Simple or seemingly pure substances found in nature can in fact be mixtures of chemical substances. For example, tap water may contain small amounts of dissolved sodium chloride and compounds containing iron, calcium, and many other chemical substances. Pure distilled water is a substance, but seawater, since it contains ions and complex molecules, is a mixture. A mixture is a material system made up of two or more different substances, which are mixed but not combined chemically.
A mixture refers to the physical combination of two or more substances in which the identities of the individual substances are retained. Mixtures take the form of alloys, solutions, suspensions, and colloids. A heterogeneous mixture is a mixture of two or more chemical substances elements or compounds , where the different components can be visually distinguished and easily separated by physical means. Examples include:. A homogeneous mixture is a mixture of two or more chemical substances elements or compounds , where the different components cannot be visually distinguished.
The composition of homogeneous mixtures is constant. Often separating the components of a homogeneous mixture is more challenging than separating the components of a heterogeneous mixture. Distinguishing between homogeneous and heterogeneous mixtures is a matter of the scale of sampling. Elements are made of only one kind of atom. The particles can be a single atom or a molecule made of only one kind of atom. There is no physical change that can separate elements into more than one kind of substance.
This makes an element a pure substance. Compounds are made of only one kind of molecule. The molecule is made up of two or more kinds of atoms. Pure substances have a sharp melting point but mixtures melt over a range of temperatures. This difference is most easily seen when the temperature of a hot liquid is measured as it cools and freezes. The graph shows the cooling curve for a sample of a compound called salol.
The horizontal part of the graph shows that the salol has a sharp melting point, so it is pure. Impure salol a mixture of salol and other substances would produce a gradual decrease over a range of temperatures as it freezes. Pure substances and mixtures The meaning of pure The word 'pure' is used in chemistry in a different way from its everyday meaning.
In chemistry: a pure substance consists only of one element or one compound a mixture consists of two or more different substances, not chemically joined together The components of a mixture can be separated without chemical reactions Mixtures and compounds Hydrogen and oxygen are both gases.
Mixture Compound Composition Variable composition - the relative amounts of the two gases can be changed. Oct 15 Mar 10 Feb 28 Oct 21 Ostensibly, compounds contain more than one type of material.
Yet both compounds and elements are considered pure substances. Pure compounds are created when elements combine permanently, forming one substance. For example, when the elements mercury and oxygen combine, and a chemical reaction takes place, mercury II oxide is created. Once mercury II oxide has been created, it is not possible to separate the mercury and oxygen used in its creation from one another.
Mixtures, on the other hand, are considered impure substances. A mixture is created when substances combine but no chemical reaction occurs. In general, a mixture can be separated into its original components, while a pure compound cannot. Of course, there are a handful of exceptions to the definition above.
Pure compounds and elements differ from mixtures in one key way: the substances in a pure compound are held together by chemical bonds.
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