(12) Environmental Science

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Oil burning, an application of the laws of conservation of mass / energy

ES12

How Can Matter Change?

Concept 2-3 when matter undergoes a physical or chemical change, no atoms are created or destroyed (the law of conservation of matter).

Matter Undergoes Physical, Chemical, and Nuclear Changes

When a sample of matter undergoes a physical change, its chemical composition, or the arrangement of its atoms or ions does not change. A piece of aluminum foil cut into small pieces is still aluminum foil. When solid water (ice) melts or liquid water boils, none of the H2O molecules are changed; instead, the molecules are organized in different spatial (physical) patterns.

Thinking About

Controlled Experiments and Physical Changes

How would you set up a controlled experiment (Core Case Study) to verify that when water changes from one physical state to another its chemical composition does not change?

In a chemical change, or chemical reaction, the arrangements of atoms, ions, or molecules change.

Chemists use shorthand chemical equations to represent what happens in a chemical reaction. For example, when coal burns completely, the solid carbon (C) in the coal combines with oxygen gas (O2) from the atmosphere to form the gaseous compound carbon dioxide (CO2). In addition to physical and chemical changes, matter can undergo three types of nuclear changes: natural radioactive decay, nuclear fission, and nuclear fusion.

We Cannot Create or Destroy Matter

We can change elements and compounds from one physical or chemical form to another, but we can never create or destroy any of the atoms involved in any physical or chemical change. All we can do is rearranging the elements and compounds into different spatial patterns (physical changes) or combinations (chemical changes). These statements, based on many thousands of measurements, describe a scientific law known as the law of conservation of matter: when a physical or chemical change occurs, no atoms are created or destroyed (Concept 2-3).

This law means there is no “away” as in “to throw away.” Everything we think we have thrown away remains here with us in some form. We can reuse or recycle some materials and chemicals but the law of conservation of matter means we will always face the problem of what to do with some quantity of the wastes and pollutants we produce.

Radiocative decay occurs when nuclei of unstable isotopes spontaneously emit fast-moving chunks of matter (alpha particles or beta particles), high-energy radiation (gamma rays), or both at a fixed rate.

Nuclear fission occurs when the nuclei of certain isotopes with large mass numbers (such as uranium-235) are split apart into lighter nuclei when struck by a neutron and release energy plus two or three more neutrons. Each neutron can trigger an additional fission reaction and lead to a chain reaction, which releases an enormous amount of energy.

Nuclear fusion occurs when two isotopes of light elements, such as hydrogen, are forced together at extremely high temperatures until they fuse to form a heavier nucleus and release a tremendous amount of energy.

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