The word oxygen may refer to an element or a simple substance. We will clarify the difference and learn experimental methods for detecting elements.
Basic Knowledge
An element is a concept representing a kind of basic component that makes up a substance. For example, in the statement “water (H2O) is made of the elements hydrogen and oxygen,” “oxygen” refers to the element.
In contrast, a simple substance refers to a specific substance (O2, H2, Fe, and so on). In the statement “oxygen was collected in a gas jar,” “oxygen” refers to the simple substance (O2).
At present, about 118 kinds of elements are known. Each element is represented by an element symbol (H, O, Na, Fe, and so on). A representative method for identifying elements present in an experiment is the flame test:
Li = red, Na = yellow, K = violet, Cu = blue-green, Ca = orange-red, Sr = crimson, Ba = yellow-green.
Element (a kind of basic component that makes up a substance; about 118 kinds)
Element symbol (an alphabetic symbol representing an element; H, C, and so on)
Simple substance (a specific substance consisting of one kind of element; O2, Fe)
Flame test (a phenomenon in which a metal shows a characteristic color when placed in a flame; Li is red)
Precipitation reaction (a precipitate forms when specific ions combine; Cl-+Ag+→AgCl white)
Allotrope (a simple substance made of the same element but having different properties; O2 and O3, diamond and graphite)
Further Study (Background and Significance)
Allotropes are an important topic in the unit on elements. Representative examples:
・Carbon C: diamond, graphite, fullerene (C60), and carbon nanotubes.
・Oxygen O: oxygen O2 and ozone O3.
・Sulfur S: rhombic sulfur, monoclinic sulfur, and plastic sulfur.
・Phosphorus P: yellow phosphorus (white phosphorus) and red phosphorus.
Although they consist of the same element, their properties differ greatly because the ways in which their atoms are bonded differ. For example, diamond is hard and an electrical insulator, while graphite is soft and electrically conductive.
Flame tests are also used to color fireworks, combining Sr (crimson), Ba (yellow-green), Cu (blue-green), and other elements. Precipitation reactions are essential for detecting ions. A standard method is to use silver nitrate for chloride ions (white AgCl precipitate) and barium chloride for sulfate ions (white BaSO4 precipitate).
- Element = a kind of basic component; simple substance = a specific substance
- There are about 118 element symbols
- Flame test: Li red, Na yellow, K violet, Cu blue-green, Ca orange-red, Sr crimson, Ba yellow-green
- Allotrope = a simple substance made of the same element but with a different structure
- Carbon allotropes = diamond, graphite, and fullerene
- Elements are detected through precipitation reactions (Cl- and Ag+)
- Allotropes have greatly different properties (diamond and graphite)
Points to Note (Easily Confused)
① Strictly distinguish an element (a kind of component) from a simple substance (a substance). Judge from the context. ② Do not confuse allotropes (simple substances with different structures) with isomers (compounds with different structures). ③ The flame-test colors “Sr = crimson” and “Ca = orange-red” are similar but different. ④ In precipitation reactions, select different reagents for detecting anions and cations.
Practice
- What color is produced in the flame test for sodium?
- Name three or more allotropes of carbon.
- What reagent should be added to detect chloride ions Cl-?