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    Home / Science lessons / Molecules, Mixtures & Compounds
    • Molecules, Mixtures & Compounds

      Molecules, Mixtures & Compounds

      The universe is made up of atoms, the tiny building blocks of matter. This computer screen is made up of atoms and so are your eyes and the rest of your body! In the last 110 years, scientists have discovered that atoms themselves are made up of even smaller particles: an atom has a central nucleus made up of protons and neutrons, and surrounded by electrons. These parts are so incredibly small that scientists can only believe that they are there, based on the behaviors that atoms display. Electron microscopes allow scientists to "see" highly magnified images of some atoms, but not the smaller particles that form the atom.

      Elements are the simplest substances in nature that cannot be broken down into smaller parts by normal chemical means. They contain only atoms of the same type, ones that have identical chemical properties. There are at least 90 naturally-occurring elements, plus man-made ones. If you look at a periodic table, you'll see the names of each of these elements and some of their properties (such as mass, or how heavy the atom is).

      Molecules & Compounds

      When atoms from different elements are joined together in groups, they form molecules. The atoms in molecules bind together chemically, which means that the atoms cannot be separated again by physical means, such as filtration. The molecule has different properties from the elements from which is was made. A water molecule is not three separate atoms, two hydrogen (H) and one oxygen (O), but it is actually a unique H2O molecule with its own set of distinct properties.

      Like elements that are formed of atoms of the same sort, compounds are formed of molecules of the same sort. The elements can be combined into about 2 million different compounds! Did you know that eggshells are made up of a calcium carbonate compound? And citric acid, which is found in oranges and other citrus fruit, is a compound of carbon, hydrogen, and oxygen atoms. Your kids might find it helpful to do a science research project finding other common compounds around your house. Make hypotheses and do research using a chemistry reference book, web site, or text book to find out the answers.

      There are more carbon compounds than compounds of any other element. Organic compound is the name for the carbon compounds found in all living things.

      Mixtures - A Bit of This and That

      All matter can be classified into two categories: pure substances and mixtures. A pure substance consists of a single element or compound. Iron is formed only of iron (Fe) atoms; table salt is formed only of sodium chloride (NaCl) molecules. A mixture, however, is made up of different compounds and/or elements. When salt is added to water to make saltwater, it becomes a mixture. The salt and water molecules do not combine to form new molecules, but only "mix" together while still retaining their identities. Air is also a mixture, containing just the right amounts of nitrogen, oxygen, and other gases for life on Earth.  

      Not all mixtures have the same composition throughout. Salt water does, but Italian salad dressing does not--the parts separate and are not perfectly blended or homogenous. Mixtures of metals are called alloys (bronze is an alloy of copper and tin); liquid mixtures (such as saltwater) are called solutions.

      The substances that make up a mixture can be separated by physical means because they have different physical properties (such as different melting points) and are not chemically bonded. A mixture can be separated into its parts in a variety of ways, including decantation (letting the sand in a mixture of water and sand settle, and then draining off the water, for example), filtering, and evaporation. You can use a kitchen funnel and coffee filter for filtration, and either use sunlight or low heat for evaporation. Try out these methods on saltwater and a sand and water mixture to see how they work and compare the results. Evaporation will work for both saltwater and sand and water solutions, but filtration will not work for saltwater. Can you think of other examples where a separating method will work for one mixture and not another? Another one to try is lemon juice, a mixture of water and citric acid; what do you think happens when it is boiled? The water evaporates and eventually leaves nothing but citric acid crystals.

      A Chemical By Any Other Name...

      The chemical formula for a molecule tells how many atoms and what types of atoms the molecule is made up of. How many atoms are in a molecule of AlSO4 (aluminum sulfate)? There is one aluminum (Al), one sulfur (S), and four oxygen (O) for a total of six atoms. What is the formula for ammonium chloride? Since it contains one nitrogen (N), four hydrogen (H), and one chlorine (Cl), the chemical formula is NH4Cl. To familiarize your kids with the periodic table and the abbreviations for elements, make a game of answering questions like this. Figure out the number of atoms for CaOH2, FeCl2, K3FeCN6, and the chemical formulas for a compound with two sodium, one calcium, and three oxygen atoms and a compound with one iron and three chlorine atoms.

      Have you ever wondered what to call a chemical compound, such as CaOH2? There are a few general rules that can help you. Start with the name of the first atom, Ca. That's the element calcium. Then, take the next atom and replace the ending with "ide." Oxygen (O) becomes "oxide". As with many rules, though, there are exceptions! In this case, the first part of the name for the last atom, hydrogen (H), is tacked on so that the correct chemical name is calcium hydroxide. Here's one your kids should be able to figure out with just a periodic table: what is the chemical name for NaCl?  

      Prefixes, the part added to the beginning of a word, can help you figure out names and formulas, too. Mono means one, di means two. So a compound with two of the same molecule (represented by a subscript 2) would have "di" before that molecule name. What would you call CO2? How many oxygen atoms are in carbon monoxide? What is the chemical formula for dicarbon dihydride?

      A mole is a measure of chemical substances. There are 602,300 million trillion (6.023 x 1023) atoms or molecules in one mole. A mole of any two elements will have a different mass, but the same number of particles. If you look up gold (Au) on a periodic table, you'll see it has an atomic mass of 107.9. To measure out one mole, you should weigh out 107.9 grams of gold. The atomic mass of sodium (Na), on the other hand, is 23.0 and so one mole is 23 grams of sodium. Both gold and sodium have the same number of particles in one mole, though: 6.023 x 1023.

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    By: Rohit
    Date: Feb 03, 2017

    can u pls help with examples of compounds that can only be separated with heating

    By: maria
    Date: Oct 06, 2015

    this is realy interesting.

    By: Kaylee
    Date: Sep 12, 2015

    Really helps with homework

    By: raven
    Date: Sep 08, 2015

    helpful for homework

    By: T Brewington
    Date: Sep 07, 2015

    Thank you for the information, but as a science teacher for middle school, I would like you to recheck this sentence in your article. 

    “When atoms from different elements are joined together in groups, they form molecules. “

    To be accurate, I believe you need to delete the word “DIFFERENT” since molecules can also be two atoms from the same element combined such as H + H = H2 (2 should be subscript ), a molecule of hydrogen.

    By: izzy
    Date: Jul 12, 2015


    By: tiz
    Date: Mar 28, 2015

    Just excellent.

    By: Oliwia
    Date: Dec 04, 2014

    Hello, i am trying to cite a website in science can you tell who is this by that would really help

    {HST adds:
    Here is the format of a typical online citation when the author is not named:

    “Article name.” Site name. Date Accessed <web address>.

    An example:

    “What is the Scientific Method?” Home Science Tools. Accessed 21 September 2012

    Hope this helps!}