Science 9: Unit B Topic 3: The Elements

Scientific Laws, Models, and Theories (Redux) Science is based on two foundations: To be considered scientific, your idea or theory must be backed up by evidence, and it must also make predictions that can be tested in an experiment. Many theories can twist evidence around to support it, but if it cannot make an accurate and testable prediction, then it’s not real science.

Scientific Theories What is a Theory? A theory is an explanation of how and why a natural phenomenon occurs. It can never be proven correct, but it can be accepted if it is supported by evidence and its predictions are proven correct through experiments. If one experiment proves the theory wrong, then it must be improved, changed, or thrown out completely from science.

Scientific Law What is a Law? A law is different than a theory in that it describes rather than trying to explain what is happening. Because it is describing something that is real, laws are considered true, and theories, are not. For example: Scientific Law: All matter with mass is pulled towards the center of the ground. Scientific Theory: There is an invisible force, called gravity that pulls matter together. The more mass of an object, the stronger its gravitation pull.

Scientific Model What is a Model? – A model is a simplified explanation of a more complex theory. A model is often used to help teach the theory to students and to the general public while simplifying things for scientists doing work on the theory. For example, the particle model of matter is an over-simplification of how matter is made up, however it is a useful starting point in learning about chemistry and matter.

Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction or physical change of a substance. This means that the mass of the chemical reactants (materials reacting) must equal the products of the chemical reaction. The matter may change, but the masses must be equal.

Law of Definite Proportions Law of Definite Proportions – The masses of elements in a pure substance always occur in the same proportions. The mass of pure water is always 8 parts oxygen to 1 part hydrogen. (or 89% oxygen to 11% hydrogen). This law helps prove that a compound is made up of a fixed ratio of elements.

Models of Matter Since the ancient Greeks, various scientists and thinkers have tried to answer the question: What makes up the universe?

Ancient Greeks/Aristotle Details: All matter is made up of a combination of 4 elements: water, air, fire, and earth. Matter is continuous and not made up of smaller particles. Limits: If matter is not made up of smaller particles than why does it break apart and change in a chemical reaction? Model does not agree with evidence.

Dalton Atomic Model (The Billiard Ball Model) Details: All matter is made up of small, indivisible particles called atoms. Different kinds of materials contain different types of atoms. Limits: Scientists noticed that atoms could give off negatively charged particle rays. Since the atom is neutral, the atom must contain negative and positive parts. This does not agree with Dalton’s predictions

Plum Pudding Model Details: An atom is made up of a thin positive layer (the pudding) with electrons embedded around and inside the positive mass (the plums) Limits: Model could not explain why positive particles would sometimes bounce back from atoms. Model predicted that high speed particles should easily pass through the thin positive layer.

Planetary Model Details: An atom is made up of small dense, positive centre called a nucleus which contains protons and neutrons; and negative electrons orbiting the nucleus like planets around the sun. Limits: This model predicts that all matter should self-destruct as negative electrons should collapse on the positive nucleus. Obviously this does not happen and this model is wrong.

Bohr-Orbital Model Details: Electrons move around in special areas called orbitals where they do not lose energy and crash into the nucleus. Limits: Could not explain why orbitals exist or how they work etc. It is almost more of a law than a theory.

Electron Cloud Model Details: Electrons are not really particles at all, but particle-waves. Electrons only exist in orbitals because only in these areas do the electron waves fit perfectly. Limits: Because electrons are now particle-waves we cannot predict their exact locations and speeds, only where they will likely be with 90% accuracy.

Scientific Reasoning There are two ways scientific theories are created: deductive and inductive reasoning.

Deductive Reasoning Scientist starts with a prediction/ hypothesis and looks for evidence and designs an experiment to test it out. Deduction started with ancient Greeks. There is a higher chance that the scientist will find only what she is looking for and the theory will be false.

Inductive Reasoning Scientists begin with observations and design a hypothesis and experiment to test it out. This method is more scientifically rigorous in that you must adapt your hypothesis to evidence, not the other way around. First designed by Sir Francis Bacon.