(IGP) GS Paper 1 - General Science - "Gist of Chemistry from NCERT Books"

Subject - General Science
Chapter : Gist of Chemistry from NCERT Books

Acid, Base and Salts

Acid

Acids are compounds that contain Hydrogen (Hydrochloric, HCl; Sulphuric, H2SO4; Nitric, HNO3). However, not all compounds that contain Hydrogen are acids (Water, H2O; Methane, CH4). Acids are usually compounds of non metals with Hydrogen and sometimes Oxygen. Acids can be classified in various ways, depending on the factors mentioned below:

• Classification Based on the Strength of the acid.

• Classification Based on the Basicity of the Acid.

• Classification Based on the Concentration of the acid.

• Classification Based on the presence of Oxygen.

Bases and Alkalis

A Base is a substance that gives OH- ions when dissolved in water. Bases are usually metal hydroxides (MOH). Examples include Sodium Hydroxide, NaOH, Calcium Hydroxide, Ca(OH)2. The solution of a base in water is called an alkali. The strength of a base depends on the concentration of the hydroxyl ions when it is dissolved in water.

1. Strong Base: A base that dissociates completely or almost completely in water is classified as a strong base. The greater the number of hydroxyl ions the base produces, the stronger is the base. Examples: Sodium hydroxide: NaOH, Potassium hydroxide: KOH, Calcium hydroxide: Ca(OH)2.

2. Weak Base: A base that dissociates in water only partially is known as a weak base. Examples: Magnesium hydroxide: Mg(OH)2, Ammonium hydroxide: NH4OH.

pH

A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali.

Salts

A Salt results when an acid reacts with a base. Both are neutralised. The H+ and OH- ions combine to form water. The non metallic ions of the acid and the metal ions of the base form the salt.

Important salts used in everyday life and industrial applications are Sodium chloride (NaCl), Sodium carbonate, (Na2CO3), Sodium Bicarbonate, (NaHCO3), Sodium Hydroxide (NaOH)

Chemical Bonding

Chemical bonds are what hold atoms together to form the more complicated aggregates that we know as molecules and extended solids. The forces that hold bonded atoms together are basically just the same kinds of electrostatic attractions that bind the electrons of an atom to its positively-charged nucleus. chemical bonding occurs when one or more electrons are simultaneously attracted to two nuclei.

Chemical Reactions and Equations

• A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. The change may conveniently be represented by a chemical equation.

• Chemical reactions occur when different atoms and molecules combine together and split apart. For example, if Carbon (C) is burnt in Oxygen (O2) to form Carbon Dioxide, a Chemical Reaction occurs. This reaction can be written: C + O2—> C O2. This is called a Chemical Equation. The substances on the left hand side of the equation are called the Reactants. The substances on the right hand side are called the Products.

Solution

• A solution is a homogeneous mixture of two or more substances. The major component of a solution is called the solvent, and the minor, the solute. Lemonade, soda water etc. are all examples of solutions. We can also have solid solutions (alloys) and gaseous solutions (air).

• The particles of a solution are smaller than 1 nm (10-9 metre) in diameter. So, they cannot be seen by naked eyes. The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.

Alloys

• Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper.

• Non-homogeneous systems, in which solids are dispersed in liquids, are called suspensions. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye.

Organic Chemistry

• Organic chemistry is that branch of chemistry which deals with the study of compounds of carbon with hydrogen (hydrocarbons), and their derivatives.

• Organic Chemistry is essentially the chemistry of Carbon. Carbon compounds are classified according to how the Carbon atoms are arranged and what other groups of atoms are attached.

• Hydrocarbons: The simplest Organic compounds are made up of only Carbon and Hydrogen atoms only. Even these run into thousands! Compounds of Carbon and Hydrogen only are called Hydrocarbons.

For Detail Description, Analysis and More MCQs of the Chapter Buy this Study Notes:

Properties of Gases

• First, we know that a gas has no definite volume or shape; a gas will fill whatever volume is available to it. Contrast this to the behavior of a liquid, which always has a distinct upper surface when its volume is less than that of the space it occupies.

• The other outstanding characteristic of gases is their low densities, compared with those of liquids and solids. The most remarkable property of gases, however, is that to a very good approximation, they all behave the same way in response to changes in temperature and pressure, expanding or contracting by predictable amounts. This is very different from the behavior of liquids or solids, in which the properties of each particular substance must be determined individually.

• All gases expand equally due to equally due to equal temperature difference.

• Diffusion of gases: The phenomenon in which a substance mixes with another because of molecular motion, even against gravity- is called diffusion.

• The pressure of a gas: The molecules of a gas, being in continuous motion, frequently strike the inner walls of their container. As they do so, they immediately bounce off without loss of kinetic energy, but the reversal of direction (acceleration) imparts a force to the container walls. This force, divided by the total surface area on which it acts, is the pressure of the gas.

• The unit of pressure in the SI system is the pascal (Pa), defined as a force of one newton per square metre (1 Nm–2 = 1 kg m–1 s–2.

