Matter and its Properties
Substances and Mixtures
A substance is a form of matter that has a definite (constant) composition and distinct
properties. Substances differ from one another in composition and can be identifi ed by their
appearance, smell, taste, and other properties. A mixture is a combination of two or more
substances in which the substances retain their distinct identities. Mixtures do not have constant
composition. Therefore, samples of air collected in different cities would probably differ in
composition because of differences in altitude, pollution, and so on.
Mixtures are either homogeneous or heterogeneous. Any mixture, whether homogeneous
or heterogeneous, can be created and then separated by physical means into pure components
without changing the identities of the components. After separation, the components of the
mixture will have the same composition and properties as they did to start with.
Elements and Compounds
Substances can be either elements or compounds. An element is a substance that cannot
be separated into simpler substances by chemical means. To date, 117 elements have been
positively identified. Most of them occur naturally on Earth. The others have been created by
scientists via nuclear processes. Chemists use symbols of one or two letters to represent the
elements. The fi rst letter of a symbol is always capitalized, but any following letters are not. For
example, Co is the symbol for the element cobalt, whereas CO is the formula for the carbon
monoxide molecule.
Atoms of most elements can interact with one another to form compounds. Water is made
up of two parts hydrogen and one part oxygen. This composition does not change, water is a
compound, a substance composed of atoms of two or more elements chemically united in fixed
proportions. Unlike mixtures, compounds can be separated only by chemical means into their
pure components.
Elements, Compounds, and Mixtures: An Atomic Overview
Based on its composition, matter can be classified into three types—elements,
compounds, and mixtures. Elements and compounds are called substances, matter with a fi xed
composition; mixtures are not substances because they have a variable composition.
 Elements. An element is the simplest type of matter with unique physical and chemical
properties. It consists of only one kind of atom and, therefore, cannot be broken down
into a simpler type of matter by any physical or chemical methods. Each element has a
name, such as silicon, oxygen, or copper. A sample of silicon contains only silicon atoms.
The macroscopic properties of a piece of silicon, such as color, density, and
combustibility, are different from those of a piece of copper because the submicroscopic
properties of silicon atoms are different from those of copper atoms; that is, each element
is unique because the properties of its atoms are unique. Several elements occur in
molecular form: a molecule is an independent structure of two or more atoms bound
together. Oxygen, for example, occurs in air as diatomic (two-atom) molecules.
 Compounds. A compound consists of two or more different elements that are bonded
chemically. That is, the elements in a compound are not just mixed together: their atoms
have joined in a chemical reaction.
 All compounds have two defining features:
o 1. The elements are present in fixed parts by mass (fixed mass ratio). This is so
because each unit of the compound consists of a fixed number of atoms of
each element. For example, consider a sample of ammonia. It is 14 parts
nitrogen by mass
and 3 parts hydrogen by mass because 1 nitrogen
atom has 14 times the mass of 1 hydrogen atom, and each ammonia molecule
consists of 1 nitrogen atom and 3 hydrogen atoms:
Ammonia (NH3) gas is 14 parts N by mass and 3 parts H by mass. 1 N
atom has 14 times the mass of 1 H atom.
Each ammonia molecule consists of 1 N atom and 3 H atoms
o 2. A compound’s properties are different from the properties of its elements.
Figure below shows a striking example: soft, silvery sodium metal and yellowgreen, poisonous chlorine gas are very different from the compound they form—
white, crystalline sodium chloride, or common table salt!
 Mixture consists of two or more substances (elements and/or compounds) that are
physically intermingled, not chemically combined. Because a mixture is not a substance,
the components can vary in their parts by mass. On the atomic scale, a mixture consists of
the individual units of its component elements and/or compounds. It makes sense, then,
that a mixture retains many of the properties of its components. Saltwater, for instance, is
colorless like water and tastes salty like sodium chloride. Unlike compounds, mixtures
can be separated into their components by physical changes; chemical changes are not
needed. For example, the water in saltwater can be boiled off, a physical process that
leaves behind solid sodium chloride.
