[{"data":1,"prerenderedAt":202},["ShallowReactive",2],{"topic-classification-of-matter":3},{"topic":4,"prev":185,"next":193},{"id":5,"slug":6,"title":7,"shortTitle":7,"lesson":5,"lastAlignmentAudit":8,"category":9,"description":10,"metaDescription":11,"objectives":12,"conceptSections":31,"workedExample":108,"oerSources":118,"conceptSummary":121,"selfStudyQuestions":122,"relatedTopicSlugs":145,"relatedElements":148,"relatedReferences":153,"howTo":154,"conceptProbe":163,"practiceTeaser":167,"gatedContent":175,"seoKeywords":178,"objectiveCount":184},2,"classification-of-matter","Classification of Matter","2026-07-15","fundamentals","Understand the classification of matter: elements, compounds, mixtures, and the states of matter.","Learn about matter classification: elements, compounds, homogeneous and heterogeneous mixtures, and physical vs chemical changes. Free chemistry study guide.",[13,16,19,22,25,28],{"id":14,"text":15},"2.1","Classify matter as a pure substance (element or compound) or mixture",{"id":17,"text":18},"2.2","Distinguish between homogeneous and heterogeneous mixtures",{"id":20,"text":21},"2.3","Describe the macroscopic properties of solids, liquids, and gases",{"id":23,"text":24},"2.4","Differentiate between physical and chemical properties",{"id":26,"text":27},"2.5","Differentiate between physical and chemical changes",{"id":29,"text":30},"2.6","Identify extensive and intensive properties and give examples of each",[32,44,56,68,80,92,104],{"heading":33,"content":34,"relatedObjectives":35,"deepDive":37},"States of Matter: Solids, Liquids, and Gases","\u003Cp>\u003Cstrong>Matter\u003C/strong> is anything that has mass and occupies space. On Earth, it exists primarily in three states:\u003C/p>\u003Cul>\u003Cli>\u003Cstrong>Solids\u003C/strong> have a definite shape and a definite volume. Their particles are packed tightly in fixed positions and vibrate in place. Examples: ice, iron, table salt.\u003C/li>\u003Cli>\u003Cstrong>Liquids\u003C/strong> have a definite volume but take the shape of their container (with a flat or slightly curved upper surface due to gravity). Their particles are close together but can slide past one another. Examples: water, mercury, ethanol.\u003C/li>\u003Cli>\u003Cstrong>Gases\u003C/strong> have neither definite shape nor definite volume — they expand to fill their container completely. Their particles are far apart and move freely. Examples: oxygen, nitrogen, carbon dioxide.\u003C/li>\u003C/ul>\u003Cp>A fourth state, \u003Cstrong>plasma\u003C/strong>, occurs in stars, lightning, plasma torches, and other sufficiently energetic environments. Plasma is a gas-like state containing appreciable numbers of electrically charged particles.\u003C/p>\u003Cp>Important distinction: \u003Cstrong>mass\u003C/strong> measures the amount of matter in an object and does not change with location, while \u003Cstrong>weight\u003C/strong> measures the gravitational force on that mass and varies (e.g., less on the Moon).\u003C/p>",[36],12,[38,41],{"label":39,"body":40},"Why this matters: shipping a gas across an ocean","\u003Cp>Natural gas is impractical to ship overseas in tanker vessels while it remains a gas: a tanker would carry relatively little substance for its volume. Cool it to about \u003Cspan class=\"nowrap\">−162 °C\u003C/span> and it condenses into a liquid (LNG) occupying roughly 1/600 of the volume, so one ship can carry far more natural gas. Entire industries are built on moving substances between states: liquefied gases, molten metals, freeze-dried food. The state of matter is not a trivia fact; it is a lever engineers pull constantly.\u003C/p>",{"label":42,"body":43},"Why your mass survives a trip to the Moon","\u003Cp>An astronaut with a mass of \u003Cspan class=\"nowrap\">80 kg\u003C/span> has that same mass on Earth, on the Moon, and floating in orbit: the amount of matter does not change with location. Weight does change, because it is the gravitational force acting on that mass (about one sixth as strong on the Moon).