[Chemistry Class Notes] on Pure Substances Pdf for Exam

Pure substances are formed by only one kind of particle that may have a fixed or constant structure. These substances are divided into elements and compounds further. Elements are substances that cannot be broken down into simpler ones. An element cannot be transformed into a new element even by using any physical or chemical means. They mostly incorporate metals, nonmetals, or metalloids.

On the other hand, compounds are also pure substances formed when two or more distinct elements are combined chemically with a fixed ratio of its atoms. However, compound substances can be broken down into further separate elements using chemical methods.

This page discusses in detail pure substances, their types and characteristics along with illustrating examples of the same.

Examples of Pure Substances

A few examples of pure substances include the following:

• Steel, 

• Iron, 

• Gold, 

• Diamond, 

• Water, 

• Copper, and many more.

Please note that air is also often considered a pure substance. However, some of the compounds like water, salt, crystals or baking soda are also considered pure substances.

Phase and Characteristics of Pure Substances

A phase can be defined as distinct molecular structural arrangements where it is spread homogeneously throughout the matter and can easily be differentiated and isolated from others due to its easily identifiable boundary surfaces. A pure substance thus exists in three different phases, namely, solid, liquid and gases. 

Within a principle phase, it is possible for the substance to acquire various phases, each with a different molecular structure. For example, Ice may exist in many different phases at high temperatures. Whereas graphite and diamond are the two distinct phases that carbon may exist in the solid phase. Also, molecular bonding is observed to be strongest in the solid phase and weakest in the liquid phase.

1. Solid: 

In solids, molecules are arranged in a three-dimensional pattern called lattice throughout the matter. Though they continuously oscillate about their same equilibrium position, they are not able to move relative to each other.

2. Liquid: 

The molecular spacing in the liquid is quite similar to that of the solid phase except that the spacing is larger in the liquid phase than that of the solid phase. In the liquid phase, the molecules are no longer at a fixed position like that of the solid phase. 

3. Gas: 

In a gaseous state, no molecular order exists. Gas molecules move randomly and continuously as they are far apart from each other and collide with each other and with the wall of the container.

Characteristics and properties of the pure substance are as follows:-

1. They contain only one type of atom or molecule and are generally homogeneous in nature.

2. They have a uniform composition throughout.

3. The melting and the boiling point for these components are fixed. 

4. The pure substance that is participating in the chemical reactions always results in the formation of predictable products.

Types of Pure Substances

The pure substances on the basis of the chemical composition have been divided into two categories namely:-

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1. Elements 

2. compounds

1. Elements:

Elements are pure substances that cannot be broken down into simpler substances by any physical or chemical means as they have only one kind of atom in the entire composition. Thus when gold is broken down it still remains gold and hence is considered a pure substance and an element. It is further classified as metals, nonmetals and metalloids.

Characteristics of elements are as follows:-

1. It is a pure substance that is homogeneous in nature and is made up of only one kind of element. Such as iron, sulfur, aluminium. Therefore they all are considered as elements. 

2. They cannot be broken down into simpler substances by any physical means or chemical methods such as heat or electricity or chemical reaction with other substances. Thus when a piece of iron is heated it melts into iron only. 

3. An atom is the smallest unit of an element that is responsible for all its properties. Hence the atom of iron shows all the properties of the metal iron. 

4. Elements possess a very sharp boiling and melting point. 

2. Compounds:

A pure substance, basically composed of two or more elements and chemically combined in a fixed proportion is called a compound. Therefore, water is known as a compound as it is made up of a combination of two elements oxygen and hydrogen.  

Properties of the compounds are as follows:-

1. It is made up of the same types of molecules and thus is homogeneous in nature. 

2. The components of the molecules cannot be separated by the physical methods but it is possible to separate them through various chemical and electrochemical methods and therefore the water can be separated into hydrogen and oxygen through the method of electrolysis.  

3. A compound comprises fixed compositions. 

4. A compound exhibits a sharp melting and boiling point. 

5. A compound displays a distinctive property of its own that is not similar to the properties of its components.

What is a Mixture?

The mixture is a material composed of two or more different substances which are combined physically. A mixture can be usually separated back to its original components. Impure substances are also referred to as mixtures.

Mixtures are unlike chemical compounds because:

1. The substances of a mixture can be separated using some physical methods like filtration, distillation.

2. Mixtures always have variable compositions, whereas compounds have a fixed and definite composition.

3. When mixture forms, there is only a little or no energy change.

Some examples of mixtures include:

• Crude oil – Mixture of some organic compounds, mainly hydrocarbons

• Air – Mixture of different gases like nitrogen, oxygen, carbon dioxide, neon, and so on

• Gunpowder – Mixture of potassium nitrate, sulfur, and carbon

• Ink – Mixture of coloured dyes which can be separated using chromatography

• Blood – Mixture of numerous abundant substances including red and white blood cells; plasma, containing glucose, water, albumin, and so on

• Mineral Oils – Mixture of branched, straight-chained, and cyclic heavy alkanes

• Soda – Mixture of sugar, water, and added flavours.

Characteristics of Mixtures

Fewer characteristics of Mixtures include:

1. The mixture is an impure substance

2. It has no formula

3. Mixtures can be fixed in any ratio

4. Mixtures can be of either homogeneous or heterogeneous

5. Their constituents can be separated easily using physical methods

Differences Between Pure Substances and Mixtures

A few of the points that differentiate mixtures and pure substances include: