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Pure substances and mixtures

Classify samples from composition and distinguish chemical purity from uniform appearance.

Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.

1. What you will learn

Classify a stated sample as an element, compound or mixture, and explain what composition and physical-property evidence can establish.

2. Start with the sample

You already recognize that materials have properties such as color, state and texture. This lesson asks a different question: which substances are actually present? No laboratory work is required. Every exercise supplies a composition or an idealized description, and you use that evidence to classify the material. A sample means the material under discussion, not its container or everything nearby.

3. Words for classifying matter

4. Count substances before counting elements

Begin with what the sample contains. If it contains more than one substance, classify it as a mixture. If it contains one substance, it is pure in this introductory model. Only then ask whether that substance contains one element or chemically combined elements. One elemental identity gives an element; chemically combined elements give a compound. This order matters because a mixture can itself contain elements, compounds or both.

A sample containing only water molecules is a pure compound. Every molecule contains hydrogen and oxygen, but the sample does not contain separate hydrogen gas and oxygen gas merely because those element names appear in the formula. A sample containing water and dissolved sugar contains two substances and is a mixture. Both component substances are compounds. Calling the sample a mixture does not change the chemical classification of either component.

An element does not have to consist of isolated atoms. Oxygen molecules contain pairs of oxygen atoms, yet oxygen is still an element because only one elemental identity is present. A bottle containing oxygen and nitrogen is a mixture of elemental substances. Count distinct substances, not the number of atoms grouped together in each molecule.

The classification is about composition, not quality. A pure substance can be hazardous, and a useful everyday product can be a mixture. Water suitable for a stated purpose need not be chemically pure water. Likewise, the word natural does not tell you how many substances are present. In the exercises, rely on the supplied chemical description rather than advertising language or an assumption about the material's use.

5. What a formula tells you

A molecular formula names the elements and the number of atoms of each in one molecule. Water has two hydrogen atoms and one oxygen atom per molecule. Ten water molecules contain twenty hydrogen atoms and ten oxygen atoms, but they still represent one molecular substance. Changing the amount does not create a new component.

A sample containing water molecules and carbon dioxide molecules contains two molecular substances. Oxygen appears in both formulas, so there are three distinct elements overall: hydrogen, oxygen and carbon. The counts of molecules, substances and elements answer different questions. Writing each count with its label is a reliable way to avoid switching between them midway through a solution.

Not all compounds consist of separate molecules. Sodium chloride is an ionic solid; its formula describes the ratio of ions in an extended structure rather than a little isolated molecule in the crystal. That difference does not make it a mixture of sodium metal and chlorine gas. Its ions form a chemically defined compound with properties different from those elemental substances. The bonding units later in the course explain that distinction in more detail.

6. Uniform appearance is not chemical purity

Salt dissolved in water can make a clear, uniform solution. A small sample from one part of a well-mixed solution can have the same composition as a sample from another part, yet both water and dissolved salt remain present. Homogeneous describes uniformity; pure describes the number of substances. These are different classifications and should not be used as synonyms.

A sample containing visible layers can give direct evidence of nonuniformity, but visibility is not required for a mixture. Air contains multiple gases that cannot be distinguished by eye. Many common alloys also appear uniform even though their compositions can vary. Brass is commonly described as a copper-zinc alloy; changing the relative amounts changes the material without defining a single fixed molecular formula for brass.

The introductory classification of familiar alloys as mixtures is useful, but do not extend it into the claim that every alloy is merely unbonded atoms or that metals never form compounds. Metallurgical systems can include solid solutions, multiple phases and intermetallic compounds. The exercises specify the composition needed for the answer. A shiny appearance alone does not reveal which microscopic structure a real alloy has.

7. Separation and decomposition answer different questions

A separation changes which substances are collected together. Recovering components from a mixture does not require turning every component into a new substance. For example, water can evaporate from a salt solution while salt remains. This supports a mixture description because the original sample contained multiple components. It does not mean the components exerted no forces on one another: dissolved ions interact strongly with water.

Chemical decomposition changes a substance into other substances. The fact that water can be chemically converted into hydrogen and oxygen does not prove that it originally contained those elemental gases as a mixture. The products of a reaction are not automatically the components of its starting material. Confusing decomposition with separation is a common source of wrong classifications.

In practice, a proposed separation also depends on the properties of the components and the equipment. This lesson does not ask you to perform any operation or decide whether an unknown sample is safe to heat. It asks you to interpret supplied evidence. If a report says only that something changed on heating, without identifying the starting or resulting substances, the classification may remain uncertain. Do not force a definite answer from incomplete information.

