Pure Substances and Mixtures

Matter refers to anything that possesses mass and also has weight. It can be distinctly divided into two groups: pure substances and mixtures. Pure substances are also further broken down into elements and compounds. Mixtures generally represent a physically combined structure which can be separated into their original constituents.

A chemical substance is made up of only one  type of atom or molecule. A mixture is made up of different types of atoms or molecules that are not chemically bonded. In this article you will read about pure substances, characteristics of pure substance, mixtures and its characteristics and their examples.

Pure Substance

A pure substance is something that is composed of only one kind of matter. They can be either one single element or one single compound, but each sample of this substance when examined should contain exactly the similar thing with a fixed, definite set of properties. An exception to this is that pure oxygen gas comprises molecules but it is still considered an element, instead of a compound, since the molecules are composed of a single type of element. Compounds generally consist of one or more elements.

Characteristics of Pure Substance

Pure substances are generally homogeneous since they are composed of only one type of atom or molecule. These substances have definite properties and composition. Composition of a pure substance does not vary greatly. Pure substances possess a uniform composition throughout.

The substances possess a fixed boiling and melting point. They have a definite colour, mass, density, volume, hardness, electrical conductivity and heat capacity. They cannot be separated into other kinds of matter via any physical process. Pure substances possess the ability to change their states without even altering or changing the identity of the substance. These substances usually participate in a chemical reaction to form known products.

Example of Pure Substance

The freezing and melting point of water is 0°C and a boiling point of 100°C. Water is colourless, tasteless and odourless in nature.

Gold is considered pure only at 24 carat. It has a yellow appearance, solid at room temperature and is considered as a good conductor of electricity. Gold is also malleable and ductile in nature.

Mixtures

When we take two or more pure substances and mix them together, we define this as a mixture. Mixtures can usually be broken down again into components of pure substances, since the bonding between the atoms of the constituent substances do not arise in a mixture. Instead a compound may possess very different properties from the elements that compose it, in mixtures the substances retain their individual properties. For example sodium is usually a soft shiny metal whereas chlorine is a pungent green gas. These two elements combine together to form a compound, namely sodium chloride (i.e. table salt) . It is a white, crystalline solid having none of the properties of both sodium or chlorine. If, however, we mix table salt with ground pepper, we would still be able to see the individual grains of each of them.

Characteristics of Mixtures

The constituents of a mixture do not occur in a definite ratio. For example in ocean water some regions are saltier than others. The amount of carbon dioxide and water vapour present in air differs from place to place.

No chemical reaction takes place between the constituents of a mixture. Therefore, the forces that hold the constituents of a mixture together are weak and, thus, the constituents can be separated only by simple methods. For example, a mixture of sand and water can be separated easily via filtration.

The constituents of a mixture have the ability to retain their original properties. For example, the ability of air to support combustion is due to the presence of oxygen, which is one of its constituents. The chemical properties of table salt (sodium chloride) remains the same, whether it is part of a mixture (i.e. seawater) or present by itself.

Types of mixtures

Homogeneous mixture – the components of a homogeneous mixture possess a uniform composition, and are not visible  separately. The word ‘homo’ means the same and it indicates that when two substances combine extremely well with one another, they form a uniform mixture. As seen in sugar and water they do not react chemically and form another compound.

Heterogeneous mixture – The constituents of a heterogeneous mixture do not possess a uniform composition and can be viewed separately without losing their identity. For example, when we mix sulfur powder with iron dust, we can easily see the two separately. We can even separate the iron dust by using a magnet.

Example of Mixture

Though water is a pure substance, if we put sand into a glass of water, it will turn into a mixture. Every component of a mixture possesses the ability of separation from one another. We can always separate the sand from water via filtering it.

If we take a mixture of salt and water, then this can be separated via evaporating the water, to get salt in the container. Air is also a mixture of different gases like those of carbon dioxide, oxygen, nitrogen and water vapour etc. Blood is a mixture composed of different types of blood cells and plasma.

Pure Substances vs Mixtures

There are some similarities and differences between pure substances and mixtures while comparing both of them. Some of their differences are discussed below: Purity of pure substances are usually pure whereas mixtures are impure.

One cannot separate pure substance via a physical process but the mixtures can be possibly separated via evaporation, magnetic separation, etc. Pure substances usually have a fixed composition whereas the mixtures have a variable composition.

Pure substances have a definite melting and boiling point while mixtures have changing melting and boiling points. Characteristics of a pure substance are fixed while mixtures possess varying physical and chemical properties. Pure substances are composed of a single element whereas a mixture is generally a combination of two substances or elements.

Conclusion

Matter is classified broadly into two distinct categories: pure substances and mixtures. A pure substance represents a form of matter that possesses a constant composition and properties that remain   constant throughout the sample. Mixtures are generally physical combinations of two or more elements and or compounds. Mixtures are further grouped into homogeneous or heterogeneous. Elements and compounds both are two distinct forms of pure substances. Compounds are substances that are generally made up of more than one type of atom. Elements are the simplest form of substances composed of only a single type of atom.