Something made of just one substance, like gold or water, is a pure substance; something with two or more pure substances mixed together, like sugar water or air, is a mixture.
A pure substance is made of one kind of particle, so its melting and boiling points are fixed, while a mixture's properties change with the proportions, and its temperature keeps shifting even while it boils.
Mixtures can be separated again by using the differences in the properties of what is mixed: filtering if the particle sizes differ, distillation if the boiling points differ.
Choose gold, water, sugar water and muddy water in turn in the particle model and you can see at a glance whether the particles are of one kind or several and whether they are evenly mixed.
Pure Substances and Mixtures
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Pure substances and mixtures in the particle model
Pure substance (element)
💡 Pure Substances and Mixtures
①A pure substance is made of only one substance; elements and compounds are pure substances.
②A mixture is two or more pure substances mixed together, and each keeps its own properties.
③A pure substance has a fixed melting point, boiling point and density: pure water always boils at 100 ℃ at 1 atm.
④In a mixture these values depend on the proportions. Salt water starts boiling above 100 ℃, and its temperature creeps up while it boils.
How Matter Is Classified
Classifying matter
How matter is classified
🔍 What Sets Them Apart
①An element cannot be broken down into other substances: gold, iron and oxygen are examples.
②A compound is two or more elements joined in a fixed ratio: water is made of molecules of one oxygen atom bonded to two hydrogen atoms.
③A homogeneous mixture has the same proportions wherever you sample it.
④A heterogeneous mixture has different proportions from place to place, and some, like muddy water, settle if left alone.
Separating Mixtures
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Separating mixtures: using differences between the substances mixed
Filtration
Principle of separation
use differences in the properties of the substances mixed
Differences in particle size, boiling point, solubility, density or speed of travel with a solvent are used.
🧪 Separation Methods
①Filtration: catches solid grains larger than the pores of the filter paper (removing soil from muddy water).
②Evaporation: heats a solution to drive off only the liquid and recover the dissolved solid (getting salt from salt water).
③Distillation: heats a mixture of liquids with different boiling points and cools the gas that boils off first to collect it (separating water and ethanol).
④Chromatography: as a solvent climbs paper, each component travels at a different speed and they separate (splitting the dyes in marker ink).
Exam Key Points
Summary
pure substance: fixed melting and boiling points · mixture: they depend on the proportions
Checking whether the melting or boiling point stays fixed tells a pure substance from a mixture.
🎯 Exam Points
①Pure substances divide into elements and compounds; mixtures into homogeneous and heterogeneous ones.
②Pure substances have fixed melting and boiling points; mixtures do not.
③Filtration uses particle size, evaporation uses the fact that only the liquid escapes, distillation uses boiling points, and chromatography uses differences in speed of travel.
④Air is a homogeneous mixture of nitrogen, oxygen, argon and more.
⑤Water is made of hydrogen and oxygen, but they are bonded in a fixed ratio, so it is a compound and a pure substance.
Examples and Unit Test
Example 1
Of water (H₂O), sugar water and gold, which is a mixture of two or more substances?
1
A pure substance is made of one substance (an element or a compound); a mixture is two or more pure substances mixed together.
2
Gold is an element and water a compound, so both are pure substances; sugar water is a mixture of water and sugar.
▸ Sugar water (a mixture of water and sugar)
Water is made of two elements, but it is a compound, not a mixture: hydrogen and oxygen are not just mixed but bonded into a new substance.
Example 2
One substance stays at a constant temperature while boiling, and another keeps changing temperature. Which is the pure substance and which the mixture?
1
A pure substance has a fixed boiling point, but in a mixture the proportions change as it boils, so the temperature does not stay fixed.
2
So the one at a constant temperature is a pure substance, and the one whose temperature changes is a mixture.
▸ Constant temperature = pure substance / changing temperature = mixture
As water boils off salt water, the share of salt grows, so its boiling point creeps up while it boils.
Unit-test style
Which statement about pure substances and mixtures is correct?
①Sugar water is a pure substance
②A pure substance has a constant boiling point and melting point
③A mixture is made of only one kind of substance
④Water (H₂O) is a mixture
⑤A heterogeneous mixture has the same composition everywhere
▸ ② A pure substance has a constant boiling point and melting point
1
A pure substance is made of one substance, so its melting and boiling points are fixed; ② is correct.
2
Sugar water is a mixture (①), a mixture is two or more substances mixed together (③), water is a compound and so a pure substance (④), and a heterogeneous mixture has different proportions from place to place (⑤).