In chemistry, substances and objects are strictly distinguished. In addition, substances are organized by dividing them into pure substances and mixtures.
Basic Knowledge
A substance refers to a material itself, such as glass, water, or iron, whereas an object refers to a specific item with a shape, such as a cup, bottle, or spoon.
From the viewpoint of composition, substances are divided into pure substances, which consist of only one kind of component, and mixtures, in which two or more pure substances are combined. Pure substances are further divided into elements, which consist of one kind of element (O2, Fe, Cu), and compounds, in which two or more elements are chemically bonded (H2O, CO2, NaCl).
Most substances around us are mixtures. Typical examples include air (N2, O2, Ar, and so on), seawater (water and salts), and petroleum (a mixture of hydrocarbons).
Substance (the material itself that makes up an object, such as glass, iron, or water)
Object (something with a specific shape, such as a cup or nail)
Pure substance (consists of one kind of substance, such as water or sodium chloride)
Mixture (a combination of several substances, such as air, seawater, or petroleum)
Element (a pure substance consisting of one kind of element, such as O2, N2, or Fe)
Compound (a pure substance in which two or more elements are bonded, such as H2O or CO2)
Further Study (Background and Significance)
Pure substances have unique physical properties. For example, water (H2O) has a definite melting point of 0℃ and boiling point of 100℃ under 1 atm, whereas a mixture such as seawater does not have clearly fixed melting and boiling points (boiling-point elevation and freezing-point depression occur).
Separating a mixture into its component pure substances is called separation. Representative methods include filtration, distillation, recrystallization, extraction, chromatography, and fractional distillation. For example, fractional distillation, which separates crude oil according to differences in temperature, is used to obtain gasoline, kerosene, and light oil, while evaporation to dryness is used to obtain salt from seawater. Understanding mixtures is the starting point of chemistry and the foundation of all analytical procedures.
- Substance ≠ object (the difference between material and something with a shape)
- Pure substance = one kind of substance; mixture = several kinds
- Pure substances = elements + compounds
- An element consists of one kind of element (O2, Fe); a compound consists of two or more elements (H2O)
- Pure substances have unique melting and boiling points
- Examples of mixtures = air, seawater, petroleum, milk, and alloys
- Separation methods = filtration, distillation, extraction, recrystallization, and others
Points to Note (Easily Confused)
① An element (a kind of component) and a simple substance (a substance) are different. Whether “oxygen” means an element or a simple substance depends on the context. ② A compound and a mixture are completely different. A compound is one substance whose components are joined by chemical bonds, whereas a mixture is only physically combined. ③ Air is a mixture (N2, O2, Ar, and so on), while water is a compound (H2O). ④ Alloys (stainless steel, bronze, and brass) are treated as mixtures of metals.
Practice
- Is water (H2O) an element, compound, or mixture?
- Is air an element, compound, or mixture?
- Describe the greatest difference between a pure substance and a mixture from the viewpoint of physical properties.