Comprehensive Notes on "Is Matter Around Us Pure?"

Matter Around Us: Pure or Mixture?

Pure Substances vs. Mixtures

• Purity in Science:
  • Common understanding: 'Pure' means unadulterated.
  • Scientific perspective: A pure substance consists of a single type of particle; all constituent particles are the same in chemical nature.
  • Many everyday items labeled 'pure' are mixtures of different substances (e.g., milk: water, fat, proteins).
• Mixtures:
  • Composed of more than one pure form of matter.
  • Examples: seawater, minerals, soil.
• Separation of Mixtures:
  • Components of a mixture can be separated by physical processes (e.g., evaporation to separate sodium chloride from water).
  • Pure substances (like sodium chloride or sugar) cannot be separated into chemical constituents by physical means.
  • A pure substance has the same characteristic properties regardless of its source.

Types of Mixtures

• Mixtures vary based on the nature of their components.
• Homogeneous Mixtures (Solutions):
  • Uniform composition throughout.
  • Examples: salt dissolved in water, sugar dissolved in water.
  • Homogeneous mixtures can have variable compositions (e.g., copper sulphate solutions with different intensities of color based on concentration).
• Heterogeneous Mixtures:
  • Non-uniform composition with physically distinct parts.
  • Examples: mixtures of sodium chloride and iron filings, salt and sulphur, oil and water.

Solutions, Suspensions, and Colloidal Solutions

• Solutions:
  • Homogeneous mixtures of two or more substances.
  • Examples: lemonade, soda water.
  • Can be solid (alloys), liquid, or gas (air).
  • Particles of solute are evenly distributed.
• Alloys:
  • Mixtures of two or more metals or a metal and a non-metal.
  • Cannot be separated into components by physical methods.
  • Considered mixtures because they retain properties of constituents and have variable composition (e.g., brass: 30% zinc, 70% copper).

Components of a Solution

• Solvent: Component that dissolves the other (usually present in larger amount).
• Solute: Component that is dissolved in the solvent (usually present in lesser quantity).
• Examples:
  • Sugar in water: sugar (solute), water (solvent).
  • Tincture of iodine: iodine (solute), alcohol (solvent).
  • Aerated drinks: carbon dioxide (solute), water (solvent).
  • Air: oxygen (21%) and other gases (solutes), nitrogen (78%) (solvent).

Properties of Solutions

• Homogeneous mixture.
• Particle size less than $1 \text{ nm } (10^{-9} \text{ metre})$ (cannot be seen with naked eyes).
• Do not scatter light (path of light not visible).
• Cannot be separated by filtration.
• Stable (solute particles do not settle).

Concentration of a Solution

• The proportion of solute and solvent can vary.
• Descriptive terms: dilute, concentrated, saturated.
• Saturated Solution:
  • A solution that has dissolved as much solute as it is capable of dissolving at a given temperature.
  • No more solute can be dissolved at that temperature.
• Solubility:
  • The amount of solute present in a saturated solution at a specific temperature.
• Unsaturated Solution:
  • Contains less solute than the saturation level.
• Different substances have different solubilities in a given solvent at the same temperature.
• Expressing Concentration:
  • Mass by mass percentage: $\frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$
  • Mass by volume percentage: $\frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100$
  • Volume by volume percentage: $\frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100$
• Example Calculation: A solution contains $40 \text{ g}$ of salt in $320 \text{ g}$ of water.
  • Mass of solution = $40 \text{ g} + 320 \text{ g} = 360 \text{ g}$
  • Mass percentage = $\frac{40}{360} \times 100 = 11.1\%$

Suspensions

• Non-homogeneous systems where solids are dispersed in liquids.
• Heterogeneous mixture where solute particles do not dissolve but remain suspended.
• Particles are visible to the naked eye.

Properties of Suspensions

• Heterogeneous mixture.
• Particles can be seen with the naked eye.
• Scatter a beam of light (path visible).
• Unstable (particles settle when undisturbed).
• Can be separated by filtration.

Colloidal Solutions

• Particles are uniformly spread throughout the solution but are not dissolved.
• Appear homogeneous but are actually heterogeneous mixtures (e.g., milk).
• Particles cannot be seen with naked eyes but can scatter light.
• Tyndall Effect:
  • Scattering of a beam of light by colloidal particles.
  • Observed when light enters a room through a small hole, scattered by dust and smoke.

Properties of Colloids

• Heterogeneous mixture.
• Particle size too small to be individually seen with naked eyes.
• Scatter light (path visible).
• Quite stable (do not settle when undisturbed).
• Cannot be separated by filtration (centrifugation can be used).

Components of Colloids

• Dispersed Phase: Solute-like component or dispersed particles.
• Dispersion Medium: Component in which the dispersed phase is suspended.

Types of Colloids (Examples):

• Liquid in gas: Aerosol (fog, clouds, mist).
• Solid in gas: Aerosol (smoke, automobile exhaust).
• Gas in liquid: Foam (shaving cream).
• Liquid in liquid: Emulsion (milk, face cream).
• Solid in liquid: Sol (milk of magnesia, mud).
• Gas in solid: Foam (foam, rubber, sponge, pumice).
• Liquid in solid: Gel (jelly, cheese, butter).
• Solid in solid: Solid Sol (coloured gemstone, milky glass).

Physical and Chemical Changes

Physical Properties

• Observed and specified without changing the substance's composition (e.g., color, hardness, rigidity, density, melting point, boiling point).
• Interconversion of states (e.g., ice, water, vapor) is a physical change.

Chemical Properties

• Lead to changes in chemical composition.
• Burning is a chemical change where a substance reacts with another.
• Chemical change results in new substances and is also called a chemical reaction.

Types of Pure Substances

Elements

• Definition: A basic form of matter that cannot be broken down into simpler substances by chemical reactions (Antoine Laurent Lavoisier).
• Classified as metals, non-metals, and metalloids.
• Metals:
  • Lustrous (shiny).
  • 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: gold, silver, copper, iron, sodium, potassium. Mercury is liquid at room temperature.
• Non-metals:
  • Variety of colors.
  • Poor conductors of heat and electricity.
  • Not lustrous, sonorous, or malleable.
  • Examples: hydrogen, oxygen, iodine, carbon, bromine, chlorine.
• Metalloids:
  • Intermediate properties between metals and non-metals.
  • Examples: boron, silicon, germanium.

Compounds

• Substance composed of two or more elements chemically combined in a fixed proportion.
• Heating iron filings and sulphur powder:
  • Group I (Physical Change): Mixing iron filings and sulphur powder retains properties of constituents.
  • Group II (Chemical Change): Heating the mixture results in a compound with different properties.
  • The number of elements known at present are more than 100. Ninety-two elements are naturally occurring and the rest are man-made.
  • Majority of the elements are solid.
  • Eleven elements are in gaseous state at room temperature.
  • Two elements are liquid at room temperature—mercury and bromine.
  • Elements, gallium and cesium become liquid at a temperature slightly above room temperature $(303 K)$.

Key Differences: Mixtures vs. Compounds