MOLECULAR REASONS - 1. All natural phenomena in the world we can se...

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CHAPTER 1

MOLECULAR REASONS

ANSWERS TO QUESTIONS:

• All natural phenomena in the world we can see are the result of invisible molecular

interactions.

Examples are numerous. Some suggestions are as follows:

a) Ice melting to water.

b) A match being struck produces a flame.

c) A shirt fading when exposed to light or too many washings.

• There are three principal reasons for a non-scientist to study science.

a) Science influences society in very profound ways. In order to be able to make informed

and intelligent decisions about ethically complex problems like human cloning, genetic engineering, or climate change, it is essential to have at least some understanding of the workings of scientific principles.

b) Everyone has to some degree a responsibility in sustaining the variability of the planet for

future generations. Important decisions regarding science policy and funding are political decisions generally made by nonscientists. An informed public can influence these decisions and can help minimize the potential for misguided or perverted applications of science by ignorant or evil governments.

c) Science can be described as a particular way to understand the workings of the world and

the universe. Lacking appreciation of scientific principles, an uninformed observer will fail to appreciate completely the beauty, complexity and subtlety of nature. Learning some of the basics of science leads to a deeper, richer, and more fulfilled life.

3.

a) The bright color of the rug is the result of particular molecules in the rug. When particles

of sunlight, photons, hit the rug, the bright color molecules are destroyed or altered in some way.

b) Because water molecules are attracted to the salt crystals, the water breaks up the salt

crystals, and surrounds the individual particles of the salt, a process observed as dissolving.

• Chemistry is the science that investigates the molecular reason for the processes occurring in

our macroscopic world.

(Chemistry in Focus A Molecular View of Our World, 7e Nivaldo Tro) (Solution Manual all Chapters) 1 / 4

Chapter 1 Instructor’s Manual 2

• The scientific method involves first making observations of nature, from which patterns are

identified. From these patterns, broadly applicable generalizations called scientific laws are established. A theory or model is then constructed to provide an interpretation of the behavior of nature. The theory is then tested by further experiments and modified if necessary to correct for any errors in the theory exposed by the experiments.

• A law is a concise statement or equation that summarizes a great variety of observations,

while a theory explains the cause of the observations. A theory has been tested by experiment over a length of time and has more predictive power than a law.

• Science and art are similar mostly in their creativity and observations of the world. The

difference lies in what they do with their observations and how they are judged. Scientists take their observations and create a model of reality that is judged by experimentation for its validity. Artists observe the world and create a painting or sculpture that is judged by its creativity and workmanship.

• The Greek approach to scientific knowledge was through pure reason and intuition. Today

we approach science through experimentation.

• Galileo Inquisition

Democritus Atomos John Dalton The atomic theory Andreas Vesalius Human anatomy Empedocles Four basic elements Joseph Proust Constant composition Copernicus Sun-centered universe Ernest Rutherford The nuclear atom Thales All things are water Lavoisier Conservation of mass Boyle Criticized idea of four elements

• The two main pursuits of alchemists were the transmutation of ordinary materials into gold

and the discovery of the “elixir of life”. Alchemists contributed to modern chemistry their understanding of metals, specifically how metals combine to form alloys. Alchemists also contributed laboratory separation and purification methods, including the isolation of natural substances (pharmacological) used to treat various ailments.

• The scientific revolution began in 1543, signaled by the publication of two books. The first

book by Copernicus announced his sun-centered universe theory. The second book by Vesalius gave an accurate description of human anatomy. The reason these books mark the beginning of the scientific revolution is the methods Copernicus and Vesalius used to learn about the natural world – they both used observation instead of pure reason.

• An element is a substance that cannot be separated into simpler substances by chemical

means. A compound can be separated into simpler substances by chemical means. All compounds are made from two or more elements combined. 2 / 4

Tro 3 The gas nitrogen and the metal gold are examples of elements; they cannot be separated into anything simpler. Sugar and salt (sodium chloride) are examples of compounds; they can, with more or less difficulty, be subdivided into their elements. Heating sugar in a pan is a degree to that as one is left with a black carbonaceous deposit.

