Chemical combination

• Properties of matter:

  • Substances can be identified by the properties they possess
  • Physical properties are associated with physical changes
  • Common physical properties include boiling point, melting point, density, hardness, malleability, crystalline form, color, odor, and taste
  • Chemical properties are involved when matter undergoes a change to form new substances
• Physical changes include:

  • Changes in states of matter like melting, freezing, vaporization, liquefaction, and sublimation
  • Separation of mixtures by evaporation, distillation, fractional distillation, sublimation, and crystallization
  • Magnetization and demagnetization of iron rods
• Chemical changes are not easily reversed and involve the formation of new substances
• Examples of chemical changes:

  • Dissolution of metals and limestone in acids
  • Rusting of iron
  • Addition of water to quicklime (slaking of lime)
  • Fermentation and decay of substances
  • Changes in an electrochemical cell
• Elements can be classified as metals, metalloids, and non-metals

  • Elements are building units of compounds
  • Elements cannot be split into simpler units by ordinary chemical processes
• Characteristics of physical changes:

  • Easily reversible
  • No new substances formed
  • No change in mass
  • Involves minimal heat changes
• Characteristics of chemical changes:

  • Not easily reversible
  • New substances always produced
  • Change in mass
  • Involves a considerable amount of heat change
• Metals:

  • Malleable, ductile, sonorous
  • Great tensile strength
  • Lustrous
  • Good conductors of heat and electricity
• Non-metals:

  • Not malleable, ductile, sonorous
  • Brittle, soft, not hard
  • Non-lustrous
  • Generally non-conductors or poor conductors
• Chemical properties of metals:

  • Tend to ionize by losing electrons
  • Form basic oxides with oxygen
  • Some can displace hydrogen from acids
• Chemical properties of non-metals:

  • Tend to ionize by gaining electrons
  • Form acidic oxides with oxygen
  • Cannot displace hydrogen from acids
• Compounds:

  • Formed as a result of a chemical change
  • New substance with different properties from its component elements
  • Component elements present in a fixed ratio by mass
• Mixtures:

  • Contains two or more constituents that can be separated by physical methods
  • Constituents retain their individual identities
  • Examples include air, which is a mixture of gases
• Characteristics of a mixture:

  • Consists of two or more different substances
  • Constituents retain their individual properties
  • Can be separated using physical means
• Differences between a mixture and a compound:

  Mixture:

  • Can be homogeneous or heterogeneous
  • Constituents are not chemically bound together
  • Can be separated by physical means
  • Components can be added in any ratio by mass
  • Properties are the sum of its individual constituents

  Compound:

  • Always homogeneous
  • Component elements are chemically bound
  • Cannot be separated by physical means
  • Components are present in a fixed ratio by mass
  • Can be represented by a chemical formula
  • Properties differ from its component elements
• Separation Techniques for Mixtures:

  • Sieving: separates solids of different sizes
  • Magnetic separation: separates magnetic from non-magnetic substances
  • Sublimation: separates solids that sublime from others
  • Decantation, Filtration: separates insoluble solids from liquids
  • Centrifugation: separates components based on density
  • Evaporation: separates a solvent from a solution
  • Crystallization: separates solutes from a solution
  • Distillation: separates miscible liquids
  • Chromatography: separates solutes from a solution based on their different affinities for a mobile and stationary phase
• Evaporation can be used to recover a solid solute from a solution, where the solvent is usually sacrificed
• A water-bath or a sand-bath is used to bring about a steady rate of evaporation
• In the evaporation process, the solute required is left behind in the dish while the solvent escapes into the air as vapor
• Evaporation is not suitable for salts easily destroyed by heating
• Evaporation is used in salt-making industries
• Crystallization is used to separate salts that decompose easily on heating from their solutions
• Salt crystals obtained through crystallization are pure and usually contain water of crystallization
• To induce crystal formation in crystallization:

  • Crystals of the same salt are added into the solution to serve as seeds; or
  • The inside of the vessel containing the solution is scratched
• Crystallization is used in industries where purity of the product is important, such as in the manufacture of drugs and in sugar production
• Fractional crystallization is used to separate two or more solid solutes present in the same solution in roughly equal amounts
• The solubilities of different solutes in the given solvent must differ at different temperatures for fractional crystallization to work
• Precipitation is used to separate a solid when it is dissolved in one of two different miscible liquids with different solubilities
• Distillation is used to recover a solvent from a solution
• Distillation is used for the separation of miscible liquids with widely different boiling points
• Fractional distillation is used to separate a mixture of two or more miscible liquids into its component parts or fractions
• In fractional distillation, the fractions distil over in ascending order of their boiling points
• The apparatus for fractional distillation includes a fractionating column packed with glass beads for separation
• Distillation is used in gin distilleries and water distilleries for the manufacture of gin and distilled water respectively
• The separating funnel method is used to separate non-polar and polar solvents that do not mix together
• Chromatography uses a solvent moving over a porous, adsorbent medium to separate a mixture of solutes
• Ascending paper chromatography is a common method that uses a strip of chromatographic or filter paper to separate solutes
• In chromatography, the rate at which solutes move up the paper strip is determined by two processes: adsorption of solutes by the paper and dissolution of solutes in the solvent
• As the solvent ascends the paper, it dissolves a mixture of solutes. Some solutes are strongly adsorbed by the paper, making it difficult for the solvent to dissolve them
• Solutes that are weakly adsorbed by the paper are easily redissolved by the ascending solvent and travel quickly up the paper strip