V

Elements, Compounds, and Mixtures

pure substances:

  • a sample of matter that has definite chemical and physical properties

 

elements:

  • pure substances that cannot be separated into simpler substances by physical or chemical means

 

compounds:

  • pure substance composed of two or more different elements joined by chemical bonds

    • made of elements in a specific ratio that is always the same

    • has a chemical formula

    • can only be separated by chemical means, not physically

 

mixtures:

  • a combination of two or more pure substances that are not chemically combined

  • substances held together by physical forces not chemical

  • no chemical change takes place

  • each item retains its properties in the mixture

  • can be separated physically

 

types of mixtures:

  • homogeneous: molecules or mixed up in an even distribution

  • heterogeneous: molecules are not mixed up in an even distribution

 

is it uniform throughout?

  • If no, the matter is a heterogeneous mixture

    • considered the least mixed

    • does not appear the same throughout

    • particles are large enough to be seen and separated from the mixture

  • if yes, the matter is homogeneous

    • looks the same throughout

 

heterogeneous mixtures:

  • suspensions: a mixture in which particles are dispersed in liquid or gas and will eventually settle out

    • particles can scatter light

    • can be filtered out using a filter

 

can it be separated by physical means?

  • If yes, the matter is a homogeneous mixture or solution

    • appears the same throughout

    • well-mixed

    • particles that make up the mixture are very small and not easily recognizable

 

homogeneous mixtures:

  • solutions: a well mixed mixture

    • appears to be a single substance

  • solute: the substance being dissolved

  • solvent: the substance in which the solute is being dissolved

    • water is considered a universal solvent

    • particles do not scatter light

  • colloids: a mixture of tiny particles that are bigger than those in a solution but smaller than in a suspension

    • do not settle out overtime

    • scatter light

 

separation techniques

 

filtration:

  • used to separate heterogeneous mixtures composed of solids and liquids

  • uses a porous barrier to separate the solid from the liquid

  • liquid passes through leaving the solid in the filter paper

  • can be used to separate an insoluble substance (residue) from a soluble substance (filtrate)

 

distillation:

  • used to separate homogeneous mixtures

  • based on differences in boiling points of substances involved

  • evaporation can be used to separate a solute from the solvent in a solution

 

crystallization:

  • results in the formation of solid particles from a solution containing the dissolved substance

  • as one substance evaporates the dissolved substance comes out a solution and collects as crystals

 

centrifugation:

  • spin sample very rapidly: denser materials go to the bottom

 

chromatography:

  • separation occurs because various components travel at different rates

  • separate components of a mixture based on ability of each component to be drawn across the surface of another material

 

decantation:

  • done to separate particles from a liquid by allowing the solids to settle to the bottom of the mixture and pouring off the particle free part of the liquid