University:
Winston-Salem Forsyth County Schools
Course:
AP Chemistry
Academic year:

2014
Views:

332

Pages:

11
Author:

Nicolas Hartman

What are Substances? Chemistry Unit - 8th Grade Science What are “Substances”?  Substances can be identified as either…  Elements  Compounds  Mixtures So, what is a substance? A substance cannot be further broken down or purified by physical means. Substance – matter of a particular kind.  Each substance has its own characteristics and properties that are different from any other substance. Characteristics of Pure Substances Fixed Composition Cannot be separated into simpler substances by physical methods. (Physical Changes) Can only be changed in identity and properties by chemical methods. Properties do not vary. What is a Pure Substance? Compounds  Composed of two or more types of elements that are chemically combined.  Always in a definite ratio  Examples: H2O; CO2 Elements  Cannot be decomposed into simpler substances by chemical changes.  Examples: Hydrogen (H), Oxygen (O) What is a Mixture? Mixtures are two or more substances that are not chemically combined. Mixtures do not… Have constant boiling points. Have constant melting points. Characteristics of Mixtures Variable Composition Components retain their characteristic properties. May be separated into pure substances by physical methods. Mixtures of different compositions may have widely different properties. Homogeneous Mixtures  Homogeneous Mixtures – look the same throughout but can be separated by physical means.  Examples: Air, Steel, Clear Salt-Water. What are Solutions?  Solutions – Homogeneous mixtures that do not scatter light and are created when something is dissolved in pure water.  Can be easily separated through distilation or evaporation.  Examples: Sugar Water, Salt Water Heterogeneous Mixtures  Heterogeneous Mixtures – composed of large pieces that are easily separated by physical means. (Density, Polarity, Metallic properties)  Will not have the some composition throughout.  Components are distinguishable. Law of Conservation of Matter Law of Conservation of Mass  “Matter can not be created, nor can it be destroyed.”  Matter can be transformed into something different as long as you start and finish with the same amount.  Example: If I place log into a fire that has the mass of 50 kg, how much mass should the ash have after according to this law?  Answer: 50 kg