\u003C/p>\u003Cp>The instruments tell the story: a beam balance compares your sample against reference masses, so gravity pulls on both sides equally and it reads the same anywhere. A spring scale measures force directly, so it would read about one sixth as much on the Moon. This is why chemistry uses balances, and why we say a sample is “massed,” not “weighed,” when precision matters.\u003C/p>",{"heading":45,"content":46,"relatedObjectives":47,"deepDive":49},"Classifying Matter: Elements, Compounds, and Mixtures","\u003Cp>All matter can be classified using a simple decision tree:\u003C/p>\u003Col>\u003Cli>Is it a \u003Cstrong>pure substance\u003C/strong> (constant composition) or a \u003Cstrong>mixture\u003C/strong> (variable composition)?\u003C/li>\u003Cli>If pure: is it an \u003Cstrong>element\u003C/strong> (cannot be broken down further by chemical means) or a \u003Cstrong>compound\u003C/strong> (two or more elements chemically combined in fixed proportions)?\u003C/li>\u003Cli>If a mixture: is it \u003Cstrong>homogeneous\u003C/strong> (uniform composition at the scale being considered) or \u003Cstrong>heterogeneous\u003C/strong> (distinct regions or phases)?\u003C/li>\u003C/ol>\u003Cp>\u003Cstrong>Elements\u003C/strong> are the simplest form of pure substance — iron, gold, oxygen, carbon. About 90 occur naturally. Each element is made of only one type of atom.\u003C/p>\u003Cp>\u003Cstrong>Compounds\u003C/strong> are pure substances made of two or more elements in fixed proportions, joined by chemical bonds. Water (H₂O) is always 2 hydrogen : 1 oxygen. Compounds can be broken into simpler substances by chemical changes, but not by physical ones. Crucially, a compound’s properties differ from those of its constituent elements — sodium is a reactive metal and chlorine is a toxic gas, while sodium chloride is a familiar crystalline solid.\u003C/p>",[48],10,[50,53],{"label":51,"body":52},"Why this matters: one extra oxygen changes everything","\u003Cp>Water (H₂O) and hydrogen peroxide (H₂O₂) are built from the same two elements. The extra oxygen atom in each hydrogen peroxide molecule gives it very different chemical properties: water is safe to drink, dilute hydrogen peroxide solutions are used in products such as disinfectants and bleaches, and concentrated hydrogen peroxide is hazardous. This is the law of definite proportions doing real work: a compound is not defined by \u003Cem>which\u003C/em> elements it contains but by their exact fixed ratio. Change the ratio and you have a different substance with different properties, not a stronger or weaker version of the same one.\u003C/p>",{"label":54,"body":55},"Common mistake: calling air a compound","\u003Cp>Air contains nitrogen, oxygen, argon, and carbon dioxide, so students often file it next to compounds like water. It is not one: the gases in air are not chemically bonded to each other, and air’s composition varies from place to place (humid air over the ocean carries far more water vapor than desert air; city air holds more carbon dioxide and pollutants than a forest). Water, by contrast, is exactly 2 hydrogen : 1 oxygen everywhere in the universe.\u003C/p>\u003Cp>A quick test: does it have a chemical formula? Water is H₂O; there is no formula for air, because a mixture has no fixed ratio to write down.\u003C/p>",{"heading":57,"content":58,"relatedObjectives":59,"deepDive":61},"Homogeneous vs. Heterogeneous Mixtures","\u003Cp>Unlike pure substances, \u003Cstrong>mixtures\u003C/strong> contain two or more substances that are physically (not chemically) combined. Their composition can vary, and their components can be separated by physical means (filtering, evaporation, distillation).