Everything around us is made of atoms: the clothes we wear, the air we breathe, the water we drink, and ourselves. These atoms form roughly 92 naturally occurring elements on Earth, which then form everything we know. The result of these combinations can come in the form of elements, compounds, or mixtures, depending on their atomic makeup. While compounds and mixtures can be separated through a variety of techniques, elements cannot, as they exist in the purest form possible. Introducing students to atoms, elements, compounds, and mixtures will provide them with an important foundation that will help them grasp more complex concepts in chemistry.
Just about everything on Earth is made from combinations of 92 different types of atoms. Matter can be uncombined substances made from one type of atom, known as elements, or combined as compounds or mixtures.
An element is a substance that is made from one type of atom. The periodic table is a chart that organizes all the known elements. Elements have a wide range of different properties from hydrogen, which is a colorless gas, to mercury, which is a liquid metal at room temperature. Not all elements in the periodic table occur naturally on Earth. Some are created in a lab and only exist for fractions of a second.
A compound is a substance made of two or more types of atoms that are chemically bonded together to form molecules. There are billions and billions of different ways to combine the elements to create compounds. Compounds have a definite composition, which can be described using a chemical formula. For example, carbon dioxide has a chemical formula of CO2. This means that each molecule of carbon dioxide is made of one carbon and two oxygen atoms. Unlike mixtures, compounds have a fixed ratio of elements. The bonds in compounds can be difficult to break, and can only be broken through chemical reactions. After a chemical reaction, the molecules of the reactants rearrange to form other substances (products).
A mixture is a combination of substances that are not chemically bonded together. Mixtures can be any combination of elements and/or compounds. Examples of mixtures are sea water, air, powdered iron, powdered sulfur, and most rocks. Mixtures can be separated more easily than compounds. There are many different methods of separating mixtures depending on the properties of the substances in the mixture and whether it is a heterogeneous mixture or a homogeneous mixture like a solution. A solution is a type of mixture that involves a solid/liquid (solute) dissolved in a liquid (solvent). If a substance doesn’t dissolve into another, it is known as insoluble. Unlike compounds, mixtures are not necessarily made of fixed ratios of the component parts.
Filtering is a process which can separate liquids and insoluble solids, like water and sand. In this process, the mixture is poured through a filter, such as filter paper or a strainer, and the insoluble sand gets separated from the liquid. This happens because of a difference in the particle size; the liquid particles are small enough to pass through the filter paper while the solid particles are too big. The solid left behind is known as the residue, and the liquid that passes through the paper is known as the filtrate. Straining works using the same mechanism as filtering, just for larger particles.
Evaporation is another method for separating soluble solids from a liquid. Table salt mixed with water is an example of a salt solution. In the evaporation process, the solution is heated so the water evaporates, leaving salt crystals in the bottom of the container. Water has a lower boiling point than salt, so the water evaporates first.
Distillation can separate mixtures of liquids that have different boiling points. It works in a similar way to evaporation, but the evaporated vapor is collected and condensed back into a liquid. This method works because of the difference of varying boiling points. This method could be used to separate water and ink. Distillation is used in making some alcoholic drinks like whisky and vodka.
Magnetism can also be employed to remove magnetic material from nonmagnetic material. An example of this is sorting iron from other metals at a recycling plant.
Chromatography is a method used to separate some dissolved substances. It is often used for separating dyes and inks because of their differences in solubility. With simple paper chromatography, a spot of the ink or dye is placed near the bottom of a piece of absorbent paper. The paper is then lowered in a container of solvent so the line of the liquid is underneath the ink spot. As the solvent moves up the paper, it takes some of the colored chemical with it. The different chemicals are spread out by different amounts.
The Next Generation Science Standards push the importance of getting students to develop and use models to understand phenomena. In the real world, scientists will make models to aid their understanding of a system or part of a system. Models are used in Science to make predictions and communicate ideas or data to other people. There are a range of activities in this teacher guide that focus on that particular skill. Students will easily be able to create their own models of elements, compounds, and mixtures using the stick-and-ball smart item. This gives you a great opportunity to discuss the limitations of using models, giving students the opportunity to evaluate and refine them.