas throw off heavy elements like iridium - so the hypothesis…
Powerful scientific ideas generate many different expectations, not just one. As an example, let's return to the idea that the continents as we know them today were once joined together…
A group of scientists showed that if you remove tubule organelles from a eukaryotic cell, they can grow back. This discovery argued against the idea that these organelles evolved via…
To further test the supernova hypothesis:If a supernova had occurred, it would have also released plutonium-244 as well as iridium. Original tests looked positive but replicating the analysis showed that…
Pons' neutron results didn't agree with his heat measurements, with Jones' neutron results, or with established nuclear theory, which suggested no fusion should be occurring at all!
Chlorine monoxide is one of the products of ozone destruction. Since there is no other known source of chlorine monoxide, finding this chemical in the upper atmosphere would strongly support…
Testing the hypothesis: If some organelles evolved via endosymbiosis, we would expect to see a close genetic relationship between the organelles and bacteria.
Scientific testing occurs in two logical steps: (1) if the idea is correct, what would we expect to see, and (2) does that expectation match what we actually observe? Ideas…
When the entrance to the reward tunnel was blocked, most of the rats picked a tunnel that led in the direction of the food, supporting the idea that rats navigate…
Tolman and his colleagues trained rats in a maze which offered them many different tunnels to enter first. One of the tunnels twisted and turned but consistently led to the…
Psychologist Edward Tolman wanted to know how rats successfully navigate their surroundings - for example, a maze containing a hidden reward. Would they build mental maps of the maze as…
In the early 1900s, Ernest Rutherford and his colleagues performed this experiment to test the hypothesis that an atom's mass and positive charge are spread diffusely throughout the atom and…
Scientific arguments are formed by figuring out what we would expect to observe if a particular idea were true and then checking those expectations against what we actually observe. A…
Hypotheses are proposed explanations for a fairly narrow set of phenomena, usually based on prior experience, scientific background knowledge, preliminary observations, and logic.
Taken together, the expectations generated by a scientific idea and the actual observations relevant to those expectations form what we'll call a scientific argument.
Most of the alpha particles passed through the gold foil without changing direction much as expected, but some came bouncing back in the opposite direction.
Gregor Mendel (left) showed that if you know the genotypes of the parents in a cross, you can predict the ratios of different offspring genotypes (right).
Though the structure of this argument is consistent (hypothesis, then expectation, then actual observation), its pieces may be assembled in different orders.
Scientific arguments involve three components: the idea (a hypothesis or theory), the expectations generated by that idea (frequently called predictions), and the actual observations relevant to those expectations (the evidence).
Science is open to anyone and benefits tremendously from the expanding diversity of perspectives. However, science only works because the people involved with it behave "scientifically."
Luis Alvarez hypothesized that the asteroid had blown millions of tons of dust into the atmosphere, blotting out the sun, stopping photosynthesis and plant growth and hence, causing the global…
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