\u003C/p>\u003Cp>\u003Cstrong>Heterogeneous mixtures\u003C/strong> contain physically distinct regions or phases. Usually you can see the separate components directly; but in a \u003Cstrong>colloid\u003C/strong> — such as milk — the dispersed particles are so small that the mixture looks uniform to the naked eye, and its two-phase (heterogeneous) nature is only apparent microscopically. Examples:\u003C/p>\u003Cul>\u003Cli>Italian dressing (oil and vinegar layers)\u003C/li>\u003Cli>Granite (visible grains of quartz, mica, feldspar)\u003C/li>\u003Cli>Chocolate chip cookies (distinct chips, dough, nuts)\u003C/li>\u003Cli>Muddy water, fresh-shaken salad dressing, milk (a colloid of fat globules dispersed in water)\u003C/li>\u003C/ul>\u003Cp>\u003Cstrong>Homogeneous mixtures\u003C/strong> (also called \u003Cstrong>solutions\u003C/strong>) look uniform throughout — every sample has the same composition. Examples include salt water, sugar dissolved in water, air (a blend of N\u003Csub>2\u003C/sub>, O\u003Csub>2\u003C/sub>, Ar, and trace gases), maple syrup, gasoline, sports drinks, alloys such as brass and stainless steel, and unopened carbonated beverages (CO\u003Csub>2\u003C/sub> dissolved in water, before bubbles form).\u003C/p>\u003Cp>The key test works cleanly for true solutions: if a sample taken from any part looks and behaves identically, it’s homogeneous; if different regions look or behave differently, it’s heterogeneous. Colloids are the exception to watch for — milk looks uniform yet is heterogeneous, because its dispersed fat and protein particles are large enough to scatter a beam of light (the \u003Cstrong>Tyndall effect\u003C/strong>) even though you cannot see them individually. Note on dissolved gases: while CO\u003Csub>2\u003C/sub> stays fully dissolved in a sealed bottle, the soda is a homogeneous solution; once the bottle is opened and CO\u003Csub>2\u003C/sub> comes out as visible bubbles, the liquid and the gas are two distinct phases, so the system is then heterogeneous.\u003C/p>",[60],11,[62,65],{"label":63,"body":64},"Why this matters: what a blood test really does","\u003Cp>Blood can look uniform to the unaided eye, but it is a heterogeneous mixture: red cells, white cells, and platelets are suspended in plasma. A lab demonstrates this with a centrifuge, spinning the sample until it separates into distinct layers. That physical separation is evidence that blood is a mixture rather than a pure substance.\u003C/p>\u003Cp>The same thinking runs through pharmaceutical manufacturing: a tablet blend must distribute its active ingredient consistently enough that every dose contains the intended amount. Content uniformity is a quality-control requirement even though the solid blend can remain heterogeneous at the particle scale.\u003C/p>",{"label":66,"body":67},"Test yourself: four quick classifications","\u003Cp>Homogeneous or heterogeneous? Decide, then check below.\u003C/p>\u003Cul>\u003Cli>14-karat gold ring\u003C/li>\u003Cli>Orange juice with pulp\u003C/li>\u003Cli>Clear, filtered apple juice with no suspended particles\u003C/li>\u003Cli>Fog\u003C/li>\u003C/ul>\u003Cp>\u003Cstrong>Answers:\u003C/strong> 14-karat gold is \u003Cstrong>homogeneous\u003C/strong> at the scale considered here (an alloy with its components distributed uniformly). Orange juice with pulp is \u003Cstrong>heterogeneous\u003C/strong> (visible solid pieces in liquid). The specified clear apple juice is \u003Cstrong>homogeneous\u003C/strong> because its dissolved components are distributed uniformly after suspended particles are removed. Fog is \u003Cstrong>heterogeneous\u003C/strong>: it is a colloid of tiny water droplets dispersed in air, and it scatters headlight beams (the Tyndall effect), which is exactly how colloids give themselves away.