• Temperature scale: A reference scale with respect to which the temperatures can be measured is known as ‘scale of temperature’. Various scales of temperatures are in use. Important scales of temperature are:
  (a) Celsius scale
  (b) Kelvin scale
  (c) Fahrenheit scale

• Celsius scale: In this scale the lowest fixed point is the freezing temperature of pure substance. The upper fixed point is the boiling point of water. The interval is divided into 100 divisions all are at equal distance. Every division being denoted as one degree Celsius (oC). The Celsius scale is also called as centigrade scale because the range of temperature is divided into 100 equal divisions.

• Kelvin scale: Another type of scale which is used to define the measure of temperature is Kelvin scale. The Kelvin scale is also known as absolute scale of temperature. The lowest fixed point is taken from the lowest temperature to which a substance to be cooled such as -273oC. According to the scale, a temperature is denoted by simply K .

• Absolute zero: The temperature at which a given mass of gas does not occupy any volume or does not exert pressure is called the “absolute zero”. Absolute zero i.e., 0K or -273oC is the lowest possible temperature that can be reached. At this temperature the gas has a theoretical volume of zero. In the Kelvin scale, the lowest possible temperature is taken as zero. This temperature is called as absolute zero. At the point absolute zero there is no molecular motion and there is no heat energy. At absolute-zero all atomic and molecular motions stop. Hence the absolute zero is the lowest possible temperature which is denoted by 0K or -273o C.

• Fahrenheit Scale of Temperature: The lower and upper fixed points in this scale are considered as 32o F and 212o F respectively. The interval of 180o F is divided into 180 equal parts. Each part is known as 1o F. This is widely used by doctors.

Gas Laws

• All gases, irrespective of their chemical composition, obey certain laws that govern the relationship between the volume, temperature and pressure of the gases. A given mass of a gas, under definite conditions of temperature and pressure, occupies a definite volume. When any of the three variables is altered, then the other variables get altered. Thus these Gas laws establish relationships between the three variables of volume, pressure and temperature of a gas.

• Boyle’s Law: Robert Boyle (1627 - 1691) discovered this law in 1662 and it was named after him. It can be restated as “The product of the volume and pressure of a given mass of dry gas is constant, at constant temperature”. P ∝ 1/ V( at constant temperature) or P X V= K (where K is constant).

• Charles’ Law: “At constant pressure, the volume of a given mass of gas increases or decreases by 1/273 of its original volume at 32oF, for each degree centigrade rise or lowering in temperature.” Assume a given mass of gas has a volume of V1 at a temperature T1 Kelvin at a constant pressure, then, according to Charles’ Law we can write:
  V directly proportional to T or VT=K (Constant).

• Avogadro’s Law: This is quite intuitive: the volume of a gas confined by a fixed pressure varies directly with the quantity of gas. Equal volumes of gases, measured at the same temperature and pressure, contain equal numbers of molecules. Avogadro’s law thus predicts a directly proportional relation between the number of moles of a gas and its volume.

Some Common Elements & Compounds

Hydrogen: Symbol H, formula H2. The first element in the periodic table and the most basic and common of all elements in the universe. Over ninety percent of all the atoms in the universe are hydrogen atoms and they are the lightest of all elements. The name hydrogen comes from the Latin word “hydro” which means water. Scientists use the letter “H” to represent hydrogen in all chemical equations and descriptions.

Carbon: The sixth element in the periodic table. It is a very stable element. Because it is stable, it can be found in many naturally occurring compounds and by itself. Scientists describe the three states of carbon as diamond, amorphous, and graphite.

Nitrogen: It is the seventh element of the periodic table located between carbon and oxygen. Almost eighty percent of Earth’s atmosphere is made of nitrogen gas. Nitrogen is a clear gas that has no smell when it is in its pure form. It is not very reactive when it is in a pure molecule, but it can create very reactive compounds when combined with other elements including hydrogen (ammonia). There are 7 electrons in a nitrogen atom.

Oxygen: Symbol O, formula O2. Alone, oxygen is a colorless and odorless compound that is a gas at room temperature. Oxygen molecules are not the only form of oxygen in the atmosphere; you will also find oxygen as ozone and carbon dioxide. There are 8 electrons in an oxygen atom.
Oxygen is non-combustible but a good supporter of combustion. An oxide is a compound of two elements, one of which is oxygen. It can be liquefied and solidified. It is employed in welding process and also used in hospitals for artificial respiration. Oxygen shows a valency of -2.

Water (H2O): Water is the only substance that can exist simultaneously in all the three states of matter, i.e., solid, liquid and gaseous on this earth.

• Pure water is a colourless, odourless and tasteless liquid.

• The density of water is 1 g cm-3 at 4oC.

• The boiling point of water is 100oC at a pressure of 760 mm of Hg. The melting point of ice is 0oC at a pressure of 1 atmosphere.

• Ice has a relative density of 0.92. The specific heat capacity of water is 1 cal/g at 15oC.

• Water is called the “Universal Solvent”. Almost all substances dissolve in water to a certain extent. Hence, it known as a universal solvent. Because of this property, it is impossible to get chemically pure water on the earth.

For Detail Description, Analysis and More MCQs of the Chapter Buy this Study Notes:

<< Go Back to IGP Main Page