Railway Science (Chemistry-Miscellaneous) (Part-II)

Molecules are formed when two or more atoms join together. An example of a molecule is ozone. Alloys are made by mixing two or more elements, at least one of which is a metal.
Examples of alloys include red gold (gold and copper), white gold (Gold and about Nickel and zinc), sterling silver (silver and copper).
Element is a substance whose atoms all have the same number of protons. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and oxygen.

When milk of lime, which is a suspension of calcium hydroxide (Ca(OH)₂) in water, is exposed to the air, it reacts with carbon dioxide to form calcium carbonate (CaCO₃), water (H₂O), and heat. This reaction is known as carbonation.
The chemical equation for this reaction is: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. As the carbonation reaction occurs, the calcium carbonate is precipitated, leading to the drying or hardening of the milk of lime.
Carbonation - It is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid.

Classification of water based on the presence of Calcium carbonate (CaCO₃) in terms of mg/L (milligrams per liter): 61 to 120 mg / L - Moderately hard, 121 to 180 mg / L - Hard, More than 180 mg / L - Very Hard.
Soft water: Surface water that contains relatively low ion concentrations and is low in calcium and magnesium ions in particular. Hard water: Which contains a high concentration of calcium and magnesium ions.

Plaster of Paris is a quick-setting gypsum plaster made of fine white powder (Calcium sulphate hemihydrate) that hardens when left to dry. The setting of plaster of paris is due to hydration. On hydration it converts into gypsum (Calcium sulphate dihydrate).

When diluting acid is added to Lime water {Calcium hydroxide, Ca(OH)₂}, the salt and water are formed. It is an exothermic reaction because it is a reaction with strong acid and strong base. The free lime is insoluble in water.

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed by bacteria in decomposing the organic wastes present in water. It is expressed in milligrams of oxygen per litre of water.
Chemical Oxygen Demand (COD) measures the amount of oxygen in parts per million required to oxidise organic (biodegradable and non-biodegradable) and oxidizable inorganic compounds in the water sample.

The mass of one mole of any substance is called the molar mass. Isotopes have same number of protons but different numbers of neutrons. Avogadro's number is equal to 6.022 × 10²³ .
Ion - Any atom or group of atoms that bears one or more positive or negative electrical charges. Cations (Positively charged ions) and Anions (negatively charged ions)

It is superheated matter that the electrons are ripped away from the atoms forming an ionized gas. It comprises over 99% of the visible universe. In the night sky, plasma glows in the form of stars, nebulas, and even the auroras that sometimes ripple above the north and south poles. Plasma, the fourth state of matter (beyond the conventional solids, liquids and gases).

Atomic Mass - Sum of the mass of the protons and neutrons. Isotopes are members of a family of an element that all have the same number of protons but different numbers of neutrons. Example - Carbon ( ⁸C to ²²C), Oxygen ( ¹⁶O,¹⁷O, ¹⁸O), Nitrogen ( ¹⁴N, ¹⁵N), Hydrogen (¹H, ²H, ³H).

An attraction property of magnets used for the separation of magnetic materials from non magnetic material. Evaporation - It is a process of turning liquid into vapour.
Fractional distillation - A process of separation of a mixture into its component parts or fractions according to their boiling points. Filtration - It is used to separate insoluble impurities from the liquid.