Key Differences Between Pure and Impure Substances
Anything that has mass and that occupies some space is called matter. There are basically two types of matter, pure and impure substances. Almost everything that you see around you is all matter. However, the question which arises here is whether the matter around us is pure or not? Moreover, on what terms can you define the term purity? There would have been many instances where you would have come across sentences like the water is pure or the milk used to make the tea is pure. But is it right to say all this? Or on what grounds can you say that a substance is pure? Can you say that any substance is pure if it is smooth in consistency or feels soft or is white in color? Well, to answer all your questions we will be learning about the matter today, particularly the pure and impure substances and know the answer to the question “is matter around us pure”.
What are Pure and Impure Substances?
A substance is called to be in the pure form if it is made up of just one type of molecules or particles. Pure substances normally cannot be separated by the means of simple physical methods and are widely classified as elements and compounds. An element contains only one kind of particle that can be atoms or molecules. Similarly, compounds are pure substances when they consist of two or more elements that are combined in a fixed proportion by their weight. Additionally, pure compounds are easily decomposed to separate elements by using several suitable chemical methods.
A substance which is composed of different types of elements or molecules is called impure. These kinds of substances are often known to be mixtures. Mixtures are further classified as either homogeneous mixtures or heterogeneous mixtures based on their composition. They are mostly made pure by using different kinds of separation techniques.
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Characteristics of Pure Substance
Let us now look at some of the characteristics of pure substance.
Pure substances have a perfectly homogeneous nature.
Pure substances are made up of only one type of atoms or molecules.
Pure substances have a fixed composition.
Pure substances have a fixed density, melting point, boiling point etc.
Characteristics of Impure Substance
Let us look at the characteristics of impure substances now.
Impure substances do not have any specific properties. The properties of the resulting mixture are a result of the average properties of all its constituents.
Impure substances are formed when there is a physical change.
Impure substances have a variable composition.
The mixtures of the impure substances are either homogeneous or heterogeneous in nature.
Difference between Homogeneous and Heterogeneous Substances
Different States of Matter
The three different states of matter present on Earth are solid state, liquid state, and gaseous state. Let us look at each of them in detail.
Solids
In a solid, the particles are packed tightly together so they don't move much. The electrons of each atom are constantly in motion, so the atoms have a small vibration, however, they are fixed in their position. Because of this, the particles in a solid have very low kinetic energy.
Solids have a definite shape, as well as mass and volume, and do not conform to the shape of the container in which they are placed. Solids also have a high density which means that the particles are tightly packed together.
Liquids
In a liquid, the particles are more loosely packed than in a solid and are able to flow around each other, giving the liquid an indefinite shape. Therefore, the liquid will conform to the shape of its container.
Much like solids, liquids (most of which have a lower density than solids) are incredibly difficult to compress.
Gasses
In a gas, the particles have a great deal of space between them and have a higher kinetic energy. A gas has no definite shape or volume. If unconfined, the particles of a gas will spread out indefinitely; if confined, the gas will expand to fill its container. When a gas is put under pressure by reducing the volume of the container, the space between the particles is reduced and the gas is compressed.
Is Matter Around us Pure? - Summary
As mentioned above matter is of two different kinds, pure and impure substances. You can know the products and matter around you is pure or impure by taking a look at them and their composition. Consider, for example, salt. The constituents of all the particles in the salt are the same, that is, the particles are all NaCl. They look and taste the same as well. There is no other kind of particle present in the salt except NaCl. This means that salt is a pure substance. When you talk about impure substances, they are made up of different molecules in just one substance. Consider, for example, ghee. Ghee is made up of several vitamins, amino acids, fats, etc., and is, therefore, regarded as an impure substance. All impure substances are said to be mixtures of two or more substances or molecules.
FAQs on Is Matter Around Us Pure?
