Chemistry (Part-VII) (Railway)

• They take even millions and billions of years to be formed.

• They cannot be replenished within a given time.

• Coal and petroleum are not an unlimited resource on the Earth.

The earliest attempt to classify the elements was in 1789, when Antoine Lavoisier grouped the elements based on their properties into gases, non-metals, metals, and earths.

• In 1829, Johann Dobereiner recognised triads of elements with chemically similar properties and showed that the properties of the middle element could be predicted from the properties of the other two.

• Even the modern periodic table lists the elements in order of property (increasing atomic number).

• Henry Moseley is known as the Father of the modern periodic table.

A homologous series is a series of compounds of carbon that have different numbers of carbon atoms but have the same functional group.

• The general formula for the homologous series of alkenes is (CₙH₂ₙ) where n is the number of carbon atoms.

• Decene (C₁₀H₂₀) belongs to the homologous series of alkenes.

The reaction of CaO + H₂O → Ca(OH)₂ is described as a combination reaction.

• In this reaction, calcium oxide and water combine to form a single product, which is calcium hydroxide.

Dobereiner classified the elements into triads (group of three).

• He made the groups such that the atomic mass of the middle element

The middle element was the average atomic mass of the other two elements.

Li, Na, K is an example of Dobereiner's triad. Here, the average atomic mass of Li and K is equal to the atomic mass of Na.

Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating.

• This acid can lower the pH in the mouth.

• Tooth decay starts when the pH of the mouth is lower than 5.5.

Carboxylic acid is a class of organic compounds in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond and to a hydroxyl group (-OH) by a single bond.

• A fourth bond links the carbon atom to a hydrogen (H) atom or to some other univalent combining group.

• The carboxyl (COOH) group is so-named because of the carbonyl group (C = O) and hydroxyl group.

In the equation, SO₂ + H₂S → 3S + 2H₂O, when we count the number of each atom in the reactant side, we find H = 2, S = 2, O = 2.

• When we count the number of each atom in the product side, we find H = 4, S = 3, O = 2.

• We can then balance the number of each atom in the equation by multiplying reactant and product side with numerical values.

• If we multiply H₂S by 2 in the reactant side, the number of atoms in the product and reactant side gets the balance.

There are five toxic, non-metallic elements in Group 17 of the periodic table.

• The group consists of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).