Is Matter Around Us Pure?: Class 9 Science NCERT Chapter 2 Note

For a common person pure means having no adulteration. But, for a scientist all these things are actually mixtures of different substances and hence it is not pure.

Example: Milk is actually a mixture of water, fat, proteins etc.

A pure substance consists of a single type of particles. A substance is pure single form of the matter.

In the view of scientist “When all the constituent particles of a substance are the same in their chemical nature, it is known as pure substance“.

2.1 What is Mixture

The mixtures are constituted by more than one kind of pure form of matter.

By the physical process of evaporation we can separate dissolved sodium chloride from water.

Sugar contains only one kind of pure matter and its composition id the same throughout.

Soft drink and soil are not single pure substance.

Conclusion: A mixture contains more than one pure substance.

2.1.1 Types of Mixtures

Depending upon the nature of the components that form a mixture, we can have different types of mixtures.

2.2 What is a Solution?

A solution is a homogeneous mixture of two or more substances.

Example: Lemonade, soda water etc. are all examples of solutions.

Usually a solution as a liquid that contains either a solid, liquid or a gas dissolved in it. But, we can also have solid solutions (alloys) and gaseous solutions (air).

Alloys: Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods.

For example, brass is a mixture of approximately 30% zinc and 70% copper.

Solvent: The component of the solution that dissolves the other component in it (usually the component present in larger amount) is called the solvent.

Solute: The component of the solution that is dissolved in the solvent is called
the solute.

Properties of a solution:

• A solution is a homogeneous mixture.
• • The particles of a solution are smaller than 1 nm (10-9 metre) in diameter. So, they cannot be seen by naked eyes.
• Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
• The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.

2.2.1 CONCENTRATION OF A SOLUTION

In a solution the relative proportion of the solute and solvent can be varied. Depending upon the amount of solute present in a solution, it can be called a dilute, concentrated or a saturated solution.

Dilute and concentrated are comparative terms.

Saturated Solution: At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving, is said to be a saturated solution.

OR

When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution.
Solubility: The amount of the solute present in the saturated solution at this temperature is called its solubility.

Unsaturated solution: If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.

The concentration of a solution is the amount (mass or volume) of solute present in a given amount (mass or volume) of solution.

There are various ways of expressing the concentration of a solution, but here we will learn only three methods.

(i) Mass by mass percentage of a solution

= \frac{Mass of solute}{Mass of solution} \times 100

(ii) Mass by volume percentage of a solution

= \frac{Mass of solute}{Volume of solution} \times 100

(iii) Volume by volume percentage of a solution

= \frac{Volume of solute}{Volume of solution} \times 100

2.2.2 What is a suspension?

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye.

Properties of a Suspension

• Suspension is a heterogeneous mixture.
• The particles of a suspension can be seen by the naked eye.
• The particles of a suspension scatter a beam of light passing through it and make its path visible.
• The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration. When the particles settle down, the suspension breaks and it does not scatter light any more.

2.2.3 WHAT IS A COLLOIDAL SOLUTION?

a colloidal solution is a heterogeneous mixture.

Example, milk.

Tyndall effect: The scattering of a beam of light particles can easily scatter a beam of visible light is called the Tyndall effect.

Properties of a colloid

• A colloid is a heterogeneous mixture. • The size of particles of a colloid is too small to be individually seen by naked eyes.
• Colloids are big enough to scatter a beam of light passing through it and make its path visible.
• They do not settle down when left undisturbed, that is, a colloid is quite stable.
• They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation can be used to separate the colloidal particles.

The components of a colloidal solution are the dispersed phase and the dispersion medium. Dispersing medium: The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known as the dispersing medium.

Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase.

2.3 Separating the Components of a Mixture

Heterogeneous mixtures can be separated into their respective constituents by simple physical methods like handpicking, sieving, filtration that we use in our day-to-day life.

2.3.1 HOW CAN WE OBTAIN COLOURED COMPONENT (DYE) FROM BLUE/ BLACK INK?

The ink is a mixture of a dye in water. Thus, we can separate the volatile component (solvent) from its non-volatile solute by the method of evaporation.

2.3.2 HOW CAN WE SEPARATE CREAM FROM MILK?

Sometimes the solid particles in a liquid are very small and pass through a filter paper. For such particles the filtration technique cannot be used for separation. Such mixtures are separated by centrifugation.