Mixtures: Classification and Separation
In the natural world, matter usually occurs as mixtures. A sample of clean air, for
example, consists of many elements and compounds physically mixed together, including
O2, N2, CO2, the noble gases [Group 8A(18)], and water vapor (H2O). And living things
contain thousands of substances—carbohydrates, lipids, proteins, nucleic acids, and many
simpler ionic and covalent compounds.
There are two broad classes of mixtures:

 A heterogeneous mixture has one or more visible boundaries between the
components. Thus, its composition is not uniform, but rather varies from one region
to another. Many rocks are heterogeneous, having individual grains of different
minerals. In some heterogeneous mixtures, such as milk and blood, the boundaries
can be seen only with a microscope.
 A homogeneous mixture (or solution) has no visible boundaries because the
components are individual atoms, ions, or molecules. Thus, its composition is
uniform. A mixture of sugar dissolved in water is homogeneous, for example,
because the sugar molecules and water molecules are uniformly intermingled on the
molecular level. We have no way to tell visually whether a sample of matter is a
substance (element or compound) or a homogeneous mixture.
Although we usually think of solutions as liquid, they exist in all three physical states.
For example, air is a gaseous solution of mostly oxygen and nitrogen molecules, and wax is
a solid solution of several fatty substances. Solutions in water, called aqueous solutions, are
especiallyimportant in chemistry and comprise a major portion of the environment and of all
organisms.
Recall that mixtures differ from compounds in three major ways:
1. The proportions of the components can vary.
2. The individual properties of the components are observable.
3. The components can be separated by physical means.
Distinction Between Mixtures and Compounds
The difference between a mixture and a compound is well illustrated using
iron and sulfur as components. Any proportions of iron metal and powdered sulfur form a
mixture. The components can be separated with a magnet because iron metal is magnetic.
But if we heat the container strongly, the components form the compound iron(II) sul- de
(FeS). The magnet can no longer remove the iron because it exists as Fe2 ions chemically
bound to S2- ions.
Separation of Mixtures
We can separate a mixture into its components by taking advantage of differences in
their properties. For example, a heterogeneous mixture of iron filings and gold filings could
be sorted by color into iron and gold. A less tedious approach would be to use a magnet to
attract the iron filings, leaving the gold ones behind. We can also take advantage of an
important chemical difference between these two metals: Many acids dissolve iron but not
gold.
An important method of separating the components of a homogeneous mixture is
distillation, a process that depends on the different abilities of substances to form gases.
For example, if we boil a solution of salt and water, the water evaporates, forming a gas,
and the salt is left behind. The gaseous water can be converted back to a liquid on the walls
of a condenser,
Mixtures that are made up of insoluble solids in a liquid can be separated by filtration
or decantation. Examples of mixtures that can be separated by the said processes are
muddy water and sand in water.
How is decantation done? For instance, you have sand in water. To separate the
components of this mixture, let the sand settle down. When all the particles have settled
down, carefully pour off the liquid into another container. And presto, you will be able to get
back both the sand and water!
What about filtration? Filtration is a process that separates the insoluble solid in
liquid by letting it pass through a filter paper, which is placed inside a funnel. You may look
at the set-up below for reference.
In this setup, the small particles of the liquid will pass through the filter paper while
the bigger solid particles will be left on the filter paper. The one left on the filter paper is the
residue while the liquid that is collected is the filtrate.
Check up quiz
Mixtures
Directions: Classify each of the following as an element, compound, heterogeneous
mixture, or homogenous mixture.
12. Water _______________ 13. Carbon _______________
14. Air _______________ 15. Table salt, NaCl _____________
16. Sugar dissolved in water ___________ 17.Granite ______________
18. Homogenized milk _______________ 19. Oxygen _______________
20. Sand in water _______________