Chemistry
Pure Substances
Pure substances are materials that are made up of only one type of particle, either atoms or molecules. They have a fixed composition and distinct chemical properties. Pure substances can exist in different phases, such as solid, liquid, or gas, and can be further classified as elements or compounds based on their chemical composition.
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9 Key excerpts on "Pure Substances"
eBook - PDF- Angela Fitzgerald(Author)
- 2013(Publication Date)
- Cambridge University Press(Publisher)
A pure substance could also be made of different atoms joined (bonded) together (e.g., methane gas (CH 4 ) particles, or sodium chloride (NaCl) which exists as a lattice of sodium- and chloride-charged atoms rather than parti- cles). The way atoms exist, alone or combined with other atoms that can be the same or different with a few or many, is dependent on their structure. In summary, understanding the concept of a substance involves understanding that: Substances are pure chemical substances with a known chemical composition; • Substances can be in the solid, liquid or gaseous state depending on the • temperature conditions; Materials are commonly made up of mixtures of substances; • Mixtures are not pure, do not have a definite chemical composition and can be • made up of one or many different substances; There are 118 different types of atoms, each with a unique chemical symbol; • A substance can be made of one type of atom or different types of atoms; and • The ways atoms bond together in a substance depends on their atomic structure. • Particle ideas to explain properties of substances In inquiring about the properties of substances and the way in which sub- tances interact with other substances in chemical reactions, scientists have developed explanatory models (Jadrich & Bruxvoort, 2011). These models have changed over time. Early Greek philosophers successfully employed the four elements of fire, earth, water and air to explain the properties periodic table: A representation that shows all the known elements explanatory models: Representations of theories that are used to explain phenomena Chapter 11 Material world: Learning and teaching chemistry 195 of substances before the idea that the universe was made of particles called atoms gained acceptance in the early 1800s. Since these early times scientists have con- tinued to construct and use models where they imagine that, at the submicroscopic level, matter is made up of particles.
eBook - PDF- Claus Borgnakke, Richard E. Sonntag(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
......................................................................................................... .............. ....................................................... 30 CHAPTER TWO PROPERTIES OF A PURE SUBSTANCE being pushed by the compressor blades in the jet engine (giving thrust) to move the aircraft forward. These are just a few examples of complete thermodynamic systems in which a sub- stance goes through several processes involving changes in its thermodynamic state and therefore its properties. As your studies progress, many other examples will be used to illustrate the general subjects. 2.1 THE PURE SUBSTANCE A pure substance is one that has a homogeneous and invariable chemical composition. It may exist in more than one phase, but the chemical composition is the same in all phases. Thus, liquid water, a mixture of liquid water and water vapor (steam), and a mixture of ice and liquid water are all Pure Substances; every phase has the same chemical composition. In contrast, a mixture of liquid air and gaseous air is not a pure substance because the composition of the liquid phase is different from that of the vapor phase. Sometimes a mixture of gases, such as air, is considered a pure substance as long as there is no change of phase. Strictly speaking, this is not true. As we will see later, we should say that a mixture of gases such as air exhibits some of the characteristics of a pure substance as long as there is no change of phase. In this book the emphasis will be on simple compressible substances. This term designates substances whose surface effects, magnetic effects, and electrical effects are insignificant when dealing with the substances. However, changes in volume, such as those associated with the expansion of a gas in a cylinder, are very important. Reference will be made to other substances for which surface, magnetic, and electrical effects are impor- tant.
