My science teacher made us read the articles titled Materials all the way to Compounds and on top of that, he had given us more work with a test :(

<h2>Compounds</h2>
<p>A <strong>compound</strong> is a chemical substance that forms when two or more different elements combine in a fixed ratio. Water (\(\text{H}_{2}\text{O}\)), for example, is a compound that is made up of two hydrogen atoms for every one oxygen atom. Sodium chloride (\(\text{NaCl}\)) is a compound made up of one <a data-lesson="redox-reactions" data-subject="chemistry" data-tutorial="reactions-in-aqueous-solution" href="https://app.ulearngo.com/chemistry/reactions-in-aqueous-solution/redox-reactions">sodium</a> atom for every chlorine atom. An important characteristic of a compound is that it has a <strong>chemical formula</strong>, which describes the ratio in which the atoms of each element in the compound occur.</p>
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<h3>Definition: Compound</h3>
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<p>A substance made up of two or more different elements that are joined together in a fixed ratio.</p>
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<p>The figure below might help you to understand the difference between the terms <strong>element</strong>, <strong>mixture</strong> and <strong>compound</strong>. Iron (\(\text{Fe}\)) and sulfur (\(\text{S}\)) are two elements. When they are added together, they form a <em>mixture</em> of iron and sulfur. The iron and sulfur are not joined together. However, if the mixture is heated, a new <strong>compound</strong> is formed, which is called iron sulfide (\(\text{FeS}\)).</p>
<div class="wp-caption alignnone" id="attachment_151086" style="width: 394px"><a href="https://data.ulearngo.com/assets/2b632425-9210-48a8-8770-d1eadfd89441"><img alt="" class="size-full wp-image-151086" height="299" src="https://data.ulearngo.com/assets/2b632425-9210-48a8-8770-d1eadfd89441" width="394"/></a><p class="wp-caption-text">A mixture of iron and sulfur</p></div> <div class="wp-caption alignnone" id="attachment_151087" style="width: 309px"><a href="https://data.ulearngo.com/assets/3fb6916c-a3ce-4dc8-9ed8-6bf4da043673"><img alt="" class="size-full wp-image-151087" height="320" src="https://data.ulearngo.com/assets/3fb6916c-a3ce-4dc8-9ed8-6bf4da043673" width="309"/></a><p class="wp-caption-text">A model of the iron sulfide crystal</p></div>
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<h3>Note:</h3>
<p>The first figure above showed a submicroscopic representation of a mixture. In a submicroscopic representation we use circles to represent different elements. To show a compound, we draw several circles joined together. Mixtures are simply shown as two or more individual elements in the same box. The circles are not joined for a mixture.</p>
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<p>We can also use <a data-lesson="revising-chemical-symbols" data-subject="chemistry" data-tutorial="physical-and-chemical-change" href="https://app.ulearngo.com/chemistry/physical-and-chemical-change/revising-chemical-symbols">symbols</a> to represent elements, <a data-lesson="mixtures" data-subject="chemistry" data-tutorial="classification-of-matter" href="https://app.ulearngo.com/chemistry/classification-of-matter/mixtures">mixtures</a> and compounds. The symbols for the elements are all found on the periodic table. Compounds are shown as two or more element names written right next to each other. Subscripts may be used to show that there is more than one atom of a particular element. (e.g.\(\text{H}_{2}\text{O}\) or \(\text{NH}_3\)). Mixtures are written as: a mixture of element (or compound) A and element (or compound) B. (e.g. a mixture of \(\text{Fe}\) and \(\text{S}\)).</p>
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<h2>Example: Mixtures and Pure Substances</h2>
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<h3>Question</h3>
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<p>For each of the following substances state whether it is a pure substance or a mixture. If it is a mixture, is it homogeneous or heterogeneous? If it is a pure substance is it an element or a compound?</p>
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<p>Blood (which is made up from plasma and cells)</p>
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<p>Argon</p>
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<p>Silicon dioxide (\(\text{SiO}_{2}\))</p>
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<p>Sand and stones</p>
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<h3>Step 1: Apply the definitions</h3>
<p>An element is found on the periodic table, so we look at the <a data-lesson="periodic-table-of-elements" data-subject="chemistry" data-tutorial="atoms-molecules-ions" href="https://app.ulearngo.com/chemistry/atoms-molecules-ions/periodic-table-of-elements">periodic table</a> and find that only argon appears there. Next we decide which are <a data-lesson="compounds-2" data-subject="chemistry" data-tutorial="quantitative-aspects-chemical-change" href="https://app.ulearngo.com/chemistry/quantitative-aspects-chemical-change/compounds-2">compounds</a> and which are mixtures. Compounds consist of two or more elements joined in a fixed ratio. Sand and stones are not elements, neither is blood. But silicon is, as is oxygen. Finally we decide whether the mixtures are homogeneous or heterogeneous. Since we cannot see the separate components of <a data-lesson="medicine-the-buffer-system-in-blood" data-subject="chemistry" data-tutorial="acid-base-equilibria" href="https://app.ulearngo.com/chemistry/acid-base-equilibria/medicine-the-buffer-system-in-blood">blood</a> it is homogeneous. Sand and stones are heterogeneous.</p>
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<h3>Step 2: Write the answer</h3>
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<p>Blood is a homogeneous mixture.</p>
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<p>Argon is a pure substance. Argon is an element.</p>
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<p>Silicon dioxide is a pure substance. It is a compound.</p>
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<p>Sand and stones form a heterogeneous mixture.</p>
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<h2>Optional Activity: Using models to represent substances</h2>
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<p>The following substances are given:</p>
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<p>Air (consists of oxygen, nitrogen, hydrogen, water vapour)</p>
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<p>Hydrogen <a data-lesson="non-ideal-gas-behavior" data-subject="chemistry" data-tutorial="gases" href="https://app.ulearngo.com/chemistry/gases/non-ideal-gas-behavior">gas</a> (\(\text{H}_{2}\))</p>
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<p>Neon gas</p>
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<p>Steam</p>
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<p>Ammonia gas (\(\text{NH}_{3}\))</p>
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<p>Use coloured balls to build models for each of the substances given.</p>
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<p>Classify the substances according to elements, compounds, homogeneous mixtures, heterogeneous mixture, pure substance, impure substance.</p>
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<p>Draw submicroscopic representations for each of the above examples.</p>
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</ol>
<a href="https://data.ulearngo.com/assets/72f7b180-dcd9-4627-8b3e-8f8258dc9335"><img alt="" class="size-full wp-image-151088 alignnone" height="240" src="https://data.ulearngo.com/assets/72f7b180-dcd9-4627-8b3e-8f8258dc9335" width="320"/></a></div>
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Compounds

Chemistry generally explores the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances. 

Chemistry

Explore the arrangement of elements and learn about materials, mixtures, pure substances, and separation techniques, including chemical formulae and important concepts such as periods in the periodic table and chemical properties of groups.


Compounds

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