2. Is matter around us pure? | Class 9 Science | PDF and Web notes

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Chapter 2: IS MATTER AROUND US PURE?

In science, a pure substance means all the particles of that substance have the same chemical properties. E.g., sodium chloride, sugar, etc.

In the normal sense, unadulterated milk is pure. But in the scientific sense, it is not pure because it is a mixture of water, fat, proteins, etc.

A pure substance consists of a single type of particle, i.e., a substance is a pure single form of matter.

  WHAT IS A MIXTURE?

-    A mixture is a combination of two or more pure matters. E.g., sea water, minerals, soft drink, soil, etc.

-    Dissolved sodium chloride can be separated from water by evaporation (a physical process). However, sodium chloride is a pure substance and cannot be separated by physical process into its chemical constituents.

  TYPES OF MIXTURES

Based on the nature of components, mixtures are 2 types:

Ø Homogeneous mixture or solution: In this, the components are uniformly distributed throughout the mixture (uniform composition). E.g.,

o  Copper sulphate (CuSO₄) dissolved in water.

o  Salt dissolved in water.

o  Sugar dissolved in water.

When more CuSO₄ is added to water, the intensity of blue colour increases. This shows that a homogeneous mixture can have a variable composition.

Ø Heterogeneous mixture: It is a mixture which contains physically distinct parts and have non-uniform compositions. E.g.,

o  Mixture of sodium chloride and iron filings.

o  Mixture of salt and sulphur.

o  Mixture of oil and water.

o  Mixture of CuSO₄ & potassium permanganate.

Activity

•    Take few CuSO₄ crystals, one spatula full of CuSO₄, chalk powder (or wheat flour) and few drops of milk (or ink).

•    Add each of them in separate water samples. Stir well using glass rod.

•    In CuSO₄ solutions, particles are not visible.

•    Direct a beam of light from a torch through the beakers containing the mixture. The path of the beam of light is not visible in CuSO₄ solutions.

•    Leave the mixtures undisturbed. CuSO₄ solutions remain stable. Chalk powder (or wheat flour) and milk (or ink) particles settle over time.

•    Filter the mixture. CuSO₄ solutions have no residue on the filter paper. Chalk powder (or wheat flour) and milk (or ink) leave residue.

•    Mixture of chalk powder and water is a suspension.

•    Mixture of milk and water is a colloidal solution.

  WHAT IS A SOLUTION?

-    A solution is a homogeneous mixture of two or more substances. E.g., Lemonade, soda water, etc.

-    Besides liquid solution, there are solid solutions (alloys) and gaseous solutions (air).

-    In a solution, there is homogeneity at the particle level. E.g., lemonade tastes the same throughout. This shows that particles of sugar or salt are evenly distributed in the solution.

Alloys are mixtures of two or more metals or a metal and a non-metal. They cannot be separated into their components by physical methods. An alloy is a mixture because it shows the properties of its constituents and can have variable composition. E.g., brass is a mixture of 30% zinc and 70% copper.

A solution has 2 components:

·    Solvent: The component that dissolves the other component in the solution. It is usually present in larger amount.

·    Solute: The component that is dissolved in the solvent. It is usually present in lesser amount.

Examples for various solutions:

o  A solution of sugar in water is a solid in liquid solution. Sugar is the solute and water is the solvent.

o  In tincture of iodine, iodine (solid) is the solute, and alcohol (liquid) is the solvent.

o  Aerated drinks like soda water are gas in liquid solutions. CO₂ (gas) is solute & water (liquid) is solvent.

o  Air is a homogeneous mixture of gas in gas. Its two main constituents are oxygen (21%) and nitrogen (78%). Other gases are present in very small amount.

Properties of a Solution

•    A solution is a homogeneous mixture.

•    The particles are smaller than 1 nm (10^-9 metre) in diameter. So, they are invisible to naked eyes.

•    Due to their small size, the particles do not scatter light, so the path of light is not visible in a solution.

