What are Pure Substances in Chemistry | Classifications

Definition of Pure Substances

Pure substances are made up of only one kind of particle and have a fixed or constant structure. A pure substance, we are speaking of something that contains only one type of matter. Pure substances can either be one single element or one single compound. But every sample of this substance that you examine must contain the same thing with a fixed, definite set of properties.

Other Definition

pure substances

A pure substance or chemical substance is a material that has a constant composition (is homogeneous) and has consistent properties throughout the sample. A pure substance participates in a chemical reaction to form predictable products. In chemistry, a pure substance consists of only one type of atom, molecule, or compound. In other disciplines, the definition extends to homogeneous mixtures.

A chemical substance is a form of matter having a constant chemical composition and characteristic properties. Some references add that the physical separation method cannot separate chemical substances into their constituent elements, i.e., without breaking chemical bonds. Chemical substances can be simple substances, chemical compounds, or alloys. Chemical elements may or may not be included in the definition, depending on the expert viewpoint.

Summary

All elements are mostly pure substances. A few of them include gold, copper, oxygen, chlorine, diamond, etc. Compounds such as water, salt or crystals, and baking soda are also grouped as pure substances.

A pure substance is a sample of matter with both definite and constant composition and distinct chemical properties.

A pure substance is often referred to as a “chemical substance.” Chemical substances are often called ‘pure’ to set them apart from mixtures.

A typical example of a chemical substance is pure water; it has the same properties and ratio of hydrogen to oxygen, whether isolated from a river or made in a laboratory. Other chemical substances commonly encountered in pure form are diamond (carbon), gold, table salt (sodium chloride), and refined sugar (sucrose).

However, in practice, no substance is entirely pure. And chemical purity is specified according to the intended use of the chemical. Chemical substances exist as solids, liquids, gases, or plasma. And may change between these phases of matter with changes in temperature, pressure, and time. Chemical substances may be combined or converted to others using chemical reactions. Forms of energy, such as light and heat, are not the matter and are thus not “substances” in this regard.

Examples of Pure Substances

Followings are the examples of Pure Substances

  • In chemistry, a substance is pure if it has a homogeneous chemical composition. This only applies to a substance made up of one type of atom, molecule, or compound at the nanoscale.
  • In the more general sense, a pure substance is any homogeneous mixture. It is a matter that appears uniform in appearance and composition, no matter how small the sample size.
  • Examples of pure substances include iron, steel, and water. Air is a homogeneous mixture that is often considered to be a pure substance. Examples of pure substances include tin, sulfur, diamond, water, pure sugar (sucrose), table salt (sodium chloride), and baking soda (sodium bicarbonate). Crystals, in general, are pure substances.
  • Tin, sulfur, and diamond are examples of pure substances that are chemical elements. All elements are pure substances. Sugar, salt, and baking soda are pure substances that compound. Examples of pure substances that are crystals include salt, diamond, protein crystals, and copper sulfate crystals.
  • Depending on who you talk to, homogeneous mixtures may be considered examples of pure substances. Examples of homogeneous mixtures include vegetable oil, honey, and air. While these substances contain multiple types of molecules, their composition is consistent throughout a sample. If you add soot to air, it ceases to be a pure substance. Contaminants in water make it impure.
  • Heterogeneous mixtures are not pure substances. Examples of not pure substances materials include gravel, your computer, a mixture of salt and sugar, and a tree.

History of Pure Substances

history of pure substances

The concept of a “chemical substance” became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate.

He deduced that “All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound.”

This is now known as the law of constant composition, later with the advancement of methods for chemical synthesis, particularly in the realm of organic chemistry.

The discovery of many more chemical elements and new techniques in analytical chemistry used to isolate and purify elements and compounds from chemicals led to the establishment of modern chemistry. And the concept was defined as is found in most chemistry textbooks.

However, there are some controversies regarding this definition, mainly because many chemical substances reported in chemistry literature need to be indexed.

Isomerism caused much consternation to early researchers since isomers have the same composition but differ in the configuration (arrangement) of the atoms.

For example, there was much speculation about the chemical identity of benzene until Friedrich August Kekulé described the correct structure.

Likewise, the idea of stereoisomerism – that atoms have rigid three-dimensional structures and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances.

For example, tartaric acid has three different isomers. A pair of diastereomers with one diastereomer forming two enantiomers.

Characteristics Of Pure Substances

  • Pure substances are mostly homogeneous in nature containing only one type of atom or molecule.
  • These substances mainly have a constant or uniform composition throughout.
  • The substances have fixed boiling and melting points.
  • A pure substance usually participates in a chemical reaction to form predictable products.

Properties of Pure Substances

Common properties of pure substances include temperature, pressure, volume, mass, density, specific volume, energy, enthalpy, and entropy. These properties can be categorized as either extensive or intensive.