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2015(Publication Date)
- Openstax(Publisher)
The basic building block of matter is the atom, the smallest unit of an element that can enter into combinations with atoms of the same or other elements. In many substances, atoms are combined into molecules. On earth, matter commonly exists in three states: solids, of fixed shape and volume; liquids, of variable shape but fixed volume; and gases, of variable shape and volume. Under high-temperature conditions, matter also can exist as a plasma. Most matter is a mixture: It is composed of two or more types of matter that can be present in varying amounts and can be separated by physical means. Heterogeneous mixtures vary in composition from point to point; homogeneous mixtures have the same composition from point to point. Pure Substances consist of only one type of matter. A pure substance can be an element, which consists of only one type of atom and cannot be broken down by a chemical change, or a compound, which consists of two or more types of atoms. 1.3 Physical and Chemical Properties All substances have distinct physical and chemical properties, and may undergo physical or chemical changes. Physical properties, such as hardness and boiling point, and physical changes, such as melting or freezing, do not involve a change in the composition of matter. Chemical properties, such flammability and acidity, and chemical changes, such as rusting, involve production of matter that differs from that present beforehand. Measurable properties fall into one of two categories. Extensive properties depend on the amount of matter present, for example, the mass of gold. Intensive properties do not depend on the amount of matter present, for example, the density of gold. Heat is an example of an extensive property, and temperature is an example of an intensive property. 1.4 Measurements Measurements provide quantitative information that is critical in studying and practicing chemistry. Each measurement has an amount, a unit for comparison, and an uncertainty.
eBook - ePubEngineering Thermodynamics
Fundamental and Advanced Topics
- Kavati Venkateswarlu(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
4 Properties of Pure SubstancesLearning Outcomes After learning this chapter, students should be able to- Understand the concept of a pure substance.
- Analyze the phase change phenomena useful in so many applications such as steam power plants.
- Illustrate the p-v, T-v, and p-T property diagrams and p-v-T surfaces of Pure Substances.
- Demonstrate the procedures for determining the thermodynamic properties of Pure Substances from tables of property data.
- Describe the hypothetical substance “ideal gas” and the ideal-gas equation of state.
- Apply the ideal-gas equation of state for typical problems.
- Introduce the compressibility factor, which accounts for the deviation of real gases from ideal-gas behavior.
4.1 Pure Substances and their Phases
A substance that has a fixed chemical composition throughout its volume is called a pure substance. A substance may be a single element or compound or a mixture of various chemical elements or compounds. A pure substance can exist in more than one phase, but its chemical composition must be the same in each phase. For example, water is a pure substance when it is in solid (ice), liquid (water), and vapor (gaseous) states separately and combinedly as water possesses the fixed ratio of hydrogen and oxygen. Similarly, air is also a pure substance as it possesses a uniform mixture of its constituents throughout its volume. Liquid air and air (gaseous) are not considered as Pure Substances because their compositions vary in the different phases.A substance exists in three different phases: solid, liquid, and gas. In a solid phase, the molecules are closely arranged and hence the attractive forces of molecules on each other are so high that they keep the molecules at fixed positions. There is no relative motion of molecules in solids; however, there is a continuous oscillation of molecules about their equilibrium positions. Temperature plays an important role during these oscillations. Liquids, like solids, have almost similar molecular structures as molecules are closely spaced; however, they differ in that molecules can rotate and translate freely apart from not having fixed positions relative to each other. Gases, on the other hand, have molecules spaced far apart when compared to both solids and liquids and molecules move randomly and collide continually with each other and with the walls in which they are contained. The molecules in the gas phase have higher energy levels when compared to the same molecules in solid or liquid phases.