•    Solute particles cannot be separated from the mixture by filtration.

•    A solution is stable (solute particles do not settle down).

  CONCENTRATION OF A SOLUTION

-    In a solution, the relative proportion of the solute and solvent can be varied.

-    Based on the amount of solute, a solution can be called dilute, concentrated or saturated. Dilute and concentrated are comparative terms. E.g., A solution with few CuSO₄ is dilute. But as its quantity increases, it becomes concentrated.

Activity

•    Take 50 mL of water each in two separate beakers.

•    Add salt in one beaker and sugar or barium chloride in the second beaker with continuous stirring.

•    When no more solute can dissolve, heat the contents of the beaker to raise the temperature by about 5°C.

•    Start adding the solute again.

•    As the temperature rises, more solute can be dissolved.

-    Saturated solution: It is a solution that has dissolved maximum amount of solute at a given temperature. i.e., no more solute can dissolve at that temperature.

-    The amount of the solute present in the saturated solution at this temperature is called its solubility.

-    Unsaturated solution: It is a solution that contains solute less than the saturation level.

-    If a saturated solution is cooled slowly, some solute may crystallize out as the solution’s capacity to hold solute decreases.

-    Different substances in a given solvent have different solubilities at the same temperature.

-    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. 3 methods are given below:

(i) Mass by mass percentage of a solution:

(ii) Mass by volume percentage of a solution:

(iii) Volume by volume percentage of a solution:

Example:

A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the solution.

Solution:

Mass of solute (salt) = 40 g

Mass of solvent (water) = 320 g

Mass of solution = Mass of solute + Mass of solvent

= 40 g + 320 g = 360 g

Mass percentage of solution:

   

  WHAT IS A SUSPENSION?

-    Suspensions are non-homogeneous systems in which solids are dispersed in liquids.

-    A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.

Properties of a Suspension

•    Suspension is a heterogeneous mixture.

•    Particles of a suspension are visible to the naked eye.

•    Particles scatter a beam of light passing through it and make its path visible.

•    A suspension is unstable (solute particles settle down). They can be separated from the mixture by filtration. When the particles settle down, the suspension breaks and it does not scatter light.

  WHAT IS A COLLOIDAL SOLUTION?

-    A colloid is a type of mixture in which tiny particles are dispersed within a medium.

-    Particles are uniformly spread throughout the solution.

-    Due to the smaller size of particles, the mixture appears to be homogeneous. But a colloidal solution is a heterogeneous mixture. E.g., milk.

-    These particles can easily scatter a beam of visible light. This is called Tyndall effect (discovered by Tyndall).

(a) Solution of copper sulphate does not show Tyndall effect 

(b) mixture of water and milk shows Tyndall effect.

Other examples of Tyndall effect:

·    Light scatters off dust and smoke particles in air when a beam of light enters a room through a small hole.

·    Sunlight passing through a dense forest canopy, where mist droplets act as colloid particles dispersed in the air.

Properties of a Colloid

•    A colloid is a heterogeneous mixture.

•    Particles are too small to be seen with the naked eye.

•    Colloids are big enough to scatter light making its path visible.

•    A colloid is stable (do not settle down).

•    Colloidal particles cannot be separated by filtration, but can be separated by centrifugation.

Components of a colloidal solution:

·    Dispersed phase:  It is the solute-like component or the dispersed particles in a colloid.

·    Dispersing medium: It is the component in which the dispersed phase is suspended.

-    Colloids are classified based on the state (solid, liquid or gas) of the dispersing medium and dispersed phase.

Dispersed phase	Dispersing Medium	Type	Example
Liquid	Gas	Aerosol	Fog, clouds, mist
Solid	Gas	Aerosol	Smoke, automobile exhaust
Gas	Liquid	Foam	Shaving cream
Liquid	Liquid	Emulsion	Milk, face cream
Solid	Liquid	Sol	Milk of magnesia, mud
Gas	Solid	Foam	Foam, rubber, sponge, pumice
Liquid	Solid	Gel	Jelly, cheese, butter
Solid	Solid	Solid Sol	Coloured gemstone, milky glass

  PHYSICAL AND CHEMICAL CHANGES

-    Interconversion of states is a physical change because it does not change composition and chemical nature of the substance.

