Pure Substances
Chemistry: IB Diploma Higher Level, topic 1.6.2
Pure Substances
Matter can be classified into several categories. Two broad categories are mixtures and pure substances. A pure substance has a constant composition.
What are pure substances?
- All specimens of a pure substance have exactly the same makeup and properties.
- Any sample of sucrose (table sugar) consists of 42.1% carbon, 6.5% hydrogen, and 51.4% oxygen by mass.
- Any sample of sucrose also has the same physical properties, such as melting point, color, and sweetness, regardless of the source from which it is isolated.
Pure substances - elements
- Pure substances may be divided into two classes: elements and compounds.
- Pure substances that cannot be broken down into simpler substances by chemical changes are called elements.
- Iron, silver, gold, aluminum, sulfur, oxygen, and copper are familiar examples of the more than 100 known elements.
Compounds
- Pure substances that can be broken down by chemical changes are called compounds.
- This breakdown may produce either elements or other compounds, or both.
- Mercury(II) oxide, an orange, crystalline solid, can be broken down by heat into the elements mercury and oxygen.
Compounds - 2
- The properties of combined elements are different from those in the free, or uncombined, state.
- For example, free sodium is an element that is a soft, shiny, metallic solid. Free chlorine is an element that is a yellow-green gas.
- Sodium and chlorine combine to form sodium chloride (table salt), a compound that is a white, crystalline solid.
Effects of Impurities in a Sample
Chemically pure substances have exact and specific melting and boiling points. Adding impurities affects these values.
Decreases melting point
Increases boiling point
Reduces range of temperatures
- Reduces the range of temperatures at which the sample melts or boils.
Test yourself on Pure Substances
More impurities in a sample will...
What does it mean if a solid melts over a range of temperatures, instead of at one exact point?
How do impurities affect the melting and boiling points of samples?