5 Key excerpts on "Elemental Composition of Pure Substances"

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  eBook - ePub

  Foundations for Teaching Chemistry

  Chemical Knowledge for Teaching

  • Keith S. Taber(Author)
  • 2019(Publication Date)
  • Routledge

    (Publisher)

  8 Pure substances: Elements and compounds

  Chapter 7 looked in some detail at the notion of substance, so central to chemistry. This chapter develops this theme by first considering how a basic and familiar concept – purity – is understood in chemistry, and then by discussing the two categories of substance: elements and their compounds.

  Pure samples of substances: the meaning of ‘pure’

  In Chapter 7 it was suggested teaching about the submicroscopic nature of single substances is not straightforward, and a teaching approach starting with the simplest examples was recommended. However, even in the cases that seem quite simple, such as substances composed of discrete molecules, there may be complications that we would not wish to engage with when introducing the substance concept but which the teacher knows will need to be addressed later in the chemistry curriculum.

  This can be illustrated with a substance as ubiquitous as water. The model we have mooted for teaching is that water is a substance because a pure sample of water contains only one type of molecular species: H2 O. Water is composed of discrete molecules and only contains one type of molecule.

  The notion of a pure sample, as used earlier, is one which is very important in chemistry and might seem unproblematic in teaching because purity, things being pure, is familiar to students from their everyday lives. However, even when a concept used in chemistry seems to be common in everyday discourse, the special context of chemistry (working with single substances when most familiar materials are more complex) introduces a complication.

  Your morning fruit juice is not pure

  The chemist’s use of pure is at odds with some everyday uses of the term. A student might expect their orange juice to be pure – but orange juice is a complex mixture of different substances (Taber, 2018a): water, citric acid, ascorbic acid, various sugars and amino acids, polyphenols, and compounds of potassium and phosphorus. Different samples of orange juices will vary considerably in their precise composition. As a mixture, orange juice can be said in everyday terms to be ‘pure’ whilst having a complex and variable composition. What is meant here by pure is that something is unadulterated and only contains what it is meant to contain – and for some people that also entails that it is ‘natural’ (cf. Figure 7.1

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  eBook - ePub

  Philosophy of Chemistry

  • Dov M. Gabbay, Paul Thagard, John Woods(Authors)
  • 2011(Publication Date)
  • North Holland

    (Publisher)

  Elements

  Robin Findlay Hendry

  1. Introduction

  Chemical substances are the central kinds of chemistry, as important to understanding chemistry as the species concept—or concepts—is to understanding the biological sciences. Chemical substances are elements, compounds or mixtures: elements are just those substances that have no others as components. Elements in this sense are the building blocks of chemical composition. Here are three claims about elements: (i) in the 18th century, long before any direct investigation of atomic structure, chemists used element names with determinate extensions; (ii) membership of those extensions was conferred by having atoms with particular nuclear charges; (iii) the chemical facts that make all this so were unknown until the twentieth century, so if they are known now they must have been discovered.

  2. Historical Development

  In this section I trace the evolving conceptions of what it is to be a chemical element, concentrating on whether or not elements survive in their compounds.

  2.1. Aristotle

  In Aristotle's theory of chemical combination, elements are not assumed to be actually present when combined in a mixt. 1 Aristotle's view was developed in conscious opposition to atomism, according to which the ultimate components of things persist unchanged in more complex bodies, the differences between things being explained by their different arrangements. Aristotle argued that if elements combine to form a new substance, as opposed to merely being juxtaposed, the product must be homogeneous. However, atomism can accommodate only juxtaposition, and so cannot recognise this distinction between mere juxtaposition and genuine combination [Needham, 2009

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  Eco-facts and Eco-fiction

  Understanding the Environmental Debate

  • William H. Baarschers(Author)
  • 2013(Publication Date)
  • Routledge

    (Publisher)

  atoms that are all of the same kind. A chunk of the element iron contains only iron atoms and a piece of the element carbon contains only carbon atoms.

  This "handful" of elements does not explain the tremendous variety of chemicals we recognize around us, some nine million of them. There is a lot more to it. The next step is the notion that atoms can combine into larger particles, molecules. Molecules can be built up from atoms of the same kind, or they can contain many different kinds of atoms, atoms of different elements. A collection of molecules represents an amount that we can see and weigh, of substances we call chemical compounds. All the molecules of the compound water are built from one atom of oxygen and two atoms of hydrogen. All the molecules of the compound cholesterol consist of twenty-seven atoms of carbon, forty-six atoms of hydrogen and one atom of oxygen.

