Lecture 3: Classifying Matter

Lecture 3 : Classifying Matter We can categorize matter into multiple groups. Two wide categories are mixtures and pure materials. A pure material possesses a constant composition. All specimens of a pure material have precisely the same composition and characteristics. Any sample of sucrose (table sugar) comprises 42.1% carbon, 6.5% hydrogen, and 51.4% oxygen by mass. Any sample of sucrose also has the same physical properties, like melting point, color, and sweetness, irrespective of the source from which it is isolated. We can divide pure materials into two classes: elements and compounds. Pure substances that can not be broken down into simpler substances by chemical alterations are called elements. Iron, silver, gold, aluminum, sulfur, oxygen, and copper are familiar examples of the more than 100 known elements, of which around 90 occur naturally on the earth, and two dozen or so have been created in laboratories. Pure materials that can be broken down by chemical alterations 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 . When heated in the absence of air, the compound sucrose is broken down into the element carbon and the compound water. (The initial stage of this process, when the sugar is Becoming brown, termed caramelization—this is what gives the distinctive sweet and nutty taste to caramel apples, caramelized onions, and caramel). Silver(I) chloride is a white solid that can be decomposed into its elements, silver and chlorine, via absorption of light. This attribute is the base for the use of this compound in photographic films and photochromic spectacles (those with lenses that darken when exposed to light). The characteristics of combined elements differ from those in the free, or uncombined, condition. For instance, white crystalline sugar (sucrose) is a compound resulting from the chemical combination of the element carbon, which is a black solid in one of its uncombined forms, and the two elements hydrogen and oxygen, which are colorless gases when uncombined. Free sodium, an element that is a soft, lustrous, metallic solid, and free chlorine, an element that is a yellow-green gas, combine to form sodium chloride (table salt), a compound that is a white, crystalline solid. A mixture is composed of two or more kinds of matter that can be present in differing amounts and can be separated by physical alterations, such as evaporation .A mixture with a composition that varies from point to point is termed a heterogeneous mixture. Italian dressing is an instance of a heterogeneous mixture , Its composition can vary because we can make it from differing quantities of oil, vinegar, and herbs. It is not the same from point to point throughout the mixture—one drop may be primarily vinegar, whereas a different drop may be mostly oil or herbs because the oil and vinegar separate and the herbs settle. Additional examples of heterogeneous mixtures are chocolate chip cookies (we can see the separate bits of chocolate, nuts, and cookie dough) and granite (we can see the quartz, mica, feldspar, and more). A homogeneous mixture, also called a solution, exhibits a uniform composition and appears visually the same throughout. An example of a solution is a sports drink, consisting of water, sugar, coloring, flavoring, and electrolytes mixed together uniformly. Each drop of a sports drink tastes the same because each drop contains the same amounts of water, sugar, and other components. Note that the composition of a sports drink can vary—it could be made with somewhat more or less sugar, flavoring, or other components, and still be a sports drink. Other examples of homogeneous mixtures include air, maple syrup, gasoline, and a solution of salt in water. While there are slightly more than 100 elements, their various combinations give rise to tens of millions of chemical compounds. Each compound is characterized by a unique composition and distinct chemical and physical properties, allowing us to differentiate it from other compounds. Furthermore, elements and compounds can be combined in countless ways to create a vast array of mixtures.