Elements, Compounds and Mixtures Study Guide

Classification of Substances

• The earth mainly consists of mixtures containing elements and compounds. These substances are classified based on similarities and dissimilarities in their composition, properties, and uses.
• Pure Substances: These have a definite composition and a definite set of properties, such as boiling point, melting point, and density. They are homogeneous, meaning their composition is uniform throughout. Both elements and compounds are pure substances.
• Impure Substances (Mixtures): These are made of two or more pure substances mixed in any proportion. They do not have a definite set of properties and retain the properties of their constituents. They can be homogeneous or heterogeneous.
• Classification Hierarchy:
  • Pure Substances (Homogeneous):
    • Elements: Metals (e.g., Copper, Iron), Non-metals (e.g., Hydrogen, Carbon), Metalloids (e.g., Silicon, Arsenic), and Noble Gases (e.g., Helium, Neon).
    • Compounds: Inorganic (e.g., Water, Salt) and Organic (e.g., Sugar, Proteins, Vinegar).
  • Mixtures (Impure):
    • Homogeneous (e.g., Tap water, Air).
    • Heterogeneous (e.g., Water and Sand, Oil and Water).

Elements

• Definition: An element is a pure substance that cannot be converted into anything simpler by any physical or chemical process. It possesses unique properties and consists of extremely small particles called atoms.
• Atoms: These are the smallest units of an element. All atoms in a specific element are identical to each other but differ from atoms of other elements. For example, an aluminium sheet contains only aluminium atoms, which differ from oxygen atoms.
• Historical Timeline and Statistics:
  • Robert Boyle was the first scientist to use the term "element" in 1661.
  • Antoine Laurent Lavoisier (1743–94) established the first experimentally useful definition of an element.
  • There are currently 118 known elements: 92 are natural and 26 are artificially created.
  • Elements can exist in solid, liquid, or gaseous states.
  • Some elements are radioactive and emit harmful radiations.

Classification of Elements

• Metals: The majority of elements are metals. Examples include gold, silver, copper, aluminium, iron, zinc, tin, and lead.
  • Specific applications: Calcium is found in chalk, milk, and bones. Magnesium is in chlorophyll to capture sunlight for photosynthesis. Iron is found in haemoglobin in red blood cells to carry oxygen. Aluminium is used for chocolate wrappers.
• Non-metals: These are fewer in number. Excluding inert gases, there are only twelve: hydrogen, oxygen, nitrogen, carbon, chlorine, sulphur, phosphorus, fluorine, selenium, bromine, iodine, and astatine.
• Metalloids: These show properties of both metals and non-metals and are hard solids. Examples include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.
• Inert or Noble Gases: These do not react chemically with other elements or compounds. They are found in traces in the air. There are six: helium, neon, argon, krypton, xenon, and radon.

Symbols and Formulas of Elements

• Symbols are abbreviated forms or shorthand notations approved by the International Union of Pure and Applied Chemistry (IUPAC). A symbol represents one atom of an element.
• Rules for Writing Symbols:
  1. Usually the first letter of the name in capital (e.g., Oxygen = $O$, Hydrogen = $H$).
  2. If multiple elements start with the same letter, a second small letter is added (e.g., Hydrogen = $H$, Helium = $He$; Boron = $B$, Barium = $Ba$, Bromine = $Br$).
  3. The second letter can be any subsequent letter in the name (e.g., Carbon = $C$, Calcium = $Ca$, Chlorine = $Cl$).
  4. Some symbols are derived from Latin or Greek names (e.g., Copper = $Cu$ from Cuprum, Cobalt = $Co$ from English).
• Latin/Greek Derived Symbols:
  • Sodium: Natrium ($Na$)
  • Potassium: Kalium ($K$)
  • Iron: Ferrum ($Fe$)
  • Copper: Cuprum ($Cu$)
  • Silver: Argentum ($Ag$)
  • Gold: Aurum ($Au$)
  • Mercury: Hydrargyrum ($Hg$)
  • Lead: Plumbum ($Pb$)
  • Tin: Stannum ($Sn$)
  • Antimony: Stibium ($Sb$)
• English Derived Symbols:
  • Lithium ($Li$), Beryllium ($Be$), Fluorine ($F$), Neon ($Ne$), Magnesium ($Mg$), Aluminium ($Al$), Silicon ($Si$), Phosphorus ($P$), Sulphur ($S$), Argon ($Ar$), Chromium ($Cr$), Manganese ($Mn$), Nickel ($Ni$), Zinc ($Zn$), Arsenic ($As$), Iodine ($I$), Platinum ($Pt$), Krypton ($Kr$), Radon ($Rn$), Xenon ($Xe$), Radium ($Ra$), Uranium ($U$).

Compounds

• Definition: A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass. The smallest unit is a molecule.
• Characteristics:
  • Properties of a compound differ entirely from its constituent elements. For example, Sodium (poisonous metal) and Chlorine (greenish gas) combine to form Sodium Chloride (safe common salt).
  • Compounds can only be broken down by chemical means (e.g., electrolysis for water), not physical means.
  • They have a fixed composition and a definite chemical formula.
  • Formation involves the absorption or liberation of energy.
• Common Compounds and Formulas:
  • Water: $H_2O$
  • Sodium chloride: $NaCl$
  • Magnesium oxide: $MgO$
  • Calcium oxide (quick lime): $CaO$
  • Carbon dioxide: $CO_2$
  • Sodium carbonate: $Na_{2}CO_{3}$
  • Washing soda: Na_{2}CO_{3}  10H_2O
  • Sodium bicarbonate (baking soda): $NaHCO_3$
  • Plaster of Paris: CaSO_4  H_2O
  • Cane sugar: $C_{12}H_{22}O_{11}$
  • Silica (sand): $SiO_2$
  • Glucose: $C_6H_{12}O_6$
  • Sodium sulphate: $Na_{2}SO_{4}$