8. Melting behavior is evidence with conditions

For a pure crystalline substance undergoing an equilibrium solid-liquid transition at fixed pressure, melting occurs at a characteristic temperature. In a simple heating record, added energy can support the phase change while the temperature remains approximately constant. Many impure samples melt over a broader range, so a melting observation can be useful supporting evidence about composition.

It is not a universal purity test. Some mixtures have a particular composition that melts sharply; such a composition is called eutectic. A sharp transition therefore does not prove that a sample contains only one substance. Conversely, a complicated heating record can reflect decomposition, changing pressure, uneven heating or other effects rather than simply the presence of a mixture. The conditions and the kind of material must be considered.

For classification in this lesson, composition is the decisive evidence when it is supplied. A measured property can support or challenge a proposed identity, but one observation should not be made to answer more than it can establish. If a material looks uniform and melts sharply while component analysis identifies two substances, the composition evidence still establishes a mixture. The apparently conflicting observations reveal a limitation of the shortcut, not a reason to ignore the analysis.

9. Checking a classification

Check that your answers to the two questions agree. Element and compound are the two pure-substance categories in the introductory scheme. Mixture is not a pure-substance category. A table row saying compound and not pure is inconsistent unless the description actually means a sample containing that compound plus something else; in that case the sample itself should be classified as a mixture.

Next check which object you classified. A sealed container can hold a pure substance even though the container wall is another material. A solution can contain a pure compound as one component while the whole solution is a mixture. State the sample boundary before counting substances, and do not accidentally include the packaging or exclude a dissolved component.

Finally compare the strength of your conclusion with the evidence. A formula and a statement that it is the only substance can settle an idealized exercise. A label on an everyday product may not establish complete purity. A uniform appearance only describes what can be observed at that scale. If composition is unknown, saying that more evidence is needed is better reasoning than treating appearance as a chemical analysis.

10. An Ohio hospital training inventory

An Ohio hospital uses simplified stockroom descriptions in staff training. One entry states that a model cylinder contains only oxygen gas; another describes only water; a third describes water with dissolved salt. The first is an elemental substance, the second a compound and the third a mixture. These classifications follow from the supplied compositions rather than the shape of the containers or the fact that all three have medical uses.

If the oxygen entry instead listed a second gas, the sample would become a mixture even though oxygen remained its main component. If the water entry included dissolved additives, its classification would also change. This is why a chemical inventory must identify the material actually present. The exercise makes no claim that an arbitrary real hospital product is absolutely pure or that purity alone makes a material suitable for patient use.

11. A Wisconsin foundry's material description

A Wisconsin foundry compares two hypothetical brass recipes: seventy parts copper with thirty parts zinc, and sixty parts copper with forty parts zinc. Both are described as copper-zinc alloy mixtures in this introductory example. The fact that the proportions can differ while the material is still called brass shows why the name does not specify one fixed compound formula. Each recipe totals one hundred parts, but that arithmetic does not make either recipe a pure substance.

A purchasing record that says shiny yellow metal provides much less chemical information than the composition record. A sharp melting event would also require interpretation rather than automatically proving purity. The foundry needs composition and phase information for its engineering decisions. The learner's narrower task is to classify the stated sample and explain which supplied evidence supports that classification.

12. One substance can contain several elements

The number of elements is not the number of substances. Pure water contains hydrogen and oxygen in one compound; a mixture of hydrogen and oxygen gases contains two elemental substances. Use the chemical identities in the description rather than counting element names and stopping.

Pure also does not mean harmless, natural or suitable for a particular use. Those claims require different evidence. A useful product can be deliberately formulated from multiple substances. Likewise, a mixture does not imply that its components never interact. Interactions can change physical properties even when the sample remains a mixture. Avoid the misleading slogan that nothing is joined or bonded anywhere in a mixture: the molecules of its component compounds still contain chemical bonds.

13. An idealized oxygen sample

1. Define the sample boundary.

   $\text{Only the gas inside the model vessel is counted.}$

   The vessel itself is not part of the gas sample.

2. Read the supplied composition.

   $O_2$ molecules only.

   No second substance is stated.

3. Count the molecular substances.

   $\text{One molecular substance is present.}$

   Repeated identical molecules do not create different components.

4. Count elemental identities.

   $\text{Only oxygen is present.}$

   Two atoms in a molecule can be atoms of the same element.

5. Record the linked classifications.

   $\text{Element; pure substance: yes.}$

   The sample contains one elemental substance.

14. Water molecules in a particle inventory

1. Read the inventory size.

   $6$ molecules of $H_2O$ and nothing else.

   The description states both identity and amount.

2. Count hydrogen atoms separately.

   $6\times2=12$ hydrogen atoms.

   Each molecule has two hydrogen atoms.

3. Count oxygen atoms separately.

   $6\times1=6$ oxygen atoms.