1. From a scientific perspective, is the matter we encounter in daily life truly pure?
No, from a scientific standpoint, most matter around us is not pure. It primarily exists as mixtures. A substance is considered scientifically pure only if it consists of a single type of particle (atoms or molecules). Everyday items like milk, soil, and even air are mixtures of several different substances.
2. What is the fundamental difference between a pure substance and a mixture?
The key difference lies in their composition and properties. A pure substance consists of only one type of particle and has a fixed chemical composition and distinct properties (like a fixed melting and boiling point). Examples include elements like iron and compounds like water (H₂O). A mixture, on the other hand, consists of two or more substances that are physically combined in any ratio and retain their individual properties.
3. How can you differentiate between homogeneous and heterogeneous mixtures? Please provide examples.
A homogeneous mixture has a uniform composition and appearance throughout. The particles are indistinguishable from one another. A heterogeneous mixture has a non-uniform composition, where the different components are often visible.
- Homogeneous Example: Salt dissolved in water. Once stirred, you cannot see the individual salt particles.
- Heterogeneous Example: A mix of sand and water. You can clearly see the sand particles settled at the bottom.
4. Why is air considered a homogeneous mixture and not a compound?
Air is considered a homogeneous mixture for several reasons:
- Variable Composition: The ratio of its components (like oxygen, nitrogen, and carbon dioxide) can vary from place to place. Compounds have a fixed ratio of elements.
- No Chemical Bonds: The gases in air are mixed together but are not chemically bonded.
- Properties of Components: Air exhibits the properties of its constituent gases.
- Physical Separation: The components of air can be separated by physical processes like fractional distillation.
5. What are solutions, colloids, and suspensions?
These are three types of mixtures, primarily classified by their particle size:
- A Solution is a homogeneous mixture with very small solute particles (less than 1 nm) that do not settle and cannot be seen. Example: Sugar-water.
- A Colloid is a heterogeneous mixture with intermediate-sized particles (1-1000 nm) that remain dispersed and do not settle. They often appear homogeneous but scatter light. Example: Milk.
- A Suspension is a heterogeneous mixture with large particles (over 1000 nm) that are visible and settle down when left undisturbed. Example: Muddy water.
6. How does the Tyndall effect help in distinguishing a colloid from a true solution?
The Tyndall effect is the scattering of a beam of light by particles in a mixture. The particles in a colloid are large enough to scatter the light, making its path visible. In a true solution, the solute particles are too small to scatter light, so the beam's path is not visible. This phenomenon is a key practical test to identify a colloidal mixture.
7. What is the difference between a physical and a chemical change?
A physical change alters the form or appearance of a substance but does not create a new substance. These changes are often reversible. An example is the melting of ice into water. A chemical change results in the formation of one or more new substances with different properties. These changes are usually irreversible. An example is the burning of wood, which produces ash and smoke.
8. What is the practical importance of knowing various separation techniques?
Understanding separation techniques is vital in science and industry. For instance, distillation is used to purify drinking water from seawater and to refine crude oil into petrol and diesel. Chromatography is crucial in forensic science for analysing crime scene samples and in the pharmaceutical industry to ensure drug purity. Centrifugation is used in medical labs to separate blood components for testing.
9. If salt (a pure compound) and water (a pure compound) are both pure, why is saltwater classified as an impure mixture?
This is a great question that highlights a core concept. While both salt (NaCl) and water (H₂O) are pure compounds individually, saltwater is a mixture because the salt and water are only physically combined, not chemically bonded to form a new compound. The salt and water molecules retain their original chemical identities and can be separated by a simple physical process like evaporation, which would leave the salt behind.
10. What are the two types of pure substances as per the Class 9 syllabus?
According to the CBSE syllabus for the 2025-26 session, pure substances are classified into two main types:
- Elements: The most basic form of matter that cannot be broken down into simpler substances by chemical reactions. Examples include Oxygen (O), Iron (Fe), and Gold (Au).
- Compounds: Substances formed when two or more elements are chemically combined in a fixed ratio by mass. Examples include Water (H₂O) and Carbon Dioxide (CO₂).