Temprature

temperature of pure substances

Temperature is measured in degrees Celsius [0C] or Kelvin [K]. To convert a temperature in 0C to K, add 273.15. This means 50 0 C is the same as 323.15K. It also means that a temperature difference is expressed in degrees. Celsius is numerically the same as the difference expressed in Kelvin. The lowest possible temperature is 0K.

Temperature is an intensive variable.

Pressure

pressure in chemistry

Pressure ( P ) is the force per unit area:

P = F/A

With F in [ kN ] and A in [ m2 ], P will be in [ kN/m2]=[ kPa ].

Other common pressure units are:

atmosphere (1atm=101.325kPa) and bar (1bar=100kPa)

Pressure is an intensive variable.

Absolute or Total Pressure

absolute or total pressure of pure substances

The total pressure is the total force exerted on a surface divided by the area of the surface. In a gas, the force exerted on a surface is due to the collision of the gas molecules against the surface. There is a limit to how low total pressure can be. If no force is exerted on a surface, the total pressure is zero. The pressure of a complete vacuum is zero because no molecules are colliding with the surface of the container. Absolute pressure ( Pabs ) is the pressure measured above this zero point. The value of total pressure can never be negative.

Atmospheric Pressure

atmospheric pressure pure substances

Atmospheric pressure ( Patm) is the total pressure caused by the weight of the atmospheric air. This pressure varies according to location and weather patterns.

The average value at sea-level is 101.325kPa. The average atmospheric pressure for Potchefstroom is 87.0kPaand 99% of the time the pressure will be between 86kPa and 88kPa.

Volume and Mass

It is often convenient to combine the two extensive properties, volume and mass, to form an intensive property: specific volume ( v ). Specific volume is the total volume divided by the total mass v=V/m [ m3/kg ]. Specific volume is an intensive property. Density ( ρ ) is the inverse of specific volume.

Example

A cup holds 25 mℓ of boiling hot water at 950C. The mass of the water is 240.4g. Calculate the specific volume in [m3/kg] and the density in [kg/m]. Solution Specific volume is the total volume divided by the total mass: v=V/m. First determine the values of V and m.

V250mℓ×1ℓ/1000mℓ×1m3/1000
=250×10−6m3
m=240.4g×1kg/1000g
=240.4×10−3kg
∴v=250×10-6
/240.4×10-3
=0.00104m3 kg
And ρ=1/v=961.5kgm3 .

Classifications of Pure Substances

Pure substances are further classified as elements and compounds

Element

elements in chemistry

An element is a substance that consists of only one type or kind of atom. An element is a pure substance as it cannot be broken down or transformed into a new substance even by using some physical or chemical means.

Elements are mostly metals, non-metals, or metalloids. Element is the simplest form of a pure substance that can neither be broken into nor built from simpler substances by ordinary physical and chemical methods.

Recent studies have revealed that the simplest form of matter is the atom. In the light of this, an element may also be defined as a pure substance made of one kind of atom. Carbon, Sulphur, hydrogen, oxygen, nitrogen, iron, copper, gold, lead, etc., are familiar elements.

An element is a pure substance consisting only of atoms with the same numbers of protons in their atomic nuclei. As of 2019, there are 118 known elements, about 80 of which are stable – that is, they do not change by radioactive decay into other elements.

Some elements can occur as more than a single chemical substance (allotropes). For instance, oxygen exists as both diatomic oxygen (O2) and ozone (O3). The majority of elements are classified as metals. These are elements with a characteristic luster, such as iron, copper, and gold.

Metals typically conduct electricity and heat well, and they are malleable and ductile. Around a dozen elements, such as carbon, nitrogen, and oxygen, are classified as non-metals. Non-metals lack the metallic properties described above. They also have a high electronegativity and a tendency to form negative ions. Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals and are known as metalloids.

Compound

compound in chemistry

On the other hand, compounds are also pure substances when two or more elements are combined chemically in a fixed ratio.

However, these substances can be broken down into separate elements by chemical methods. When two or more elements chemically combine in a fixed ratio by mass, the obtained product is known as a compound.

Compounds can be defined as substances consisting of 2 or more different types of elements in a fixed ratio of their atoms.

When the elements combine, some individual property of the elements is lost, and the newly formed compound has new properties. A chemical compound is a chemical substance that is composed of a particular set of atoms or ions.

Two or more elements combined into one substance through a chemical reaction form a chemical compound.

All compounds are substances, but not all substances are compounds. A chemical compound can be either atom bonded together in molecules or crystals in which atoms, molecules, or ions form a crystalline lattice.

Compound based primarily on carbon and hydrogen atoms are called organic compounds, and all others are called inorganic compounds. Compound containing bonds between carbon and a metal are called organometallic compounds.

Compounds in which components share electrons are known as covalent compounds. Compounds consisting of oppositely charged ions are known as ionic compounds or salts.

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