Learning Objectives Explain the difference between a pure substance and a mixture. Explain the difference between an element and a compound. Explain the difference between a homogeneous mixture and a heterogeneous mixture. When we speak of a pure substance , we are speaking of something that contains only one kind of matter. This can either be one single element or one single compound, but every sample of this substance that you examine must contain exactly the same thing with a fixed, definite set of properties. If we take two or more pure substances and mix them together, we refer to this as a mixture (Figure \(\PageIndex{1}\)). Mixtures can always be separated again into component pure substances,because bonding among the atoms of the constituent substances does not occur in a mixture. Whereas a compound may have very different properties from the elements that compose it, in mixtures the substances keep their individual properties. For example sodium is a soft shiny metal and chlorine is a pungent green gas. These two elements can combine to form the compound, sodium chloride (table salt) which is a white, crystalline solid having none of the properties of either sodium or chlorine. If, however, you mixed table salt with ground pepper, you would still be able to see the individual grains of each of them and, if you were patient, you could take tweezers and carefully separate them back into pure salt and pure pepper. Mixtures fall into two types, based on the uniformity of their composition (Figure \(\PageIndex{1}\)). The first, called a heterogeneous mixture , is distinguished by the fact that different samples of the mixture may have a different composition. For example, if you open a container of mixed nuts and pull out a series of small samples and examine them, the exact ratio of peanuts-to-almonds in the samples will always be slightly different, no matter how carefully you mix them. Common examples of heterogeneous mixtures include dirt, gravel and vegetable soup. In a homogeneous mixture, on the other hand, any sample that you examine will have exactly the same composition as any other sample. Within chemistry, the most common type of homogeneous mixture is a solution which is one substance dissolved completely within another. Think of a solution of pure sugar dissolved in pure water. Any sample of the solution that you examine will have exactly the same ratio of sugar-to-water, which means that it is a homogeneous mixture. Even in a homogeneous mixture, the properties of the components are generally recognizable. Thus, sugar-water tastes sweet (like sugar) and is wet (like water). Unlike a compound, which has a fixed, definite ratio, in a mixture one can vary the amounts of each component. For example, when you add a little sugar to one cup of tea and a lot of sugar to another, each cup will contain a homogeneous mixture of tea and sugar but they will have a different taste. If you add so much sugar that some does not dissolve and stays on the bottom, however, the mixture is no longer homogeneous, it is heterogeneous; you could easily separate the two components (Figure \(\PageIndex{1}\)). Example \(\PageIndex{1}\) Identify each substance as a compound, an element, a heterogeneous mixture, or a homogeneous mixture (solution). filtered tea freshly squeezed orange juice a compact disc aluminum oxide, a white powder that contains a 2:3 ratio of aluminum and oxygen atoms selenium Given : a chemical substance Asked for : its classification Strategy: Decide whether a substance is chemically pure. If it is pure, the substance is either an element or a compound. If a substance can be separated into its elements, it is a compound. If a substance is not chemically pure, it is either a heterogeneous mixture or a homogeneous mixture. If its composition is uniform throughout, it is a homogeneous mixture. Solution A) Tea is a solution of compounds in water, so it is not chemically pure. It is usually separated from tea leaves by filtration. B) Because the composition of the solution is uniform throughout, it is a homogeneous mixture . A) Orange juice contains particles of solid (pulp) as well as liquid; it is not chemically pure. B) Because its composition is not uniform throughout, orange juice is a heterogeneous mixture . A) A compact disc is a solid material that contains more than one element, with regions of different compositions visible along its edge. Hence a compact disc is not chemically pure. B) The regions of different composition indicate that a compact disc is a heterogeneous mixture. A) Aluminum oxide is a single, chemically pure compound . A) Selenium is one of the known elements . Exercise \(\PageIndex{1}\) Identify each substance as a compound, an element, a heterogeneous mixture, or a homogeneous mixture (solution). white wine mercury ranch-style salad dressing table sugar (sucrose) Answer a: homogeneous mixture (solution) Answer b: element Answer c: heterogeneous mixture Answer d: compound Example \(\PageIndex{2}\) How would a chemist categorize each example of matter? saltwater soil water oxygen Solution Saltwater acts as if it were a single substance even though it contains two substances—salt and water. Saltwater is a homogeneous mixture, or a solution. Soil is composed of small pieces of a variety of materials, so it is a heterogeneous mixture. Water is a substance; more specifically, because water is composed of hydrogen and oxygen, it is a compound. Oxygen, a substance, is an element. Exercise \(\PageIndex{2}\) How would a chemist categorize each example of matter? coffee hydrogen an egg Answer a: a homogeneous mixture (solution), assume it's filtered coffee Answer b: element Answer c: heterogeneous mixture. Compounds and Chemical Reactions In chemistry, we refer to compounds substances that result from the combination of two or more components. For example, water is a compound composed of two hydrogen atoms bonded to one oxygen atom. In the following chapter, we will look more closely at how elements are bonded together in compounds, but first we will examine some of the properties of how to make and break compounds. The reaction between hydrogen and oxygen can be shown in what is called a chemical equation . In words, we could write the reaction as: \[\underbrace{\ce{hydrogen + oxygen}}_{\text{reactants}} → \underbrace{\ce{water}}_{\text{product}} \label{reaction}\] The substance(s) to the left of the arrow (hydrogen and oxygen) in a chemical equation are called reactants . A reactant is a substance that is present at the start of a chemical reaction. The substance(s) to the right of the arrow (water) are called products . A product is a substance that is present at the end of a chemical reaction. In the equation above, the hydrogen and oxygen are the reactants that chemically combine to form the water product. There is a standard way of writing chemical equations. The reactants are all written on the left-hand side of the equation, with the products on the right-hand side. An arrow points from the reactants to the products to indicate the direction of the reaction: \[\ce{reactants → products} \nonumber\] When appropriate, a symbol may be written above or below the arrow to indicate some special circumstance. The symbol “Δ” is often used to indicate that the reaction is to be heated. The presence of the arrow also indicates that the reaction goes in one direction under the conditions indicated. There are reactions which can be easily reversed with the arrow pointing to the left and the nature of the products and reactant switch, but we will not take those up right now. \[\ce{products <- reactants } \nonumber\] There are a wide variety of reactions possible: elements may form compounds (e.g., Reaction \ref{reaction}), compounds may form elements (water will break down in the presence of an electric current to form hydrogen gas and oxygen gas) or compounds may combine, break apart, or rearrange to form new materials. Example \(\PageIndex{3}\) In the reaction \[\text{sodium} + \text{water} → \text{sodium hydroxide} + \text{hydrogen} \nonumber\] what are the reactants? what are the products? Solution Since sodium and water are the left side of the arrow, they are the reactants Since sodium hydroxide and hydrogen are on the right side of the arrow, they are the products Summary Matter can be classified into two broad categories: pure substances and mixtures. A pure substance is a form of matter that has a constant composition and properties that are constant throughout the sample. Mixtures are physical combinations of two or more elements and/or compounds. Mixtures can be classified as homogeneous or heterogeneous. Elements and compounds are both examples of pure substances. Compounds are substances that are made up of more than one type of atom. Elements are the simplest substances made up of only one type of atom. A chemical equation describes a chemical reaction. A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Reactants are starting materials and are written on the left-hand side of the equation. Products are the end-result of the reaction and are written on the right-hand side of the equation.