Classification of Matter

Concept One: Homogeneous Substances A homogeneous substances looks to a person’s eyes as if it is uniform throughout, appearing to be of one substance.

Concept Two: Heterogeneous Substances A heterogeneous substances looks to a person’s eyes as if it is made up of two or more parts.

Concept Three: A Pure Substance A pure substance has properties that are fixed and exact which do NOT change over time or from sample to sample of the same substance. Sodium is soft, silvery in color, melts at 97.8 celsius, conducts electricity well and explodes when placed in water.

Concept Four: Impure Substances An impure substance has variable (changing) properties from sample to sample of the substance. The properties of an impure substance are NOT FIXED (melting point/boiling point varies, density varies, color varies etc.) The color of a tea solution can vary as can its strength. Even though a substance LOOKS uniform throughout (homogeneous), it is NOT NECESSARILY PURE!

Heating and Cooling Curves of Pure and Impure Substances The heating/cooling curve of a pure substance always has plateaus or flat sections where it melts or boils because it melts/boils at fixed temperatures whereas impure substances melt/boil at variable temperatures.

Every Pure Substance has Unique, Fixed Melting and Boiling Points

Pure substances are distinguished by their physical and chemical properties Physical properties are properties that substances have by themselves (Like color, density, electrical conductivity, melting point, boiling point) Chemical properties are properties that substances have when they are combined with other substances (Like fizzing with an acid, solubility with water etc.)

Physical and Chemical Changes A physical change is a change in which a substance retains its identity (becomes itself in another form) and usually can be easily reversed. A chemical change (reaction) is a change in which new substances are formed with different physical and chemical properties from the original substance(s).

A Classification System For Matter

Mixtures Mixtures are impure substances with variable properties. The four common types of mixtures are mechanical mixtures, suspensions, colloids and solutions.

Mechanical mixtures are impure, heterogeneous substances made up of different parts visible to the unaided eye.

The Tyndall Effect When light passing through a substance is visible from the side (due to particles big enough to scatter the light), this is called the Tyndall Effect. The Tyndall Effect is used to classify mixtures

The Tyndall Effect is only observed when the particles in suspension are large enough to scatter light sideways

Filtration Filtration removes large suspended particles. Being able to remove parts of a mixture by filtration is a way to distinguish different kinds of mixtures.

Distillation A Liquid mixture can be heated and the different liquids in the mixture will boil off as vapours at different temperatures, leaving dissolved solids as a residue when all the liquids have been vaporized.

Commercial Distillation Distillation is a technique used to separate components of mixtures in many commercial operations like making brandies or whiskeys or, in separating crude oil into its parts.

Chromatography Components of mixtures can be separated by chromatography in which a sample is placed in a solvent which dissolves and moves mixture parts at different rates, separating them.

Ink Chromatography

Solutions are impure homogeneous mixtures that are usually clear (may be colored). Solutions do NOT show the Tyndall Effect because their particles are too small. Solutions may be separated with distillation and with chromatography. Solutions can NOT be separated by filtration.

Alloys are solutions of two or more metals Brass is an alloy of zinc and copper. Bronze is an alloy of tin and copper. Stainless steel is an alloy of iron, carbon, chromium or nickel.

Colloids – Can’t be Separated by Filtration Colloids are impure and homogeneous. They can be clear, cloudy or opaque mixtures that have large enough particles to show the Tyndall Effect.

Suspensions Suspensions are impure heterogeneous/homogeneous mixtures whose parts can be separated by filtration. On standing, parts of suspensions often settle to the bottom because their particles are large enough to be pulled down by gravity.

Centrifuges and Suspensions Centrifuges spinning at high speeds will separate suspensions.

Classification of Impure (Mixture) and Pure Substances

Pure Substances: Elements An element is a pure homogeneous substance that can NOT be broken down into a simpler substance by chemical or physical means. An element is made up of one kind of atom. If it has molecules, the molecules are combinations of one kind of atom only.

Two Classes of Elements: Metals and Nonmetals Most elements are metals that typically are shiny, malleable (mold like pasticine), ductile (can be drawn into wires) and good conductors of heat and electricity. Some elements are nonmetals which typically are dull, crumbly, poor conductors of heat and electricity, and may be gases.

Compounds A compound is a homogeneous pure substance (fixed properties) that is made up of molecules which have more than one kind of atom. Compounds CAN be broken down into simpler pure substances (elements)

Compounds can be broken down into simpler elements. When orange mercuric oxide is heated, gaseous oxygen and liquid mercury are formed.

Two Main Classes of Compounds: Ionic and Covalent Ionic compounds are made up of positive and negative ions. These compounds are formed from metallic atoms and nonmetallic atoms when the metallic atoms give one or more electrons to the nonmetallic atoms. Covalent compounds are made up of neutral molecules formed when nonmetallic atoms share electrons with other nonmetallic atoms.

Particle Patterns in Substances

What would you classify foggy air as?

What would you classify paradichlorobenzene as? PDB breaks down into carbon, chlorine and hydrogen. Cooling curve of PDB shows a long flat line at 52.4 Celsius

The Law of Definite Proportions When elements combine to form a compound, they combine in a definite proportion by weight.

When carbon combines with oxygen, two compounds are possible. In one the weight ratio is 16:12 (1.33:1) and in the other, 32:12 (2.67:1)