Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Pure Substance asks what the sample is, what property is being used, and whether a new substance is formed.
Common stuck point:Students often know a formula related to pure substance but skip the recognition step: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample? That leads to a correct-looking substitution attached to the wrong chemical model.
Sense of Study hint:Ask: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample?
Worked Examples
Example 1
easy
Distinguish between a pure substance and a mixture. Give two examples of each.
Answer
Pure: Au, H2O.Mixtures: salt water, air.
First step
1
A pure substance has a fixed composition and distinct properties: elements (e.g., gold) and compounds (e.g., water H2O).
Full solution
2
A mixture is a combination of two or more substances in variable proportions: (e.g., salt water, air).
3
Mixtures can be separated by physical means; pure substances cannot (compounds require chemical methods).
Classification of matter starts with distinguishing pure substances from mixtures. Pure substances have uniform and definite composition throughout.
Example 2
medium
How can you determine whether a clear liquid is a pure substance or a mixture?
Example 3
medium
How does a pure substance's boiling-point graph look compared to a mixture's?
Example 4
medium
Two unlabeled clear liquids are heated. Liquid A boils at exactly 78.4 °C the entire time. Liquid B's temperature drifts from 70 °C to 95 °C while boiling. Which is the pure substance?
Example 5
hard
Explain why distilled water is a pure substance while tap water is not.
Example 6
hard
Sea water tastes salty, looks clear, and is uniform throughout. Why is it classified as a mixture rather than a pure substance?
Example 7
hard
A student claims 'any colorless liquid that boils at 100 °C is pure water.' Is this true?
Example 8
challenge
A 10.0 g sample is analyzed and found to contain 4.00 g carbon and 6.00 g oxygen. Could this be a pure compound, and which compound is consistent with the data?
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easy
Is orange juice a pure substance or a mixture? Explain.
Example 2
hard
Both distilled water and steel are uniform in appearance and properties. Explain why distilled water is classified as a pure substance while steel is a mixture.
Example 3
easy
Is distilled water (H2O) a pure substance?
Example 4
easy
Is a bar of pure gold (Au) a pure substance?
Example 5
easy
Is table salt, NaCl, a pure substance?
Example 6
easy
Is pure oxygen gas, O2, a pure substance?
Example 7
easy
Is freshly squeezed orange juice a pure substance?
Example 8
easy
Is carbon dioxide, CO2, a pure substance?
Example 9
easy
A sample contains only N2 molecules. Pure substance or mixture?
Example 10
easy
Is a diamond (pure carbon, C) a pure substance?
Example 11
medium
Steel is iron mixed with about 1% carbon, uniform throughout. Pure substance or not, and why?
Example 12
medium
A clear liquid boils completely at exactly 100 °C at 1 atm. What does the constant boiling point suggest?
Example 13
medium
You have a beaker labeled 'H2O2' (pure hydrogen peroxide). Pure substance or mixture? Justify.
Example 14
medium
Air is filtered and dried but still contains N2, O2, Ar, and CO2. Pure substance or mixture?
Example 15
medium
Brass is copper and zinc melted together uniformly. Classify it and name the larger-amount idea.
Example 16
medium
A student says 'sugar water is pure because it looks perfectly clear.' Correct them.
Example 17
medium
Graphite (pure C) and a graphite pencil 'lead' (graphite plus clay) are compared. Which is the pure substance?
Example 18
medium
A flask holds only CH4 (methane) molecules. State the classification and the number of elements involved.
Example 19
challenge
Sample X melts sharply at one temperature; Sample Y melts gradually over a 6 °C range. Both look identical. Which is more likely pure, and what is the reasoning chain?
Example 20
challenge
You are given pure water, salt water, and pure NaCl. Rank them as pure substance or mixture, and explain why salt water is the odd one out.
Example 21
challenge
Argue whether 'pure 24-karat gold jewelry' truly meets the chemistry definition of a pure substance, while 18-karat gold does not.
Example 22
medium
A label reads 'distilled water.' How does distillation make it a pure substance, unlike tap water?
Example 23
easy
Is pure copper wire (Cu) a pure substance?
Example 24
easy
Is salt water a pure substance?
Example 25
easy
Is air a pure substance or a mixture?
Example 26
easy
Is sugar (C12H22O11) a pure substance?
Example 27
easy
Is a sample of pure nitrogen gas (N2) a pure substance?
Example 28
medium
Is brass (an alloy of copper and zinc) a pure substance?
Example 29
medium
Is graphite (pure carbon) a pure substance?
Example 30
medium
Is steel a pure substance?
Example 31
medium
A solution of sugar dissolved completely in water — pure substance, homogeneous mixture, or heterogeneous mixture?
Example 32
medium
Is liquid bromine (Br2) a pure substance?
Example 33
hard
A test gives a sample a sharp single melting point. Is the sample most likely a pure substance or a mixture?
Example 34
hard
Is 24-karat gold a pure substance? Why or why not?