eBook - ePub- Dov M. Gabbay, Paul Thagard, John Woods(Authors)
- 2011(Publication Date)
- North Holland(Publisher)
Ostwald [1907 , 166-170], if the properties of two coexisting phases remain invariant during a phase change, the system is called hylotropic; if not it is a solution (phase of variable composition). If it is hylotropic over a range of temperatures as the pressure varies, it is a pure chemical substance. If it is hylotropic only at a particular temperature and pressure, it is a special kind of solution, for example an azeotropic mixture. If it is hylotropic over all pressures and temperatures, except the most extreme ones, it is a simple substance; if not it is a compound.20 For critical but sympathetic discussion of their proposals see [Psarros, 1999 , 133n151; Schummer, 1996, 185n11], and the article on ‘Wald’ in Part 2 of this Volume.21 Of course such criteria break down if we move to the nanometer scale. Nevertheless it is assumed that a hybrid crystal in which two polymorphs coexist allows for separation in principle.22 Although the term “solution” originated with the observation that many liquids may form homogeneous mixtures, it is now also used for the solution of solids in one another. An ideal solution (of which there are few if any) has properties linear in composition. Save under the most exceptional circumstances, there can be only one gas/vapour phase.Against this operational background a pure substance can be defined as a substance of which properties such as density do not change during a phase conversion (as in boiling a liquid or melting a solid), which takes place at one constant temperature. Thus the melting point of a pure substance is a constant of nature, 23 even though it refers to one substance only and there are many such unique constants characterising (pure) chemical substances.23 Assuming the existence of absolutely Pure Substances (which may not exist).Operationally a substance is pure if it is perfectly homogeneous after being subjected to successive modes of fractionating which are as different as possible and when attempts at further purification produce no further change in properties. As already noted in the first section, later refinements may show that what was once thought to be a pure substance is, after all, not pure. As noted by many writers in the field, purity is a matter of negotiation or consensus. Different applications require different standards of purification. Different separation techniques (crystallisation, electrophoresis, and so on) set different standards of purity. 24
eBook - PDF- John Kenkel(Author)
- 2010(Publication Date)
- CRC Press(Publisher)
1.3 Classification of Matter 1.3.1 Introduction More than 13 million material substances exist . The study of the composi-tion, structure, properties, and changes of this large number of substances is accomplished by creating classifications and subclassifications . The vast majority of substances as they are found in nature are not pure . In fact, they are often extremely complex mixtures . Examples are ocean water, oil, dirt, and ores . Even air and the fresh water found in lakes and wells can actually be complex mixtures . For example, ocean water con-tains many, many substances besides water . A large number of industrial, household, and laboratory activities are dedicated to purifying or separat-ing mixtures, substance by substance . Examples are industrial processes, such as the refining of oil into component chemicals, household processes such as the removal of minerals from water with the use of water soften-ers and distillers, and laboratory processes, such as the use of a sophisti-cated chromatography instrument to analyze for the quantity of a pesticide residue in river water . In many of these purification schemes, extremely Pure Substances often result and are available as consumer products or for further industrial or laboratory processing . Thus, while complex mixtures are commonplace in our lives, the same can be said about Pure Substances . Classification according to purity (i .e ., mixtures vs Pure Substances) is our starting point . All matter can be classified as either a mixture or a pure substance . All Pure Substances can be classified as either a compound or an element . The flow diagram in Figure 1 .1 is a pictorial representation of this . We will now define these terms and give additional details and examples . 5 Properties and Structure of Matter 1.3.2 Elements and Compounds There are 109 elements in existence, and more than 13 million compounds .
eBook - PDFChemistry
An Industry-Based Introduction with CD-ROM
- John Kenkel, Paul B. Kelter, David S. Hage(Authors)
- 2000(Publication Date)
- CRC Press(Publisher)
Thus, while complex mixtures are commonplace in our lives, the same can be said about Pure Substances, and there is no dependence on temperature or other parameters. Classification according to purity (mixtures vs. Pure Substances) would thus seem to be a good starting point with our scheme. All of matter can be classified as either a mixture or a pure substance. All Pure Substances can be classified as either a compound or an element. All mixtures can be classified as homogeneous or heter-ogeneous. The flow diagram in Fig. 1.1 shows a pictorial representation of this. We will now define these terms and give additional details and examples. 1.3.2 Elements and Compounds Elements and compounds consist of just one material substance. There are 109 elements in existence and over 13 million compounds. Many of the elements are familiar to us. Examples are various metals such as iron, copper, aluminum, and lead. Also familiar are elements that are not metals. Examples of nonmetals are helium, hydrogen, oxygen, and iodine. For convenience, all elements have been assigned symbols . A symbol consists of one or two letters, the first a capital letter and the second, if used, a lower case letter. These letters are derived from either the English name of the element or the Latin name. For example, “Al” is the symbol for aluminum, and “I” is the symbol for iodine. In these two examples, the symbol is either the first letter or the first two letters of the English name. The symbol “Fe” represents iron, the Latin name for which is “ferrum.” Other examples are “Cu” (from “cuprum,” the Latin word for copper), “Na” (from the “natrium,” the Latin word for sodium), and “Pb” (for “plumbum,” the Latin word for lead). All 109 elements are listed in a table that chemists have developed, known as the periodic table . The periodic table contains the names and symbols of all the elements as well as some numeric information.
