Chemistry is a branch of science that deals with properties, transformation, and composition of matter.
Taking limestone, for instance, its chemical composition is CaCO3, it reacts with dilute hydrochloric acid (dil. HCl) to give calcium chloride (CaCl2), water (H2O), and carbon dioxide (CO2).
$$CaCO_3+ dil. HCl \rightarrow CaCl_2 + CO_2 + H_2O$$
Chemistry is further divided into following groups:
1) Physical chemistry
2) Inorganic chemistry
3) Organic chemistry
5) Industrial chemistry e.t.c.
Simply, matter is anything that occupies space, has mass, and offers resistance. Matter can be sensed by us via our sense organs like nose, eyes, skin etc. Examples are- duster, book, table, desk, water, gas etc.
Mass: The amount of matter contained in a body is known as its mass. It can be determined by the help of a beam balance. Its value is constant. This means, regardless of the place, the mass of the body remains the same.
Weight: The force exerted by the gravitational force on an object is called its weight. Weight is not a constant value and may differ from place to place. The value of weight depends on the value of gravitational force 'g'. This signifies that higher the value of 'g', higher will be its weight and vice-versa.
The chart below shows the simple classification of matter:
Physically, matter is classified into three states. They are widely known as:
1) Solid: The matter which has a fixed shape, size, volume, and mass is called solid. The intermolecular force in solid is very strong. This causes the strong association between the molecules and atoms. Hence, solids take fixed shape and size. Example- Iron, book, pen etc.
2) Liquid: The matter which has fixed volume but has no fixed shape is called liquid. The intermolecular force in a liquid is intermediate. This means the molecules are associated with each other in neither too strong nor too weak way. This way they are mobile in nature and hence take the shape of the vessel they are kept in. Examples are- Water, milk, oil etc.
3) Gas: The matter which has neither fixed volume nor fixed shape is called gas. The intermolecular force in gas is very weak. This causes the attraction between the molecules to be weak. Thus, they are not able to retain a fix shape and size. Example- Hydrogen, Nitrogen, Oxygen etc.
Chemically, matter is divided into pure and impure (mixture) substances.
A) Pure chemical substance: A pure chemical substance is composed of only one kind of matter and cannot be split physically into more than one kind of substances. Example: Iron, water etc.
Pure chemical substances are further divided into :
⇒Elements: Elements are the pure chemical substances, which can neither be split into simpler substances nor can be prepared by combining two or more substances by any simple method. Hydrogen, Oxygen, Carbon are some examples of elements. There are around 118 elements in the periodic table so far. Out of these 118 elements, 98 elements are natural as they are found in the earth naturally whereas the rest are man made or artificial. Elements are further divided into:
⇒Compounds: Compound may be defined as the pure chemical substance which is produced by the combination of two or more than two elements in definite proportion by weight. The compounds are homogeneous and chemical and physical properties of compounds are entirely different from the component elements. Compounds can be split into two or more elements by suitable chemical methods. For example- Hydrogen sulphide (H2S), which is composed of two elements, hydrogen and sulphur in the ratio 1:16 by mass.
B) Impure (Mixture) chemical substances: Mixture is a combination of two or more substances, compounds in any ratio so that components do not lose their characters. The two or more substances, which when brought together do not undergo any chemical changes and retain their identity, the resulting mass is known as a mixture. Air, salt solution, mud etc are examples of a mixture. Mixtures are of following two types:
⇒ Homogeneous mixture: A homogeneous mixture contains the same composition throughout the sample. Examples- Sodium chloride solution, air mixture.
⇒ Heterogeneous mixture: A heterogeneous mixture contains different composition and properties in different part of the mixture. Example- Mudwater, salt water.
|Physical Change||Chemical Change|
|1)||It is a temporary change and can be easily reversed. Eg- $$Water \rightleftharpoons vapour$$||It is a permanent change and cannot be reversed. Eg- Paper burnt into ashes cannot be brought back into paper.|
|2)||In a physical change, there is no new chemical substance formed.||In a chemical change, there is a formation of new chemical substance.|
|3)||There is no change of mass during the physical change.||The mass of product is different from that of the reactants.|
|4)||Generally, energy change during a physical change is very small.||The energy change during a chemical change is rather high.|
Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).
Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.
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