• A pure substance cannot be separated into simpler substances by physical means, such as

filtration, chromatography, crystallization or distillation. A mixture consists of two or more pure substances, and can be separated into those individual components by selection of the appropriate physical methods.The compounds sugar and sodium chloride in question 12 are examples of pure substances; they cannot be further purified by any method. A mixture could be made by dissolving sugar in water. We could recover the sugar simply by evaporating the water away. Another example of a mixture is air, which is composed of several gases – nitrogen, oxygen etc.

• A homogeneous mixture is completely uniform on the molecular scale. Air and a solution of

sugar in water are examples of homogeneous mixtures. Heterogeneous mixtures are not uniform on a molecular scale. If sand is thrown into the solution of sugar, the sandy regions are clearly distinguishable from the non-sandy ones.

• The three states of matter are distinguished by the strength of the interactions between the

molecules relative to their thermal energy. (For the sake of brevity we will refer here only to molecules, but it should be remembered that substances may be composed of atoms, molecules or ions) The relative strength of the molecular interactions decreases in the sequence solid – liquid – gas. In a solid, the interactions are strong enough to prevent free movement of the molecules and they are locked into rigid arrangements (lattices). In a liquid, the molecular energies are strong enough to loosen the grip of the intermolecular forces sufficiently to allow free movement of molecules; but the intermolecular forces still maintain enough control to keep the molecules together. In a gas, molecular energies have increased to the extent that now all inter-molecular shackles are broken and gas molecules behave completely independently. This molecular picture explains the obvious properties of solids, liquids and gases. Solids are rigid and dense; liquids flow freely yet have a density similar to the solid; gases have a very low density and are confined only by their container.

16.

a) Solid: incompressible, fixed volume, fixed shape

b) Liquid: incompressible, fixed volume, variable shape

c) Gas: compressible, variable shape, variable volume

• John Dalton, using the laws of Lavoisier and Proust and the data from his own experiments,

combined a number of ideas to formulate the “atomic theory.” Dalton’s atomic theory was

based on three parts:

 First, each element is composed of particles called atoms, which can neither be created nor destroyed. Second, all atoms of the same element have the same mass and other properties. These properties are unique characteristics of each element, and thus differ from other elements. 3 / 4

Chapter 1 Instructor’s Manual 4  Third, atoms of different types can combine to form compounds in simple whole number ratios. For example, the compound carbon dioxide is formed from one carbon atom and two oxygen atoms. The numbers, 1 and 2, are simple whole numbers.

• Rutherford examined atomic structure through his gold foil experiment. By shooting alpha

particles at a thin sheet of gold foil, Rutherford tested his idea that atoms were either soft, like blueberry muffins, or hard, like billiard balls. The results of his experiment were surprising. The majority of particles passed though the gold foil without deflection (as expected), but some deflected, scattering at angles both large and small. A small portion bounced back in the direction they had come from.

• The only way to explain the results of the gold foil experiment was to propose a new model

of the atom in which most of the atom must be empty space. This structure would allow most of the alpha particles to pass through the gold foil with little or no deflection. However, the atom must also contain a nucleus, a dense positively charged central core containing most of the mass. In the experiment, whenever an alpha particle came close to a nucleus, or hit it head on, it experienced a large repulsive force causing it to be scattered. Furthermore, since the atom is electrically neutral, it must contain an equal number of negative charges (electrons) and positive charges. The exact identity of the positive charge was later established to be the proton. Rutherford purposed the electrons were outside the nucleus.

• A black hole is a very dense form of matter where the structures of atoms have broken down,

to form “solid” matter. The large mass and small size associated with the black hole causes a strong gravitational field, which allows mass and light to enter but not leave the black hole.Thus, because black holes neither reflect nor emit light, they are referred to as “black”. If atoms did not have their characteristic structure of a tiny dense nucleus surrounded by a large volume of "empty" space, then all matter should have the enormous density of a black hole.

SOLUTIONS TO PROBLEMS:

21.

a) Water boiling and bubbles forming represent an observation. It would be made during an

experiment on heating water.

• “Two grams of hydrogen combine with sixteen grams of oxygen to form eighteen grams

of water” represents an observation. It would be the result of one experiment.

• “Chlorine and sodium readily combine in a chemical reaction that emits much heat and

light” is also an observation.

• “The properties of elements vary periodically with the mass of their atoms” is a law. The

relationship between element properties and size statement was derived from examination of many observations. It is often referred to as the periodic law.

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