   Each molecule has one oxygen atom.

4. Distinguish elements from substances.

   $\text{Two elements; one molecular substance.}$

   Hydrogen and oxygen are chemically combined in each water molecule.

5. Classify the entire sample.

   $\text{Compound; pure substance: yes.}$

   All stated molecules have the same chemical identity.

15. A solution that looks like one liquid

1. State the reported components.

   $\text{Water and dissolved sugar remain present.}$

   The report identifies two substances.

2. Interpret the uniform appearance.

   $\text{The solution is homogeneous at the observed scale.}$

   Well-mixed components need not be visibly distinct.

3. Classify the whole solution.

   $\text{Mixture; pure substance: no.}$

   Uniformity does not reduce two substances to one.

4. Classify a component separately.

   $\text{Water is a compound within the mixture.}$

   A component's classification differs from that of the whole sample.

5. Check the proposed formula.

   $\text{The solution needs a composition description, not one fixed water formula.}$

   The amount of sugar can change without turning the mixture into pure water.

6. State the evidence boundary.

   $\text{The conclusion follows from composition, not from clarity alone.}$

   Appearance by itself would leave the number of substances unresolved.

16. A model vessel contains only carbon dioxide molecules. Classify the sample and its purity.

1. Count the named substances.

   $CO_2$ only: one molecular substance.

   The description excludes other components.

2. Count the element identities.

   $\text{Carbon and oxygen: two elements.}$

   Different element symbols identify different kinds of atom.

3. Your turn: work this step out. Its working is at the end of the packet.

   State the sample classification.

17. Guided practice

Match each idealized sample to its classification using the composition stated beside it. A classification may be used more than once.

18. Guided practice

A sealed idealized sample contains only $5$ molecules of water, $H_2O$. Complete the count of hydrogen atoms, oxygen atoms and molecular substances.

1. Count all hydrogen atoms.

   $\text{Hydrogen atoms:}$ _h

   Each molecule contributes a pair of hydrogen atoms.

2. Count all oxygen atoms.

   $\text{Oxygen atoms:}$ _o

   Each molecule contributes one oxygen atom.

3. Count the molecular substances.

   $\text{Distinct substances:}$ _s

   All the molecules have the same identity.

19. Guided practice

An analyst is evaluating the brass of a door handle. Which finding would establish that the sample is a mixture rather than one compound?

• Component analysis identifies two distinct substances still present in the sample.
• Elemental analysis detects more than one chemical element.
• The material can undergo a reaction producing simpler substances.
• The material looks uniform and has a sharp melting event.

20. Practice

Classify these idealized samples using the supplied composition, and decide which are pure substances. First, the copper of a length of electrical wire: every atom in it is a copper atom, and no chemical change will turn copper into anything simpler. Second, the water in a bottle of distilled water: hydrogen and oxygen are joined in it, always two hydrogen atoms to every oxygen atom, and the result behaves like neither of them. Third, the air in this room: nitrogen, oxygen, argon and carbon dioxide are distinct substances present together rather than one chemically defined compound.

21. Practice

A particle inventory contains $4$ molecules of carbon dioxide, $CO_2$, and $5$ molecules of water, $H_2O$, with no reaction. How many distinct molecular substances, distinct elements and total atoms are present?

22. Practice

A stockroom log describes an idealized sample of the bronze of a cast statue. Select the two statements that bear on its chemical classification rather than its purchasing history.

This task has no paper form; do it on a device.

23. Somewhere new

An Ohio hospital's training inventory uses the following idealized compositions. Classify each sample and its purity: the medical oxygen in a green cylinder: there is one kind of atom in the cylinder and nothing else with it; the dry ice packing a vaccine box: it is solid carbon dioxide, one substance with carbon and oxygen joined in it; the surgical steel of a scalpel blade: the specified steel is an alloy mixture containing iron, chromium and carbon, not a single compound. Classify the stated material, not its packaging.

24. Lesson test

Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.

25. Test question

Classify these idealized samples using the supplied composition, and decide which are pure substances. First, the copper of a length of electrical wire: every atom in it is a copper atom, and no chemical change will turn copper into anything simpler. Second, the salt in a laboratory bottle of sodium chloride: sodium and chloride ions form one ionic compound in a one-to-one ratio; its properties differ from those of sodium metal and chlorine gas. Third, the crude oil in a tanker: it is hundreds of different molecules sharing a tank, which is exactly why a refinery can boil them apart without any reaction taking place.

26. What you can do now

Can you distinguish the number of substances from the number of elements, and explain why a uniform appearance or sharp melting event alone cannot prove purity?

Working for the steps left to you

16. A model vessel contains only carbon dioxide molecules. Classify the sample and its purity., step 3