1) Explain why it is not possible for a scientist to state that a scientific theory or law is absolutely and forever correct.
It is not possible for a scientific theory or law to be absolutely and forever correct. This is because new information is constantly being discovered. With this new information comes new facts that may contradict previously proposed theories or laws. If information does come into conflict with a theory, the theory must either be modified to include this new information, or disregarded and a new theory instituted in its place. Take, for example, the Molecular Theory of Gases. This theory did not take into account what happened to gas at very low or high temperatures or pressures. Eventually, new facts were discovered and a new theory took the place of the old one. This new theory took into account for the ideas the old one did not. Another example is the old phlogiston theory. It was believed that everything that burned gave off phlogiston. This, as we know today, is not true. Dues to Lavoisier’s experiments he found that instead of an object giving off phlogiston as it burned, it actually took in air. This discovery is the basis for our modern theory of burning. So, as you can see, a theory or law can never be entirely true, and can, in almost all cases, have an exception.
2) What clues could you use to determine whether a change is a physical change or a chemical change?
There are many differences between physical and chemical changes, and just as many clues you could use to figure out which has occurred. In a physical change, no chemical changes take place. No new substance is formed from a physical change. But during such a change, the appearance of a substance may change. Color, for example, could be a clue that a physical change has occurred. If a substance changes color, but still is composed of the same chemical properties as before, you could pretty much safely assume the change was physical. The magnetizing of iron, for example, could represent a physical change. The iron has undergone no chemical change, because it’s still iron. Chemical change occurs, however, when a new substance is formed. The properties of the new substance must differ from those of the original. Take, for example, the rusting of iron. The reaction that occurs between the water and iron forms a new substance known as rust. A chemical change has taken place. Another example of chemical change could be the souring of milk. These are just some of the clues you could use to differentiate between a physical and chemical change. It mainly depends on whether a new substance was formed or not.
3) Lavoisier’s conclusion “that mass is not created nor destroyed in chemical changes” had a large impact on understanding chemical reactions. Give examples of two applications from this that can be made to chemical reactions.
Lavoisier believed that mass could neither be created nor destroyed by a chemical reaction. Before he did an experiment to prove that this statement was true, it was believed that mass could be gained or lost during a chemical change. Scientists before Lavoisier had burned tin to prove that after burning the tin weighed more than before. Lavoisier tried his experiment of burning tin in a sealed container, and, what a surprise!, it didn’t gain weight after it had been burner. But as soon as he removed that lid from the container air rushed in and replaced the gas that had combined with the tin during burning and only then did the tin gain weight. So Lavoisier concluded that mass could neither be created nor destroyed by any chemical change, because the tin only gained weight after it combined with the air. I believe another example of mass being neither created nor destroyed is in the boiling of water. People may have believed that it just disappeared as the water evaporated or turned to steam. But it’s not true. As the water is boiled and evaporates, it turns itself into air, thus nothing is lost or destroyed. Lavoisier’s theory holds true for most normal chemical changes, but there are those rare occasions when the mass does change because of the small, almost undetectable traces of energy given off. But for all practical purposes, Lavoisier is correct.
4) What specific observations in the foil experiment led Rutherford to suggest that the positive charge in an atom must be located in a very small part of the atom?
We now know that the atom has a very small, positively charged nucleus. It was due to an experiment by Rutherford that led us to learn this. His experiment was designed to shoot a beam of alpha particles directly through a piece of gold foil. The alpha particles, after passing through the foil would be seen as flashes of light on a screen that surrounded the foil. Rutherford observed that a majority of these particles went directly through the foil. He then concluded that the atom consisted mostly of empty space! But some of the alpha particles were reflected by the foil, some by more than a 90º angle! Thus Rutherford concluded that the atom must have a very small, massive nucleus, and that the nucleus must be positively charged. If the nucleus were not positively charged the alpha particles would not be reflected. Rutherford then designed a model of the atom based on his experiments, (his experiments included trying different kinds of foil and getting similar results.) His model showed the atom with a small positively charged nucleus that had electrons orbiting around it. Later, flaws would be found in his model atom, but it was a brand new idea at that time.
5) Rutherford’s model of electrons moving around a positive nucleus was contrary to the accepted theory of moving electric charges. What prediction did Rutherford’s theory make for the electrons and how did the Bohr model overcome this contradiction?
Rutherford’s model and theory of the atom were correct in many ways, but they also had their faults. If an atom was really designed the way Rutherford suggested, then eventually the electrons that were moving around the nucleus would lose energy a little at a time, ending up with crashing into the nucleus itself. This, of course, was an obvious problem, considering electrons definitely did not do this. Liens Bohr, however, came up with a solution. He designed his own model of the atom where in the electrons had constant, specific orbits, called energy levels. Let’s think of a ladder for a moment. Say only two people could stand on one rung of the ladder. If the people gained energy they would move to higher energy fields, but only two people to a rung. As they lost energy, they would slowly return to their original rungs. This concept is, essentially, what electrons do. Their energy levels are the rungs on the ladder. As they gain energy they are said to enter an excited state. As they lose energy, they return to a ground state. An atom is energetically unstable with its electrons in an excited state, so it will always return to its ground state.
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