Applications

• Used in diagnostic laboratories for blood and urine tests.
• Used in dairies and home to separate butter from cream.
• Used in washing machines to squeeze out water from wet clothes.

2.3.3 HOW CAN WE SEPARATE A MIXTURE OF TWO IMMISCIBLE LIQUIDS?

Applications

• To separate mixture of oil and water.
• In the extraction of iron from its ore, the lighter slag is removed from the top by this method to leave the molten iron at the bottom in the furnace.

2.3.4 HOW CAN WE SEPARATE A MIXTURE OF SALT AND CAMPHOR?

• We know that camphor changes directly from solid to gaseous state on heating.
• So, to separate such mixtures that contain a sublimable volatile component from a non-sublimable impurity. The sublimation process is used.
• Some examples of solids which sublime are ammonium chloride, naphthalene and anthracene.

2.3.5 IS THE DYE IN BLACK INK A SINGLE COLOUR?

Chromatography: This process of separation of components of a mixture is known as chromatography.

• Kroma in Greek means colour. This technique was first used for separation of colours, so this name was given.
• Chromatography is the technique used for separation of those solutes that dissolve in the same solvent.
• With the advancement in technology, newer techniques of chromatography have been developed.

Applications

To separate,

• colours in a dye
• pigments from natural colours
• drugs from blood.

2.3.6 HOW CAN WE SEPARATE A MIXTURE OF TWO MISCIBLE LIQUIDS?

Distillation: The process which is used for the separation of components of a mixture
containing two miscible liquids that boil without decomposition and have sufficient
difference in their boiling points are called distillation.

To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K.

2.3.7 HOW CAN WE OBTAIN DIFFERENT GASES FROM AIR ?

Air is a homogeneous mixture and can be separated into its components by fractional distillation.

The flow diagram (Fig. 2.11) shows the steps of the process.

2.3.8 HOW CAN WE OBTAIN PURE COPPER SULPHATE FROM AN IMPURE SAMPLE?

Crystallisation is a process that separates a pure solid in the form of its crystals from a solution.

Crystallisation technique is better than simple evaporation technique as –

• Some solids decompose or some, like sugar, may get charred on heating to dryness.
• Some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.

Applications

• Purification of salt that we get from sea water.
• Separation of crystals of alum (phitkari) from impure samples.

2.4 Physical and Chemical Changes

The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point etc. are the physical properites.

Both water and cooking oil are liquid but their chemical characteristics are different.

• They differ in odour and inflammability.
• We know that oil burns in air whereas water extinguishes fire.
• It is this chemical property of oil that makes it different from water.
• Burning is a chemical change.
• During this process one substance reacts with another to undergo a change in chemical composition.
• Chemical change brings change in the chemical properties of matter and we get new substances.
• A chemical change is also called a chemical reaction.

2.5 What are the Types of Pure Substances?

On the basis of their chemical composition, substances can be classified either as elements or compounds.

2.5.1 ELEMENTS

Robert Boyle was the first scientist to use the term element in 1661. Antoine Laurent
Lavoisier (1743-94), a French chemist, was the first to establish an experimentally useful definition of an element.

Elements can be normally divided into metals, non-metals and metalloids.
Metals usually show some or all of the following properties:

• They have a lustre (shine).
• They have silvery-grey or golden-yellow colour.
• They conduct heat and electricity.
• They are ductile (can be drawn into wires).
• They are malleable (can be hammered into thin sheets).
• They are sonorous (make a ringing sound when hit).

Examples of metals are gold, silver, copper, iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature.

Non-metals usually show some or all of the following properties:

• They display a variety of colours.
• They are poor conductors of heat and electricity.
• They are not lustrous, sonorous or malleable.

Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc. Some elements have intermediate properties between those of metals and non-metals, they are called metalloids; examples are boron, silicon, germanium etc.

2.5.2 COMPOUNDS

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

Mixtures

1. Elements or compounds just mix together to form a mixture and no new compound is formed.
2. A mixture has a variable composition.
3. A mixture shows the properties of theconstituent substances.
4. The constituents can be seperated fairly easily by physical methods.

Compound

1. Elements react to form new compounds.
2. The composition of each new substance is always fixed.
3. The new substance has totally different properties.
4. The constituents can be separated only by chemical or electrochemical
   reactions.