Definition of compound: In chemistry, a compound is a pure substance that is composed of two, or more different elements, chemically combined with one another in a definite ratio.
In other words, a compound is a unique substance that can be broken down (or decomposed) into simpler substances by some suitable chemical reaction or change.
A molecule of a compound is formed when two or more atoms of different elements combine together chemically in a fixed proportion by mass. For example, water is a compound of hydrogen and oxygen elements present in a fixed proportion 1 : 8 by mass.
Thus, the compound water has a definite composition of 11.2% hydrogen and 88.8% oxygen. Other examples of some compounds are ammonia, carbon dioxide, sugar, methane, etc. You can see the molecules of water and carbon dioxide in the below Fig.
From the figure, it is clear that a molecule of water comprises two hydrogen atoms and one oxygen atom. Similarly, a carbon dioxide molecule contains two oxygen atoms combined with one carbon atom. Thus, the atoms of different elements are present in a compound in a fixed proportion or ratio, and this proportion is characteristic of a specific compound.
Properties of Molecular Compounds
There are the following properties of molecular chemical compounds in chemistry that you should keep them in mind. They are:
1. We cannot separate the constituents of a compound into simpler substances by mechanical or physical methods. Chemical methods can separate them. For instance, if we bring a magnet near a sample of iron sulphide, we cannot separate the iron present in the iron sulphide.
2. The properties of a compound are different entirely from those of its constituent elements. In other words, the properties of a compound differ entirely from the properties of the elements from which it has made up.
For instance, hydrogen and oxygen are gases, whereas the compound formed by their combination, i.e., water molecule is a liquid under ordinary atmospheric conditions. Thus, the properties of water differ totally from the properties of hydrogen and oxygen both. However, it is interesting to note that hydrogen is combustible, oxygen is a supporter of combustion, but water (made up of both hydrogen and oxygen) has normally used as a fire extinguisher.
Another example we can take NaCl that is very different from the sodium and chloride elements used in its formation. Sodium element consists of sodium atoms is a soft, and silvery metal that can cut easily with knife. Its melting point is 97.5℃ and it reacts violently with water. Element chlorine, Cl2 is a yellow green gas at room temperature and very toxic. Its boiling point is -34℃.
The compound NaCl composed of Na and Cl is a translucent, brittle, and colorless crystal, having a melting point of 800℃. It does not react with water as sodium does. Also, it is not toxic to human as chlorine is.
3. Compounds containing over four elements are rare.
4. The constituent elements in a compound are in definite ratio by weight. In water, hydrogen and oxygen are present in a definite ratio 1:8 by weight.
5. A compound is a homogeneous substance that is the same throughout in properties and composition.
6. It has fixed melting point and boiling point. For example, ice melts at 0℃.
7. When a compound forms, energy change occurs. For example, iron and sulphur react only when we supply heat. Similarly, the process of photosynthesis is an example of endothermic reaction, whereas burning candle is an example of exothermic reaction.
Types/Classification of Chemical Compounds
We can classify chemical compounds into two types. They are:
- Organic compounds
- Inorganic compounds
Organic compounds: The compounds obtained from living sources or things are called organic compounds. The term organic is now applied to hydrocarbons (compounds of hydrogen and carbon) and their derivatives. There are a very large number of organic compounds that occur naturally or can be made synthetically.
Some familiar examples of organic compounds are carbohydrates, fats (lipids), proteins, and nucleic acids, which are very essential for human life. Other examples of organic compounds are petroleum and natural gas, which are the main components of fossil fuels.
Inorganic compounds: The compounds obtained from non-living things such as minerals and rocks are called inorganic compounds. The compounds of all elements except hydrocarbons and their derivatives involves in this category. The number of organic compounds is very large compared to inorganic compounds.
Some familiar examples of common everyday inorganic compounds are water, sodium chloride (salt), sodium bicarbonate (baking soda), calcium carbonate (dietary calcium source), phosphoric acid, sulphuric acid, nitric acid, and ammonia.
