On the basis of chemical nature, the matter is known to exist either as a pure substance or as a mixture of two or more pure substance. A substance is a kind of matter that cannot be separated into other kinds of matter by any physical process.
A pure substance defined as one which contains only one kind of atoms or molecules. Such as water. On the bases of constitution it is of two types:
- Substance which is made of only one kind of atoms is called an element while which is made up of only one kind of molecules is called a compound.
Characteristics of pure substance
- A pure substance is uniform in its composition.
- A pure substance cannot be separated into different materials by any physical process.
- A pure substance contains one type of atoms or molecules and has a definite composition.
- A pure substance has fixed melting and boiling points.
Impure substances are commonly called mixtures. A mixture is a material containing two or more elements or compounds that are in close contact and are mixed in any proportion.
- A mixture may be homogeneous or heterogeneous.
- The composition of a mixture is variable.
- A mixture does not have a definite melting point or a boiling point.
- Energy is neither absorbed nor evolved during the formation of a mixture.
- The properties of a mixture are the properties of its constituents.
Mixtures can be homogeneous or heterogeneous. A homogeneous mixture has a uniform composition throughout its mass. For example, sugar or salt dissolved in water, alcohol in water, etc. While in a heterogeneous mixture the composition is not uniform throughout its mass. Different portions of a heterogeneous mixture show different properties. There are visible sharp boundaries. Example: Oil and water, salt and sand, etc.
Types of mixtures
A heterogeneous mixture is the mixture that consists of physically distinct parts, each with different properties. Examples of heterogeneous mixture are mixture of potassium dichromate an iron filling and salt and sugar that have been stirred together.
A heterogeneous mixture of salt and sugar is said to be composed of two different phases: one of the phases is salt, the other is sugar. In heterogeneous mixture, the composition is not uniformed throughout and sometimes the different component can be observed. For example, the mixture of salt in sugar, grains and pulses along with some dirt (often stone) pieces, are heterogeneous mixture.
A homogeneous mixture is a mixture that is uniform in its properties throughout given samples. When sodium chloride is dissolved in water. Air is a gaseous solution, principally of two elementary substances, nitrogen and oxygen, which are physically mixed but not chemically combined.
In a homogeneous mixture, the component completely mixed with each other and its composition is uniformed throughout. Sugar solution and air are thus, the examples of homogeneous mixtures.
Characteristics of mixture
- Constituents of a mixture can be present in any ratio.
- Mixture may be heterogeneous or homogeneous.
- Constituents of a mixture can be separated by easy mechanical means.
Examples of Mixtures
- Coke is a mixture of water, sugar, flavourings, and carbon dioxide gas.
- Milk is a mixture of water with various proteins, fats, and other substances.
- Stainless steel is a mixture (alloy) of iron, carbon, chromium, and nickel. Carbon gives hardness to the mixture. Chromium and nickel give a silvery look to the mixture.
A compound is a combination of two or more elements in a definite proportion. The ratio of elements in a particular compound is always same. It can be decomposed into its elements by using some physical or chemical methods. Such as H2O is a compound where one Oxygen atom combines with two Hydrogen atoms. Compounds are mainly of two types.
- Organic compounds: Organic compounds are the compounds which are obtained from plant and animal sources. They are the carbon compounds (molecules containing carbon). E.g. Methane
- Inorganic compounds: Inorganic compounds are compounds obtained from non living sources. Eg. Minerals
Characteristics of compound
- Compounds always contain a definite proportion of the same elements by mass. Water as a compound, always contains hydrogen and oxygen in the ratio 1:8 by mass.
- The properties of compounds are totally different from the elements from which they are formed. For example, while water is normally used for extinguishing fire, its elements are not. Hydrogen is combustible and oxygen is a supporter of combustion.
- Compounds are homogeneous.
- During the formation of a compound, energy in the form of heat, light or electricity is either evolved or absorbed. Coal when burnt, gives heat and light energies.
Properties of Compounds
- The ratio of elements in a compound is fixed.
- The individual properties of the components are not retained in a compound. Both sodium and chlorine are poisonous but their compound, table salt (NaCl) is absolutely essential to life.
- It takes large inputs of energy to separate the components of a compound.
Examples of Compounds
- Water (H2O)
- Sugar (C12H22O11)
- Aluminium Chloride (NH4Cl)
- Aluminium Oxide (Al2O3)
- Nitric Acid (HNO3)
Characteristics of Compounds and Mixtures
|Made up of atoms of elements in a fixed proportion
|Made up of elements, or compounds, or both in any proportion
|Particles are of the same kind
|Particles are of different kinds
|May or may not be homogeneous
|Components can be separated only by chemical means
|Components can be separated by physical means
|Energy is always evolved or absorbed
|Generally no energy is evolved or absorbed
|Components cannot be seen separately
|Components may or may not be seen separately
|Always involves a chemical change
|Involves only physical change
|Entirely different from those of the constituents
|No property of their own. Show the average properties of all the constituents
Suspension is a mixture in which the particles are suspended in a dispersion medium. They contain relatively large particles.
- Example: Clay in water.
Water is a compound and not a mixture
- The components hydrogen and oxygen cannot be separated by physical methods such as filtration, evaporation.
- Hydrogen and oxygen are present in a fixed proportion of 1: 8.
- Energy changes accompany the formation of a compound i.e., heat and light are given out.
- Properties of water are entirely different from the constituents, hydrogen and oxygen.s
- The boiling point of water is 100oC at 76 cm Hg i.e., one atmospheric pressure.
Differences between mixtures and compounds
|A mixture can be separated into its constituents by physical processes (filtration, evaporation, sublimation, distillation)
|A compound cannot be separated into its constituents by physical processes. It can be separated by chemical means
|A mixture shows the properties of its constituents
|A compound has a new set of properties different from its constituents
|Composition of a mixture varies and the constituents are present in any proportion by weight. It does not have a definite formula
|The composition of a compound is fixed and the constituents are present in fixed proportions by weight. It has a definite formula
|The constituents do not react chemically, thus no energy changes take place
|Chemical reactions take place and energy changes in the form of heat and light are involved
|A mixture does not have a fixed melting point and boiling point Examples: air, sand and salt
|A compound has a fixed melting point and boiling point Examples: H2O (water), FeS (iron sulphide)