NCERT Class 9 Science: Is Matter Around Us Pure? Comprehensive Study Notes

The Scientific Concept of Purity

• General Definition: In common parlance, the word ‘pure’ implies that a substance has no adulteration. For a typical consumer, pure milk, pure ghee, or pure salt means they are free from contaminants.

• Scientific Definition: For a scientist, most substances regarded as pure by consumers are actually mixtures of different substances. For instance, milk is a mixture of water, fat, and proteins.

• Pure Substance: Scientifically, a substance is considered pure when all its constituent particles are identical in their chemical nature. A pure substance consists of a single type of particle and is a pure single form of matter.

• Prevalence of Mixtures: Most matter in the environment exists as mixtures of two or more pure components. Examples include sea water, minerals, and soil.

Understanding Mixtures

• Definition: Mixtures are constituted by more than one kind of pure form of matter, known as a substance.

• Nature of a Substance: A substance cannot be separated into other kinds of matter by any physical process. While sodium chloride can be separated from water via evaporation, it remains a substance itself and cannot be separated into its chemical constituents by physical means.

• Uniformity: Substances like sugar are considered pure because they contains only one kind of pure matter and their composition is the same throughout.

• Characteristics: Whatever the source of a substance, it will always exhibit the same characteristic properties. Therefore, a mixture contains more than one substance.

Types of Mixtures

• Homogeneous Mixtures (Solutions): These mixtures have a uniform composition throughout. Examples include:

  • Salt dissolved in water.

  • Sugar dissolved in water.

  • Copper sulphate dissolved in water (though the intensity of color can vary depending on the concentration).

• Heterogeneous Mixtures: These mixtures contain physically distinct parts and have non-uniform compositions. Examples include:

  • Sodium chloride and iron filings.

  • Salt and sulphur.

  • Oil and water.

• Experimental Observation (Activity 2.1): Different amounts of solute (like copper sulphate) produce solutions with varying color intensities but uniform composition, highlighting that homogeneous mixtures can have variable composition.

Solutions and Their Properties

• Definition: A solution is a homogeneous mixture of two or more substances. Homogeneity exists at the particle level (e.g., lemonade tastes the same throughout).

• Types of Solutions:

  • Solid Solutions: Alloys.

  • Gaseous Solutions: Air.

  • Liquid Solutions: Common solutions containing solids, liquids, or gases dissolved in a liquid.

• Alloys: These are mixtures of two or more metals, or a metal and a non-metal, that cannot be separated into their components by physical methods. They are considered mixtures because they retain the properties of their constituents and have variable compositions.

  • Example: Brass is an alloy consisting of approximately $30\%$ zinc and $70\%$ copper.

• Components of a Solution:

  • Solvent: The component that dissolves the other component, usually present in a larger amount.

  • Solute: The component that is dissolved in the solvent, usually present in a lesser quantity.

• Examples of Solutions:

  • Sugar in water: Solid in liquid solution. Solute: Sugar; Solvent: Water.

  • Tincture of Iodine: Iodine (solid) as solute dissolved in alcohol (liquid) as solvent.

  • Aerated drinks (Soda water): Carbon dioxide (gas) as solute and water (liquid) as solvent.

  • Air: A homogeneous mixture of gases. Main constituents are nitrogen ($78\%$) and oxygen ($21\%$).

• Properties:

  1. It is a homogeneous mixture.

  2. Particle diameter is smaller than $1\,nm$ ($10^{-9}\,metre$), making them invisible to the naked eye.

  3. Because particles are so small, they do not scatter a beam of light; the path of light is not visible.

  4. Solute particles cannot be separated by filtration.

  5. The solution is stable; particles do not settle when left undisturbed.

Concentration of a Solution

• Qualitative Terms: Solutions can be dilute, concentrated, or saturated based on the amount of solute.

• Saturated Solution: A solution that has dissolved as much solute as it is capable of dissolving at a specific temperature. No more solute can be dissolved at that temperature.

• Unsaturated Solution: A solution where the amount of solute is less than the saturation level.

• Solubility: The mass of the solute present in a saturated solution at a given temperature.

• Solubility and Temperature: Different substances in a given solvent have different solubilities at the same temperature. Solubility generally changes with temperature variations.

• Concentration Calculations:

  • $\text{Concentration of solution} = \frac{\text{Amount of solute}}{\text{Amount of solution}}$

  • $\text{Concentration of solution} = \frac{\text{Amount of solute}}{\text{Amount of solvent}}$

• Mass by Mass Percentage:

  • $\text{Mass percentage} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$

• Mass by Volume Percentage:

  • $\text{Mass by volume percentage} = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100$

• Numerical Example (Example 2.1): A solution with $40\,g$ of salt and $320\,g$ of water:

  • $\text{Mass of solution} = 40\,g + 320\,g = 360\,g$

  • $\text{Mass } \% = \frac{40}{360} \times 100 = 11.1\%$

Suspensions

• Definition: A suspension is a non-homogeneous system in which solid particles are dispersed in a liquid but do not dissolve. They remain suspended throughout the bulk of the medium.

• Properties:

  1. It is a heterogeneous mixture.

  2. Particles are visible to the naked eye.

  3. Particles scatter a beam of light, making the light path visible.

  4. The mixture is unstable; particles settle down when left undisturbed.

  5. Solute particles can be separated by filtration.

  6. When particles settle, the suspension breaks and no longer scatters light.

Colloidal Solutions

• Definition: A colloid or colloidal solution is a mixture where particles are uniformly spread. Due to the small size of particles, it appears homogeneous but is actually heterogeneous.

• Tyndall Effect: The scattering of a beam of visible light by colloidal particles. Named after the scientist who discovered it.