Examples of Compound and Their Molecular Formula
Here, we have listed some commonly used compounds and their molecular formulas in table form that you should remember for the examination.
| Compound Name | Molecular Formula |
|---|---|
| Water | H₂O |
| Carbon Dioxide | CO₂ |
| Methane | CH₄ |
| Sodium Chloride (Salt) | NaCl |
| Ammonia | NH₃ |
| Glucose | C₆H₁₂O₆ |
| Hydrochloric Acid | HCl |
| Sulfuric Acid | H₂SO₄ |
| Acetic Acid (Vinegar) | CH₃COOH |
| Ethanol (Alcohol) | C₂H₅OH |
| Calcium Carbonate | CaCO₃ |
| Baking Soda (Sodium Bicarbonate) | NaHCO₃ |
| Oxygen (Diatomic Oxygen) | O₂ |
| Nitrogen (Diatomic Nitrogen) | N₂ |
| Potassium Nitrate (Saltpeter) | KNO₃ |
| Calcium Hydroxide (Slaked Lime) | Ca(OH)₂ |
| Magnesium Sulfate (Epsom Salt) | MgSO₄ |
| Phosphoric Acid | H₃PO₄ |
| Nitric Acid | HNO₃ |
| Sodium Hydroxide (Caustic Soda) | NaOH |
| Benzene | C₆H₆ |
| Aluminum Oxide | Al₂O₃ |
| Hydrogen Peroxide | H₂O₂ |
| Carbon Monoxide | CO |
Difference between Element and Compound
In this section, we will understand the basic difference between element and compound, which is one of the most commonly discussed topics in the chemistry subject. Here, we have listed the differences in the tabular form that will help you to understand better.
| Feature | Element | Compound |
|---|---|---|
| Definition: | A pure substance that cannot be broken down (or decomposed) into simpler substances by chemical means. | A unique substance that can be broken down (or decomposed) into simpler substances by some suitable chemical reaction or change. |
| Relatedness: | It consists of only one type of atom. | It consists of two or more different types of atoms. |
| Example: | Hydrogen (H), Oxygen (O), Gold (Au). | Water (H₂O), Sodium Chloride (NaCl), Carbon Dioxide (CO₂). |
| Structure: | Exists as single atoms or in forms such as diatomic molecules. For example, noble gases, O₂, N₂, etc. | Has a definite chemical formula and structure, representing the ratio of atoms. For example, the water (H₂O) consists of 2 H and 1 O. |
| Visibility: | Elements can be visible as solids, liquids, or gases. For example, mercury is liquid, whereas oxygen is gas. | Compounds can be visible as solids, liquids, or gases, depending on their state at room temperature. For example, water is a liquid, whereas salt is a solid. |
| Chemical Properties: | Element has unique chemical properties that distinguish them from one another. | Compound has distinct properties that differ from the properties of constituent elements. For example, water has different properties than hydrogen and oxygen. |
| Physical Properties: | The common physical properties of elements are melting point, boiling point, density, color, conductivity, etc. | Compound has its own set of physical properties, which can differ significantly from those of its elements that form it. For example, sodium is a metal, chlorine is a gas, but NaCl is a solid. |
| Formation: | Naturally occurring or synthesized in laboratories. | Formed through chemical reactions. |
| Examples in Nature: | Found as pure substances in nature. | Found as combinations in nature. For example, water, air, etc. |
Frequently Asked Questions based on Chemical Compounds
1. What is a compound in chemistry?
A compound in chemistry is a unique substance made up of two or more different elements in a definite ratio by weight.
2. What are five compounds used in our daily life?
Five compounds used in our everyday life are: common salt, water, soap, baking soda, and sugar.
3. Is it necessary the physical and chemical properties of a compound similar to those of elements from which it comprised?
No, it is not necessary that physical and chemical properties of a compound are the similar to those of elements of which they made up of.
4. What are the familiar examples of compound in chemistry?
The familiar examples of compound in chemistry are hydrochloric acid (HCl), acetic acid (CH3COOH), sulphuric acid (H2SO4), ammonia (NH3), nitric acid (HNO3), calcium carbonate (CaCO3), sodium bicarbonate (NaHCO3), etc.
5. What is stoichiometric and non-stoichiometric compounds in chemistry?
Molecule having a definite atomic composition are called the stoichiometric compounds. For example, H2O, CH4, NH3, etc.
Certain molecules in which atomic composition is variable are called non-stoichiometric compounds. For example, the composition of cuprous sulphide may vary from Cu1.7S to Cu2S.
In this tutorial, you have learned about what is the simple definition of a compound in chemistry and its types, properties with some familiar examples. I hope that you will have understood the basic definition of compounds and enjoyed study.
Thanks for reading!!!