\u003C/p>",{"heading":69,"content":70,"relatedObjectives":71,"deepDive":73},"Physical vs. Chemical Properties","\u003Cp>Every substance has characteristic \u003Cstrong>properties\u003C/strong> that help identify it. These fall into two categories:\u003C/p>\u003Cp>\u003Cstrong>Physical properties\u003C/strong> can be observed or measured \u003Cem>without changing\u003C/em> the substance’s chemical identity:\u003C/p>\u003Cul>\u003Cli>Color, odor, density, melting point, boiling point, hardness, electrical conductivity, temperature, length, mass, volume\u003C/li>\u003Cli>Some require a state change to observe (melting point of iron), but the substance’s identity is unchanged\u003C/li>\u003C/ul>\u003Cp>\u003Cstrong>Chemical properties\u003C/strong> describe a substance’s ability to undergo \u003Cem>chemical changes\u003C/em> — transformations into different substances:\u003C/p>\u003Cul>\u003Cli>Flammability (can it burn?), toxicity, acidity, reactivity with oxygen (does it rust?)\u003C/li>\u003Cli>\u003Cstrong>Heat of combustion\u003C/strong> — the energy released when a substance is burned in oxygen; a chemical property because it can only be measured during the combustion reaction itself\u003C/li>\u003Cli>Iron forms a flaky rust layer in moist air. Chromium also reacts with oxygen, but the thin, adherent chromium oxide layer slows further oxidation; this passivation behavior is a chemical property.\u003C/li>\u003C/ul>\u003Cp>The distinction matters: physical properties help you identify a substance; chemical properties tell you what it can \u003Cem>become\u003C/em>. You can observe a physical property by inspection, but you can only observe a chemical property by attempting (or observing) a chemical reaction.\u003C/p>",[72],13,[74,77],{"label":75,"body":76},"Why this matters: the labels on a tanker truck","\u003Cp>The diamond-shaped placards on tanker trucks and chemical drums are a public catalog of properties. “Flammable,” “corrosive,” and “oxidizer” are chemical properties: they describe what the cargo can \u003Cem>do\u003C/em> in a reaction. The shipping papers alongside them list physical properties: state, density, boiling point, which tell responders how the material will behave if it spills (float or sink, evaporate or pool). Emergency crews read both kinds within seconds of arriving, because the two kinds of properties answer two different questions: what is it like, and what can it become?\u003C/p>",{"label":78,"body":79},"Common mistake: melting the sample does not make it chemical","\u003Cp>Measuring iron’s melting point requires melting the iron, and that feels like “changing” it, so students mark it chemical. It is physical: liquid iron is still iron, and it freezes right back. The identity never changed.\u003C/p>\u003Cp>The reliable tell: observing a chemical property \u003Cem>uses up\u003C/em> some of the sample. To learn whether ethanol is flammable, you must burn some, and what burned is gone, converted to carbon dioxide and water. If you can observe the property and keep every atom of the original substance as that substance, the property is physical.\u003C/p>",{"heading":81,"content":82,"relatedObjectives":83,"deepDive":85},"Physical vs. Chemical Changes","\u003Cp>Changes in matter also fall into two categories:\u003C/p>\u003Cp>\u003Cstrong>Physical changes\u003C/strong> alter a substance’s form or state but \u003Cem>not\u003C/em> its chemical composition. The same substance is present before and after:\u003C/p>\u003Cul>\u003Cli>Ice melting to water (H₂O remains H₂O)\u003C/li>\u003Cli>Sugar dissolving in coffee\u003C/li>\u003Cli>Grinding a solid into powder\u003C/li>\u003Cli>Magnetizing or demagnetizing a metal\u003C/li>\u003C/ul>\u003Cp>\u003Cstrong>Chemical changes\u003C/strong> (chemical reactions) produce one or more \u003Cem>new substances\u003C/em> with different properties:\u003C/p>\u003Cul>\u003Cli>Wood reacting with oxygen during burning, producing gases such as CO₂ and H₂O while leaving mineral-rich ash\u003C/li>\u003Cli>Iron rusting (Fe + O₂ + H₂O → hydrated iron oxides)\u003C/li>\u003Cli>Food digesting, a banana browning, milk souring\u003C/li>\u003C/ul>\u003Cp>Clues that a chemical change has occurred: color change, gas production (bubbles), formation of a precipitate (solid from solution), energy release or absorption (heat, light). However, these are clues, not proof — some physical changes also produce bubbles (boiling) or color changes (dissolving a dye).