  We can use a simple form of shorthand to describe these molecules. We use the first, or the first and another letter of the Latin name of each element to describe it. For example, we use C for carbon, Co for cobalt, and Cd for cadmium. Then we add subscript numbers to indicate how many atoms there are of each kind. The combination leads to the idea of chemical formulae. So, in chemistry shorthand, water becomes H2 O and cholesterol is C27 H46 0. In the case of water this formula is enough to describe the molecule, for cholesterol it is not. For cholesterol we have to add a drawing of the geometry of the molecule, the arrangement of all the atoms in space. Such a drawing that represents the architecture of a molecule is called a structural formula. A properly drawn structural formula is a sufficient and complete description of that molecule.

  Berzelius (1779-1848), a Swedish chemist, divided chemicals into two large categories. One contained those chemicals that were directly derived from living things, either from plants or animals. He postulated that a "vital force" was essential for the formation of these chemicals from living organisms. Naturally, Berzelius called these chemicals "organic chemicals." All other chemicals, not derived from living organisms, were therefore

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  eBook - ePub

  Barron's Science 360: A Complete Study Guide to Chemistry with Online Practice

  • Mark Kernion, Joseph A. Mascetta(Authors)
  • 2021(Publication Date)
  • Barrons Educational Services

    (Publisher)

  PART III

  USING ATOMS AND MOLECULES

  Passage contains an image

  4 CHEMICAL FORMULAS

  WHAT YOU WILL LEARN

  Upon completing this chapter, you will be able to: •Articulate and apply the Laws of Definite Composition, Multiple Proportions, and Conservation of Mass •Write chemical formulas for common chemical substances from their names •Write the names of common chemical substances from their formulas •Write the names of common acids from their formulas and vice versa •Write a simple balanced reaction equation using chemical formulas including phase information

  Laws of Definite Composition and Multiple Proportions

  Near the end of the eighteenth century, the French chemist Joseph Proust argued that the chemical elements combined only in particular ways by mass to produce chemical compounds. As chemical knowledge grew, this concept became generally accepted and is known today as the Law of Definite Composition (sometimes referred to as the Law of Definite Proportions). This idea was one of the foundations of the atomic theory. In the early nineteenth century, chemist John Dalton theorized that all matter is comprised of atoms (small, discrete bits of matter). This is the atomic theory.

  The Law of Definite Composition, with its specificity in describing the particular masses of the elements that combine to form compounds, naturally hints at the particulate nature of matter. John Dalton himself developed the Law of Multiple Proportions, embellishing the idea that matter is specific, discrete, and particular. Dalton noted that when two elements combined to form two different compounds, they did so in such a way that the ratio of the masses of the second element, when combined with equal amounts of the first, would always turn out to be small whole numbers. Both of these laws then pointed to the fixedness or discreteness (or the atomic nature) of matter.

  Examples of both laws can be seen in the familiar compounds carbon monoxide and carbon dioxide. The masses of carbon and oxygen required to make 100.0 grams of both compounds are shown:

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  eBook - ePub

  General Chemistry for Engineers

  • Jeffrey Gaffney, Nancy Marley(Authors)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  − 1 ) a conversion factor between the number of moles and the number of fundamental particles in a substance.

  Chemical equation  the symbolic representation of a chemical reaction using symbols of the elements and chemical formulas.

  Complete ionic equation  a chemical equation for a reaction in aqueous solution that shows all the dissolved ionic species present during the reaction.

  Conservation of mass  the principle that atoms are neither created nor destroyed in a chemical reaction.

  Empirical mass  the sum of the atomic masses in the empirical formula multiplied by the number of atoms of each element.

  Empirical formula  the simplest positive integer ratio of atoms present in the compound.

  Flocculation  a process in which the fine particles of a precipitate aggregate to form larger particles.

  Formula equation  a chemical equation which shows all the ionic compounds and their states during an aqueous reaction, but does not specifically show their ionic forms in solution.

  Gravimetric analysis  an experimental technique that involves the quantitative determination of a substance based on the measurements of mass.

  Hydrocarbons  compounds made up of only carbon and hydrogen.

  Limiting reactant  the reactant whose amount determines, or limits, the amount of the products formed.

  Molar mass  the mass of one mole of a substance.

  Mole  

  (mol) the mass of any substance which contains the same number of fundamental particles as there are atoms in exactly 12.000 g of 12

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