Is Matter Around Us Pure? Class 9 Notes Science Chapter 2

Pure Substances

• Pure substances, like elements or compounds such as diamond or carbon dioxide, consist of a single type of entity and cannot be broken down further through chemical or physical methods. 
• Elements, in particular, are pure materials made up of only one type of atom, ensuring consistency in composition and properties. 
• These substances exhibit homogeneity, with constant boiling and melting points. 
• In chemical reactions, pure substances play a role, producing predictable outcomes due to their stable nature.

Mixtures

Properties

• It may be homogeneous or heterogeneous.
• The properties of constituent substances are retained.
• No new compound is formed after mixing.
• Constituents of a mixture can be separated by simple physical processes.
• It does not have a fixed melting and boiling point.

Solution

A solution is a homogeneous mixture of two or more substances (e.g., Lemonade, soda water). 

Table:Difference between Solute and Solvent.

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Concentration of Solution

• The amount of solute that has dissolved in a specific amount of solvent or solution is measured as solution concentration.
• A concentrated solution is one that has a significant amount of dissolved solute in it. 
• A diluted solution is one that has a small amount of dissolved solute in it.

Alloys

Examples:

• Steel – a combination of iron (metal) and carbon (non-metal).
• Bronze – a combination of copper (metal) and tin (metal).
• Brass – a mixture of copper (metal) and zinc (metal).

Suspension

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Ex: Chalk in water, smoke in the air

Properties of Suspension :

1. It is a heterogeneous mixture.
2. Particles of a suspension are visible to the naked eye.
3. The size of the particles is greater than 1000 nm.
4. It is an unstable mixture. Solute settles down at the bottom over the period.
5. If the solution is passed through filter paper, the solute and solvent get separated.
6. It scatters light when light is passed through the solution i.e. it shows the Tyndall effect.

Colloidal Solution

A colloid solution is a heterogeneous mixture in which the size of particles lies between the true solutions and suspensions. 

• Colloidal particles can easily scatter a beam of visible light. 
• This phenomenon is called the Tyndall effect. 

Properties of Colloidal Solution:

1. The particles of colloids can’t be seen by the naked eye individually.
2. It is a heterogeneous mixture and thus solute and solvent can’t be separated by filter paper.
3. The size of particles is smaller than suspensions but greater than solutions (1 nm to 100 nm).
4. It is a stable mixture. Particles do not settle down at the bottom over a period of time.
5. They do not settle down when left undisturbed which means colloid is quite stable. 

Physical and Chemical Change

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Types of Pure Substances

The pure substance is divided in two types based on their chemical composition:

(i)  Elements

Properties of Metals

Note:Mercury is the only metal that is liquid at room temperature.

Properties of Non-Metals

Metalloids: Elements having intermediate properties between those of metals and non-metals are called metalloids.  Examples are boron, silicon, germanium etc.

(ii) Compounds

Characteristics

• The properties of a compound differ from those of its constituents.
• The compound has a fixed melting point and boiling point.
• A compound is a homogeneous substance.
• Constituent elements can be separated by chemical processes.

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Difference between Compounds and Mixtures

Summary