\u003C/p>\u003Cp>The \u003Cstrong>law of conservation of matter\u003C/strong> applies to all changes: in a closed system, the total mass before a change equals the total mass after. Matter is neither created nor destroyed; an apparent mass change in an open system can occur when matter enters or leaves.\u003C/p>",[84],14,[86,89],{"label":87,"body":88},"Why this matters: your kitchen runs on both kinds of change","\u003Cp>Melting butter is a physical change: cool it and you get butter back. Browning a steak or caramelizing sugar is chemical: the heat drives reactions that build hundreds of new flavor compounds, and no amount of cooling will un-brown the crust. Cooking succeeds or fails on this distinction. Whipping cream (physical) can be over-whipped into butter and buttermilk (a physical separation), but burning the garlic (chemical) is forever. Easy reversibility is a helpful hint that a change was physical, but it is not the real test: shattering a glass is physical yet unfixable, and plenty of chemical reactions reverse. The reliable criterion is whether the chemical identity of the substances changed, and cooks learn by experience what chemists state as a rule.\u003C/p>",{"label":90,"body":91},"Common mistake: bubbles do not always mean a reaction","\u003Cp>“Gas production” appears on every list of chemical-change clues, so students see bubbles and declare a reaction. But boiling water bubbles furiously while remaining pure H₂O: the bubbles are water vapor, a state change only. An opened soda fizzes because dissolved CO₂ is coming out of solution, also physical.\u003C/p>\u003Cp>Compare baking soda meeting vinegar: the CO₂ in that foam did not exist as CO₂ before the mixing; it was \u003Cem>produced\u003C/em> by a reaction. The question is never “are there bubbles?” but “is the gas a new substance, or an old one changing state or escaping solution?”\u003C/p>",{"heading":93,"content":94,"relatedObjectives":95,"deepDive":97},"Extensive vs. Intensive Properties","\u003Cp>Properties can also be classified by whether they depend on the \u003Cem>amount\u003C/em> of substance present:\u003C/p>\u003Cp>\u003Cstrong>Extensive properties\u003C/strong> are additive for subsystems and scale with the amount of matter:\u003C/p>\u003Cul>\u003Cli>Mass (more matter = more mass)\u003C/li>\u003Cli>Volume (more matter = more volume under the same conditions)\u003C/li>\u003Cli>Amount of substance and internal energy\u003C/li>\u003C/ul>\u003Cp>\u003Cstrong>Intensive properties\u003C/strong> are independent of the amount of matter:\u003C/p>\u003Cul>\u003Cli>Temperature (a cup and a gallon of milk at 20 °C are both at 20 °C — combining them doesn’t change the temperature)\u003C/li>\u003Cli>Density (a gram of gold and a kilogram of gold both have the same density, 19.3 g/cm³)\u003C/li>\u003Cli>Color, melting point, boiling point\u003C/li>\u003C/ul>\u003Cp>Intensive properties are especially useful for \u003Cstrong>identifying substances\u003C/strong> because they don’t depend on sample size. If you measure an unknown metal’s density and get 19.3 g/cm³, that strongly suggests gold — regardless of whether you have a gram or a kilogram of it.\u003C/p>\u003Cp>A practical way to tell them apart is to imagine combining two identical samples under the same conditions. Additive quantities such as mass, volume, and internal energy double; intensive properties such as temperature and density remain the same.\u003C/p>",[96],15,[98,101],{"label":99,"body":100},"Why this matters: how a jeweler spots a fake diamond","\u003Cp>A gemologist handed a suspicious stone does not need to scratch or burn it. Density settles it: diamond is \u003Cspan class=\"nowrap\">3.52 g/cm³,\u003C/span> while cubic zirconia, the most common imitation, is around \u003Cspan class=\"nowrap\">5.7 g/cm³,\u003C/span> noticeably denser. Because density is intensive, the test works on any size stone, from a chip to a showpiece. Intensive properties are nature’s ID card: melting point, density, and boiling point identify a substance no matter how much of it you have, which is why reference tables list them and why your worked example matched an unknown liquid to ethanol.