Mixtures

• Definition: Impure substances formed by mixing two or more pure substances (elements or compounds) in any proportion without chemical combination. Components retain their individual properties.
• Characteristics:
  • Components are loosely held; no new substance is formed.
  • No fixed proportion of components. A lime juice solution can vary in sourness based on the ratio of juice to water.
  • No specific set of properties; components are easily recognized (e.g., rice and wheat).
  • Physical methods can separate components (e.g., evaporation of salt solution).
  • Melting and boiling points are not fixed (e.g., pure water boils at 100C, but salty water boils at a higher temperature).
  • Formation involves no energy exchange.
• Types Based on Distribution:
  • Homogeneous: Constituents are uniformly distributed and cannot be seen separately (e.g., salt solution, air, alloys like brass and bronze).
  • Heterogeneous: Constituents are not uniformly distributed and can be recognized separately (e.g., soil, oil and water, smoke).

Types of Mixtures by State

• Solid + Solid: Heterogeneous (Sand and salt); Homogeneous (Alloys like brass).
• Solid + Liquid: Heterogeneous (Sand and water); Homogeneous (Sugar in water).
• Liquid + Liquid: Heterogeneous (Oil in water); Homogeneous (Alcohol and water).
• Gas + Liquid: Homogeneous (Aerated drinks).
• Gas + Gas: Homogeneous (Pure air).
• Solid + Gas: Heterogeneous (Smoke/soot in air).

Separation Techniques for Solid-Solid Mixtures

• Hand-picking: Used for small quantities where substances are large enough to be seen (e.g., stones from rice).
• Winnowing: Uses wind to separate light solids from heavier ones (e.g., rice from husk).
• Magnetic Separation: Used when one component is magnetic (e.g., iron filings from sulphur).
• Gravity Separation: Based on density differences in water. Heavier particles sink (sand), while lighter ones float (sawdust).
• Sublimation: Used when one component changes directly from solid to vapour on heating. Examples of sublimable solids: Camphor, naphthalene, iodine, and ammonium chloride.
• Solvent Extraction: Uses a solvent to dissolve one component while leaving the other behind (e.g., using water to separate salt from sand).

Separation Techniques for Solid-Liquid Mixtures

• Heterogeneous Mixtures:
  • Sedimentation and Decantation: Heavy insoluble solids settle as sediment. The clear liquid (supernatant) is poured out (decantation).
  • Filtration: Passing a mixture through a filter (e.g., filter paper, charcoal). The liquid that passes through is the filtrate; the solid left behind is the residue.
• Homogeneous Mixtures:
  • Evaporation: Converting liquid to vapour to leave the solid behind (e.g., getting salt from sea water).
  • Distillation: Heating a solution to evaporate the liquid and then condensing the vapour to get a pure liquid (distillate). Used for tap water purification.

Separation Techniques for Liquid-Liquid and Other Mixtures

• Fractional Distillation: Separates miscible liquids based on different boiling points (difference must be 25C or more). Used for water and alcohol, and for refining crude petroleum (petrol, kerosene, diesel).
• Separating Funnel: Used for immiscible liquids with different densities. The heavier liquid (e.g., water) forms the bottom layer and is drained first, leaving the lighter liquid (e.g., kerosene) behind.
• Gas-Liquid Separation: Boiling can remove dissolved gases (e.g., air escapes from boiled water, making it tasteless).

Chromatography

• Definition: A technique to separate complex mixtures of coloured or colourless components based on differences in adsorption rates on a stationary phase while moving in a mobile phase.
• Phases:
  • Stationary Phase: The material the substances adhere to (e.g., Whatman filter paper, silica gel).
  • Mobile Phase: The solvent that carries the substances (e.g., water, ethyl alcohol, acetic acid).
• Process: A drop of mixture (e.g., ink) is placed on paper and dipped in solvent. Components dissolve and rise; more soluble components move faster, creating distinct spots called chromatograms.
• Advantages and Uses: Separates very small quantities and components with similar properties. Used for separating pigments from natural colours, drugs from blood, and dyes from ink.

Questions & Discussion

• Objective Questions:
  • Q: Atoms of different kinds combine to form molecules of?
  • A: A compound.
  • Q: Which is an inert gas?
  • A: Helium ($He$).
  • Q: Molecular formula of common salt?
  • A: $NaCl$.
  • Q: Which is a metalloid?
  • A: Silicon.
  • Q: Quicklime is a compound of?
  • A: Calcium and oxygen.
  • Q: Pure liquid is obtained from a solution by?
  • A: Distillation.
  • Q: Components of crude petroleum separated by?
  • A: Fractional distillation.
• Short Answer Identification:
  • Haemoglobin Metal: Iron.
  • Chalk Metal: Calcium.
  • Chlorophyll Metal: Magnesium.
  • Chocolate Wrapper Metal: Aluminium.
• Case Study:
  • Q: What kind of substance is dough (flour, water, salt)?
  • A: A mixture.
  • Q: Chemical name for common salt and a separation method?
  • A: Sodium chloride; Evaporation.
  • Q: Why is fruit cream heterogeneous?
  • A: Components (different fruits and cream) are not uniformly distributed and are easily recognized.
  • Q: Elements in sugar?
  • A: Carbon, hydrogen, and oxygen.