-    Ice, water and water vapour have different physical properties but are chemically the same.

-    Water & cooking oil are liquids but differ in chemical properties. They differ in odour and inflammability. Oil burns in air but water extinguishes fire.

-    Burning is a chemical change. Chemical changes (chemical reaction) alter chemical properties and composition of matter forming new substances.

-    Physical & chemical changes in candle burning:

·    Physical changes: Solid wax melts into liquid and liquid wax turns into gas.

·    Chemical changes: Combustion of wax into CO₂, H₂O and energy.

  WHAT ARE THE TYPES OF PURE SUBSTANCES?

  ELEMENTS

-    Robert Boyle (1661) first used the term element.

-    Antoine Laurent Lavoisier (1743–94), a French chemist, gave an experimentally useful definition of an element. According to him, an element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions.

-    Elements are 3 types: metals, non-metals & metalloids.

-    Properties of Metals:

·    Have a lustre (shine).

·    Have silvery-grey or golden-yellow colour.

·    Conduct heat and electricity.

·    Ductile (can be drawn into wires).

·    Malleable (can be hammered into thin sheets).

·    Sonorous (make a ringing sound when hit).

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

-    Properties of Non-metals:

·    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: hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.

-    Metalloids: These are elements having intermediate properties between metals and nonmetals. E.g., boron, silicon, germanium, etc.

•    There are more than 100 elements known at present.

•    92 elements are natural. Others are man-made.

•    Majority of the elements are solid.

•    11 elements are in gaseous state at room temperature.

•    2 elements are liquid at room temperature - mercury & bromine. Elements, gallium & cesium become liquid at a temperature slightly above room temperature (303 K).

  COMPOUNDS

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

What do we get when two or more elements are combined?
(An experiment using iron and sulphur)

	Mixture of iron and sulphur	Compound of iron and sulphur
Preparation	Mix and crush 5 g of iron filings and 3 g of sulphur powder in a China dish.	Mix and crush the same substances in a China dish and heat strongly till red hot. Allow to cool.
Showing magnet	Shows magnetism (attracts towards the magnet).	Does not show magnetism.
Physical/chemical change	Physical change	Chemical change
Properties	Same as that of its constituents.	Different properties compared to the combining elements.
Texture and colour	Granular. Color is a mix of gray of iron filings and yellow of sulphur powder.	Solid mass. Dark colour.
Can separate the components of the material obtained?	Yes	No
Add carbon disulphide. Stir well and filter. Add dilute sulphuric acid or dilute hydrochloric acid.	Produces hydrogen gas. It is colourless, odourless and combustible.	Produces hydrogen sulphide (a colourless gas with the smell of rotten eggs).

 

Differences between Mixtures and Compounds

Mixtures	Compounds
1.  Elements or compounds just mix together to form a mixture and no new compound is formed.	Elements react to form new compounds.
2.  A mixture has a variable composition.	Composition of each new substance is always fixed.
3.  Shows the properties of the constituent substances.	New substance has totally different properties.
4.  The constituents can be separated easily by physical methods.	The constituents can be separated only by chemical or electro-chemical reactions.

 

Matter (Solid, Liquid or Gas)
Pure substance		Mixtures (No Fixed Composition)
Elements	Compounds	Homogeneous	Heterogeneous
Cannot be broken down to simpler substances	Have fixed composition. Can be broken down into elements by chemical or electrochemical reactions.	Uniform composition	Non-uniform composition
E.g., Cu, O, Fe, H, Hg etc.	E.g., water, methane, sugar, salt etc.	E.g., sugar in water, salt in water, sulphur in carbon disulphide, water in alcohol etc.	E.g., sand and salt, sugar and salt, water in oil etc.

 

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