\u003C/p>",{"label":102,"body":103},"Common mistake: confusing temperature, internal energy, and heat","\u003Cp>A bathtub of lukewarm water can have far more internal energy than a teacup of boiling water, even though the teacup is hotter. Temperature is intensive and tracks the thermal state; internal energy is extensive and depends on how much material is present as well as its state. \u003Cstrong>Heat\u003C/strong> is energy transferred because of a temperature difference, not energy stored inside a sample.\u003C/p>\u003Cp>The combining test sorts the properties: combine two identical samples of \u003Cspan class=\"nowrap\">20 °C\u003C/span> water and you have more water, more mass, and more internal energy (all extensive), but the combined sample remains at \u003Cspan class=\"nowrap\">20 °C\u003C/span> (intensive).\u003C/p>",{"heading":105,"content":106,"relatedObjectives":107},"Classifying Matter: A Decision Flowchart","\u003Cp>When asked to classify a sample of matter, work through these questions in order:\u003C/p>\u003Col>\u003Cli>\u003Cstrong>Can physical methods separate it into different substances\u003C/strong> (for example, by filtering, distilling, or sorting)? If \u003Cem>yes\u003C/em>, it is a \u003Cstrong>mixture\u003C/strong>. If \u003Cem>no\u003C/em>, it is a \u003Cstrong>pure substance\u003C/strong>.\u003C/li>\u003Cli>If a mixture: \u003Cstrong>is the composition uniform at the scale being considered?\u003C/strong> Uniform → \u003Cstrong>homogeneous\u003C/strong> (e.g., saltwater). Distinct regions or phases → \u003Cstrong>heterogeneous\u003C/strong> (e.g., granite).\u003C/li>\u003Cli>If a pure substance: \u003Cstrong>can it be broken down into simpler substances by chemical means?\u003C/strong> Yes → \u003Cstrong>compound\u003C/strong> (e.g., H\u003Csub>2\u003C/sub>O decomposes into H\u003Csub>2\u003C/sub> and O\u003Csub>2\u003C/sub> by electrolysis). No → \u003Cstrong>element\u003C/strong> (e.g., gold cannot be simplified further).\u003C/li>\u003C/ol>\u003Cp>This hierarchy is definitive for a specified sample at a specified scale. A common mistake is confusing \u003Cem>phase\u003C/em> with \u003Cem>classification\u003C/em> — ice, liquid water, and steam are all the same compound (H\u003Csub>2\u003C/sub>O) in different states, not different types of matter.\u003C/p>",[48,60],{"title":109,"problem":110,"steps":111,"answer":117},"Classifying Matter and Its Properties","A student has a clear, colorless liquid. She measures its density as 0.79 g/mL and finds it boils at 78.4 °C. It burns with a blue flame when ignited. Classify each observation as a physical or chemical property, and determine if the substance is likely a pure substance or a mixture.",[112,113,114,115,116],"Density (0.79 g/mL): measured without changing identity → physical property (intensive)","Boiling point (78.4 °C): the substance changes state but not composition → physical property (intensive)","Burns with a blue flame: the substance reacts with oxygen to form new substances → chemical property (flammability)","A sharp boiling point supports the pure-substance interpretation, although this observation alone is not definitive","Compare with known values: density 0.79 g/mL and BP 78.4 °C match ethanol (a compound, C₂H₅OH)","Density and boiling point are physical properties; flammability is a chemical property. The sharp boiling point and matching reference values support identifying the sample as pure ethanol.",[119,120],"OpenStax Chemistry 2e, Ch 1.2: Phases and Classification of Matter (CC BY 4.0)","OpenStax Chemistry 2e, Ch 1.3: Physical and Chemical Properties (CC BY 4.0)","",[123,125,127,130,132,134,136,138,141,143],{"question":124},"What is a pure substance?",{"question":126},"What is a mixture and what