  • Examples: A beam of light entering a dusty room through a small hole; sunlight passing through a dense forest canopy where mist (water droplets in air) acts as a colloid.

• Components of a Colloid:

  • Dispersed Phase: The solute-like or dispersed particles.

  • Dispersion Medium: The component in which the dispersed phase is suspended.

• Properties:

  1. Heterogeneous mixture.

  2. Individual particles are too small to be seen by naked eyes.

  3. Particles are large enough to scatter light (Tyndall effect).

  4. They are stable (do not settle when left undisturbed).

  5. They cannot be separated by filtration.

  6. Separation: Can be separated using a special technique called centrifugation.

Classification of Colloids (Table 2.1)

Separation Techniques for Mixtures

• Simple Physical Methods: Handpicking, sieving, and filtration are used for some heterogeneous mixtures.

• Evaporation: Used to separate a non-volatile solute from a volatile solvent.

  • Example: Obtaining dye from blue/black ink. The water evaporates, leaving the dye residue on a watch glass.

• Centrifugation: Based on the principle that denser particles are forced to the bottom and lighter particles stay at the top when spun rapidly.

  • Applications: Diagnostic blood and urine tests, separating butter from cream in dairies/homes, squeezing water from wet clothes in washing machines.

• Separating Funnel: Used to separate a mixture of two immiscible liquids based on their densities.

  • Applications: Separating oil and water; extracting iron from its ore (removing lighter slag from molten iron).

• Sublimation: Separates a sublimable volatile component from a non-sublimable impurity.

  • Substances that sublime: Ammonium chloride, camphor, naphthalene, and anthracene.

• Chromatography: Used to separate solutes that dissolve in the same solvent (Greek kroma means colour).

  • Technique: Components move at different speeds on a filter paper as solvent rises. The more soluble component rises faster.

  • Applications: Separating colours in a dye, pigments from natural colours, and drugs from blood.

• Distillation: Used for miscible liquids with a boiling point difference of more than $25\,K$. The component with the lower boiling point vaporizes and is then condensed and collected.

• Fractional Distillation: Used for miscible liquids with a boiling point difference of less than $25\,K$. Uses a fractionating column packed with glass beads to provide surface area for repeated condensation and cooling.

  • Applications: Separating gases from air, refining petroleum products.

• Crystallisation: Process of separating a pure solid in the form of its crystals from a solution.

  • Advantages over Evaporation: Prevents decomposition or charring of solids (like sugar) and removes impurities that might remain after filtration.

  • Applications: Purifying salt from sea water, separating alum (phitkari) from impure samples.

Separation of Gases from Air

• Process: Air is a homogeneous mixture.

  1. Air is compressed by increasing pressure.

  2. Compressed air is cooled by decreasing temperature to produce liquid air.

  3. Liquid air is warmed slowly in a fractional distillation column.

  4. Gases separate at different heights based on boiling points:

     • Oxygen: Boiling point $-183^\circ C$

     • Argon: Boiling point $-186^\circ C$

     • Nitrogen: Boiling point $-196^\circ C$

Physical and Chemical Changes

• Physical Properties: Observable characteristics like colour, hardness, rigidity, fluidity, density, melting point, and boiling point.

• Physical Change: Interconversion of states without a change in chemical composition or nature. Example: Ice, water, and water vapour are chemically identical ($H_2O$).

• Chemical Change (Chemical Reaction): One substance reacts with another to undergo a change in chemical composition, resulting in new substances. Example: Oil burning in air (inflammability).

• Candle Burning: During this process, both physical change (melting of wax) and chemical change (burning of wax) occur.

Types of Pure Substances: Elements and Compounds

• Elements: Defined by Antoine Laurent Lavoisier as the basic form of matter that cannot be broken down into simpler substances by chemical reactions.

  • Metals: Lustrous, silvery-grey/golden-yellow, conductive, ductile, malleable, sonorous. (e.g., gold, iron, potassium). Mercury is the only metal liquid at room temperature.

  • Non-metals: Varied colours, poor conductors, non-lustrous, non-malleable. (e.g., hydrogen, iodine, carbon, bromine).

  • Metalloids: Intermediate properties (e.g., boron, silicon, germanium).

  • Statistics: Over 100 elements; 92 naturally occurring; 11 gaseous at room temperature; Mercury and Bromine are liquid; Gallium and Cesium liquify just above room temperature ($303\,K$).

• Compounds: Substances composed of two or more elements chemically combined in fixed proportions.

  • Activity 2.10 Result: Heating iron and sulphur creates iron sulphide ($FeS$), a compound that is non-magnetic and does not behave like its constituent elements. Dilute hydrochloric acid added to $FeS$ produces hydrogen sulphide gas (rotten egg smell), while adding it to a simple mixture of iron and sulphur produce hydrogen gas.

Summary: Mixtures vs. Compounds

Questions & Discussion

• Q: What is meant by a substance?

• A: A substance is a pure form of matter consisting of a single type of particles that cannot be separated by physical processes.

• Q: List differences between homogeneous and heterogeneous mixtures.

• A: Homogeneous mixtures have uniform composition and no visible boundaries (e.g., salt solution); heterogeneous mixtures have non-uniform composition and visible boundaries (e.g., sand and salt).

• Q: How do we make tea using chemical terminology?

• A: Water (solvent) is heated. Tea leaves (insoluble solute) are added. Sugar and milk (solutes) are dissolved. After boiling, the mixture is filtered. The liquid obtained is the filtrate, and the tea leaves left on the sieve are the residue.

• Q: Concentration Calculation: 36 g NaCl in 100 g water at 293 K.

• A: Total mass of solution = $36\,g + 100\,g = 136\,g$. Concentration ($mass \%$) = $\frac{36}{136} \times 100 = 26.47\%$.