eBook - PDF- Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2019(Publication Date)
- Openstax(Publisher)
1.2 • Phases and Classification of Matter 15 FIGURE 1.8 (a) The mass of beer precursor materials is the same as the mass of beer produced: Sugar has become alcohol and carbon dioxide. (b) The mass of the lead, lead oxide, and sulfuric acid consumed by the production of electricity is exactly equal to the mass of lead sulfate and water that is formed. Although this conservation law holds true for all conversions of matter, convincing examples are few and far between because, outside of the controlled conditions in a laboratory, we seldom collect all of the material that is produced during a particular conversion. For example, when you eat, digest, and assimilate food, all of the matter in the original food is preserved. But because some of the matter is incorporated into your body, and much is excreted as various types of waste, it is challenging to verify by measurement. Classifying Matter Matter can be classified into several categories. Two broad categories are mixtures and Pure Substances. A pure substance has a constant composition. All specimens of a pure substance have exactly the same makeup and properties. Any sample of sucrose (table sugar) consists of 42.1% carbon, 6.5% hydrogen, and 51.4% oxygen by mass. Any sample of sucrose also has the same physical properties, such as melting point, color, and sweetness, regardless of the source from which it is isolated. Pure Substances may be divided into two classes: elements and compounds. Pure Substances that cannot be broken down into simpler substances by chemical changes are called elements. Iron, silver, gold, aluminum, sulfur, oxygen, and copper are familiar examples of the more than 100 known elements, of which about 90 occur naturally on the earth, and two dozen or so have been created in laboratories. Pure Substances that are comprised of two or more elements are called compounds. Compounds may be broken down by chemical changes to yield either elements or other compounds, or both.
eBook - PDF- Bernadette Bensaude-vincent, Jonathan Simon(Authors)
- 2008(Publication Date)
- ICP(Publisher)
The physicist will consider it as natural, and capable of motion or rest according to the 136 Chemistry — The Impure Science constitutive principles of the natural body, in terms of their nature, that is matter and form, because this is how it is constituted as an object of physics. The doctor will consider the same body in terms of its capacity to receive or cause health, examining it in terms of the four primary qualities, cold, hot, dry, and wet that constitute the body’s temperament, which is responsible for health or sickness. The chemist, likewise, will consider the object in his own way, in terms of its ability to be resolved or to coagu-late and the diverse virtues in its interior that can be made manifest by the art, rendering it more useful. Thus, in so far as mercury, sulphur, and salt are the principles that render mixt bodies soluble or prone to coagulate — being the roots of the body’s internal virtues — or that they are veritable chemical substances, that is to say the principles that support and give substance to all the virtues and accidents of the compound, the chemist should proceed by means of these three principles in all his examinations, theories and operations. 10 Following the demise of his four-element theory, the lesson that Béguin draws from Aristotle is that to each science or art its own specific perspec-tive on matter. The principles of chemistry “give substance” to matter, in both senses of the term: they nourish and give physical substance to the body, while at the same time reifying the associated properties. However many principles one might accept, each one has its own specificity and function. These principles are not, however, intended to explain the prop-erties of a given compound, but rather to throw light on what happens when, for example, an acid dissolves a metal.
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