Chemical Composition and Classification of Matter| Nature of Matter - Class IX
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Chemical Composition and Classification of Matter
Chemical composition of a matter
Chemical composition of a matter tells us about the kinds of atoms or elements that make up a substance and the ratio in which they are present.
Key Remarks
* As far as physical behaviour of a matter in different states and their interconversion under different conditions are concerned, molecules are considered as the basic units of matter. But, in case of chemical behaviour, atoms are considered as the basic units of matter.
* A molecule can be broken down into smaller particles called atoms.
* Atoms are the smallest particles that retain the chemical properties of an element and take participate in chemical reactions.
* A molecule can be broken down into smaller particles called atoms.
* Atoms are the smallest particles that retain the chemical properties of an element and take participate in chemical reactions.
Classification of Matter based on its chemical composition
Based on chemical composition, matter are classified into two types:
(i) Pure substances and
(ii) Mixtures
Pure substances and Mixtures
Pure substances
The substances which contain only one kind of molecules are called pure substances.
For example, water contains only $H_2O$ molecules, so water is a pure substance. Other examples are carbon dioxide $(CO_2)$, Hydrogen gas $(H_2)$, Sodium chloride $(NaCl)$, etc.
Mixtures
The substances which contain two or more kinds of molecules are called mixtures.
For example, mixture of water and table salt (aqueous solution of salt) contains $H_2O$ and NaCl molecules, so it is a mixture. Other examples are aqueous solution of sugar, mixture of water and sand, Brass, etc.
Types of pure substances
(i) Elements
The pure substances in which a molecule is made up of only one kind of atoms are called elements.
For example, Hydrogen gas $(H_2)$ in which the molecule $H_2$ is made up of only hydrogen atom, so hydrogen gas is an element.
Other examples are Gold $(Au)$, Arsenic $(As)$, Ozone $(O_3)$, etc.
(ii) Compounds
The pure substances in which a molecule is made up of two or more kinds of atoms are called compounds.
For example, water in which its molecule $H_2O$ contains two kinds of atoms - Hydrogen and Oxygen, so water is a compound.
Other examples are Carbon dioxide $(CO_2)$, Ammonia $(NH_3)$, Sulphuric acid $(H_2SO_4)$, etc.
Key Remarks
* The number of atoms present in a molecule of an element is called its atomicity.
For example, gold $(Au)$ has its atomicity is equal to 1 and ozone $(O_3)$ has atomicity is equal to 3.
For example, gold $(Au)$ has its atomicity is equal to 1 and ozone $(O_3)$ has atomicity is equal to 3.
Types of elements
Based on the nature, elements are classified into three types:
(i) Metals
The elements that are usually shiny, hard, and good at carrying heat and electricity are metals.
Examples: iron, copper, aluminium, etc.
(ii) Non-metals
The elements that are usually dull, soft or brittle, and poor conductors of heat and electricity are non-metals.
Examples: oxygen, sulphur, carbon, etc.
(iii) Metalloids
The elements that have properties of both metals and non-metals are metalloids.
Examples: silicon, boron, germanium, etc.
Key Remarks
* Based on their atomicity, elements are also classified as:
(i) monoatomic elements: The elements in which a molecule is made up of only one atom, i.e., atomicity is equal to 1 are called monoatomic elements.
Examples: Ag, Al, Au, He, Ne, B, C, etc.
(ii) diatomic elements: The elements in which a molecule is made up of only two atoms, i.e., atomicity is equal to 2 are called diatomic elements.
Examples: $H_2$, $N_2$, $O_2$, etc. (iii) polyatomic elements: The elements in which a molecule is made up of more than two atoms, i.e., atomicity is more than 2 are called polyatomic elements.
Examples: $O_3$, $P_4$, $S_8$, etc.
* polyatomic elements can be further classified as triatomic elements (atomicity=3), tetraatomic elements (atomicity=4), pentaatomic elements (atomicity=5), and so on.
(i) monoatomic elements: The elements in which a molecule is made up of only one atom, i.e., atomicity is equal to 1 are called monoatomic elements.
Examples: Ag, Al, Au, He, Ne, B, C, etc.
(ii) diatomic elements: The elements in which a molecule is made up of only two atoms, i.e., atomicity is equal to 2 are called diatomic elements.
Examples: $H_2$, $N_2$, $O_2$, etc. (iii) polyatomic elements: The elements in which a molecule is made up of more than two atoms, i.e., atomicity is more than 2 are called polyatomic elements.
Examples: $O_3$, $P_4$, $S_8$, etc.
* polyatomic elements can be further classified as triatomic elements (atomicity=3), tetraatomic elements (atomicity=4), pentaatomic elements (atomicity=5), and so on.
Types of Mixtures
Based on the uniformity of composition, mixtures have two types:
(i) Homogeneous mixtures
The mixtures which have uniform distribution of their different types of molecules are called homogeneous mixtures.
For examples, aqueous solution of glucose, aqueous solution of salt and aqueous solution of sugar are homogeneous mixtures.
(ii) Heterogeneous mixtures
The mixtures which have non uniform distribution of their different types of molecules are called heterogeneous mixtures.
For examples, muddy water, mixture of oil and water and mixture of iron and sulphur are all heterogeneous mixtures.
Characteristics of elements
- Elements are the pure substances whose molecules are made up of only one kind of atoms.
- Based on their nature, elements may be metals, non-metals and metalloids.
- Based on their atomicity, elements may be monoatomic, diatomic or polyatomic.
- Elements cannot be broken into simpler substances by ordinary chemical methods.
- The atoms of one element combine with atoms of other elements to form compounds.
Characteristics of compounds
- Compounds are pure substances whose molecules are made up of two or more kinds of atoms.
- The constituents elements present in a fixed proportion by weight. For example, in $CO_2$, carbon and oxygen are in the ratio of 12:32 (3:8).
- The properties of the compounds are completely different from those of their constituents elements, i.e., constituents elements do not retain their properties. For example, the properties of water is completely different from those of hydrogen and oxygen. Hydrogen burns with a pop sound but water does not and oxygen is a supporter of a fire but water is not.
- The constituents elements can be separated only by chemical methods. For example, water can be decomposed into simpler substances like hydrogen and oxygen by electrolysis method.
- The formation of a compound by combining of atoms of different elements associated with significant energy changes. For example, $C+O_2 \to CO_2$ -94 kcal, i.e., in formation of $CO_2$ (carbon dioxide) from C (carbon) and $O_2$ (Oxygen) releases 94 kcal of heat.
Characteristics of Mixtures
- The constituents may be present in any proportion.
- The constituents retain their individual properties.
- Mixtures can be either homogeneous or heterogeneous.
- Separation of constituents is carried out by physical methods.
- Formation of a mixture does not involve significant energy change.
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