types of mixtures are there?",{"question":128,"hint":129},"What are the three states of matter and what distinguishes them?","Think about particle arrangement and energy.",{"question":131},"What is the difference between an element and a compound?",{"question":133},"What is the difference between a homogeneous and a heterogeneous mixture?",{"question":135},"What is a physical property? Give an example.",{"question":137},"What is a chemical property? Give an example.",{"question":139,"hint":140},"What is the difference between a physical change and a chemical change?","Consider whether new substances are formed.",{"question":142},"What is an intensive property and how does it differ from an extensive property?",{"question":144},"How can intensive properties be used to identify an unknown substance?",[146,147],"numbers-units-measurement","basic-atomic-structure",[149,150,151,152],"H","O","C","N",[],{"title":155,"steps":156},"How to Classify a Sample of Matter",[157,158,159,160,161,162],"Ask: can the sample be separated into different substances by physical means (filtering, distilling, sorting)? If yes, it is a mixture.","If it is a mixture, check whether the composition is uniform at the scale being considered. Uniform means homogeneous (solution); distinct regions or phases mean heterogeneous. Remember that colloids can look uniform to the unaided eye while remaining heterogeneous.","If it cannot be physically separated, it is a pure substance. Ask: can it be broken down into simpler substances by chemical means?","If it can be chemically decomposed, it is a compound. If it cannot, it is an element.","To classify a property, determine whether observing it changes the substance's identity. If not, it is physical; if new substances form, it is chemical.","To classify a property as extensive or intensive, mentally double the sample size. If the property doubles, it is extensive; if it stays the same, it is intensive.",{"question":164,"answer":165,"type":166},"A student heats a white solid and observes that it melts at exactly 801 °C, forming a clear liquid. Is this evidence of a physical or chemical change, and does the sharp melting point tell you anything about whether the sample is pure or a mixture?","Melting is a physical change because the substance changes state but not chemical identity. A sharp melting point is strong evidence for a pure substance, while many mixtures melt over a range. The observation therefore supports, but does not by itself prove, that this is a pure substance undergoing a physical change.","conceptual",[168,171],{"id":169,"problem":170,"type":166},"pt-2-1","Classify each of the following as an element, compound, homogeneous mixture, or heterogeneous mixture: (a) distilled water, (b) trail mix, (c) bronze, (d) nitrogen gas.",{"id":172,"problem":173,"type":174},"pt-2-2","A substance has a density of 2.70 g/cm³ and melts at 660 °C. Are these physical or chemical properties? Are they extensive or intensive? What element do these values suggest?","calculation",{"workedExampleCount":176,"hasWorksheets":177},8,true,[179,180,181,182,183],"classification of matter","elements compounds mixtures","physical chemical changes","states of matter","intensive extensive properties",6,{"id":186,"slug":187,"lesson":186,"title":188,"shortTitle":189,"description":190,"category":9,"objectiveCount":191,"problemCount":192},1,"measurement-and-significant-figures","Measurement and Significant Figures","Units & Measurement","Master the foundation of chemistry: significant figures, scientific notation, dimensional analysis, and the metric system.",9,73,{"id":194,"slug":195,"lesson":194,"title":196,"shortTitle":197,"description":198,"category":199,"objectiveCount":200,"problemCount":201},3,"atomic-structure-and-isotopes","Atomic Structure and Isotopes","Atomic Structure","Explore atomic structure: protons, neutrons, electrons, isotopes, atomic mass, and the history of atomic models.","atomic-structure",7,89,1785108608014]