CBSE NCERT Chemistry Class 11 Science – Nature of Matter
NCERT Chemistry Class 11 : Takshila Learning is one of the largest growing unit providing online coaching classes for KG to class 12th offering animated videos, recorded lectures by experienced faculty, and notes on various subjects related to school syllabus and various professional courses. Our aim is to provide quality and result oriented education with holistic guidance to students. In the following article, we will discuss “the nature of matter and laws of chemical combination” from Unit I of chemistry Class 11 science– some basic concepts.
Chemistry notes for class 11 : Matter- Any physical substance which occupies space and possesses mass is called matter.
Atoms and molecules are all composed of matter. There are three fundamental states (sometimes called phases) of matter; Solids, Liquids and Gases. It is the physical state of the molecules and atoms that makes the different states of matter.
The two other states of matter are Plasmas and Bose-Einstein condensate (BEC).
In nature, matter is found in three different forms, based on their composition. These forms are
- Elements: An element is the simplest type of matter. It has unique physical and chemical properties. An element is made up of only one kind of an atom. However, an atom is the smallest constituent unit of matter. Various elements occur in nature like oxygen (O), aluminium (Al), copper (Cu), silicon (Si), etc.The physical properties of a piece of silicon, such as colour, density, etc. is different from that of a piece of aluminium. This is because the properties of their atoms are different. Hence, each element is unique as the properties of its atoms are unique.
- Compounds: A compound is a matter that consists of two or more different elements that are bonded chemically.While, a molecule is formed when two or more atoms or elements (may be same type or different types) join together chemically. So, acompound is a molecule that contains at least two different elements. For example O2 is a molecule (diatomic) as it contains 2 oxygen atoms that are chemically bonded, but CO2 is a compound containing two different atoms, Carbon and Oxygen that are bondedchemically.
- Mixtures:A mixture is said to be a matter which consists of two or more elements or compounds that are only physically intermixed. For example, NaCl (common salt) is added to H2O (water), mixture of compounds NaCl and water is prepared.
The elements and the compounds are considered as substances. A substance is a matter whose composition is fixed. Mixtures are not considered as substances because their composition varies, but they maintain many properties of its constituent elements or compounds.
In the above example of mixture of salt and water, the components NaCl and water can vary in terms of their mass. But, this mixture is colourless like water and salty like sodium chloride.
Chemistry is the study of the transformation of matter from one form to the other. These transformations often occur as a result of the combination of two different types of matter. The combination of different elements to form compounds is governed by certain basic rules. These rules are referred to as laws of chemical combination. There are five basic laws of chemical combination that govern the chemical combinations of elements. These laws are discussed here-
- Law of Conservation of Mass
This law states that matter can neither be created nor destroyed. In other words,it can be stated that the total mass of the substances does not change in a chemical reaction. The number of substances and their properties may change, but total amount of matter does not change. Antoine Lavoisier gave this law in the year 1789 based on the data he obtained after carefully studying numerous combustion reactions. Even in complex biochemical reactions, for example, metabolism of glucose, where many complex reactions are involved, the mass is conserved.
180g +192g oxygen gas → 264g carbon di oxide + 108g water
372g material before → 372g material after
- Law of Definite Proportions
Joseph Proust, a French chemist stated that no matter what is its source, a particular compound is composed of the same elements in the same parts (fractions) by mass and mass percent.The fraction by mass or part by mass is the part of the compound’s mass that each element contributes. It is calculated by dividing the mass of each element by the mass of the compound.The mass percent is calculated as fraction by mass multiplied by 100 (mass%)Thus in a compound, each element has a fixed mass fraction and thus fixed mass percent (mass%). For example, the calcium carbonate is composed of three elements calcium, carbon and oxygen; so each element will have a fixed fraction by mass/mass percent in calcium carbonate be it obtained from sea shells, marble, or from corals.
- Law of Multiple Proportions
This law states that if two elements combine to form more than one compound, the masses of these elements in the reaction are in the ratio of small whole numbers. It means,if elements A and B react to form two compounds, the different masses of B that combine with a fixed mass of A can be expressed as a ratio of small whole numbers. This law was given by Dalton in the year 1803. For example; let us consider that carbon and oxygen form two compounds- carbon oxide 1(CO1) and carbon oxide 2 (CO2).
Both CO1 and CO2 have different physical properties. By mass analysis it is found that;
In CO1: Oxygen is 57.1 mass% and Carbon is 42.9 mass%
In CO2: Oxygen is 72.7 mass% and Carbon is 27.3 mass%
To see the phenomenon of multiple proportions, the mass percentof oxygen and carbonto find their masses in a given mass say, 100g of each compound.Divide mass of oxygen by mass of carbon in each compound to obtain the mass of oxygen that combines with fixed mass of carbon:
g of oxygen / g of carbon in CO1= 57.1/42.9= 1.33
g of oxygen/ g of carbon in CO2= 72.7/27.3= 2.66
2.66g oxygen per g of carbon in CO2/ 1.33 g of oxygen per g of carbon in CO1
It comes out to be 2:1 which is ratio of small whole numbers.
Thus we see in the above example, the ratio is 2:1 that means a given mass of carbon in CO2 contains 2 times as much oxygen as in CO1.
- Gay Lussac’s Law of Gaseous Volumes
In 1808, Gay Lussac gave this law based on his observations. This law states that when gases are produced or combine in a chemical reaction, they do so in simple ratio by volume given that all the gases are at same temperature and pressure. This law can be considered as another form of law of definite proportions. The only difference between these two laws of chemical combination is that Gay Lussac’s Law is stated with respect to volume while law of definite proportions is stated with respect to mass.
- Avogadro’s Law
Avogadro proposed this law in the year 1811. It stated that under same conditions of temperature and pressure, equal volume of all the gases contain equal number of molecules. This implies that 2 litres of hydrogen will have the same number of molecules as 2 litres of oxygen given that both the gases are at same temperature and pressure.
The laws of chemical combination discussed above form the base for quantitative study of chemical reactions.