Indicators and their colours in acid and alkaline solution

• An indicator is substances that have distinctly different colors in acidic and basic media. Many indicators are dyes which have been extracted from natural sources, for example litmus. 
• Methyl orange, a common indicator used in titrations is pink in an acid solution but changes to show a yellow colour in an alkaline solution. 
• Phenolphthalein is pale-yellow crystals; soluble in alcohol, ether, and alkalis, insoluble in water; used as an acid-base indicator (carmine-colored to alkalis, colorless to acids) for titrations. 

Electrolysis of molten lead (II) bromide

During the electrolysis of molten lead(II) bromide, PbBr₂, bromine gas, Br₂ is released at the cathode while the lead metal is formed at the anode.  

Pb²⁺ and Br^- ions are present in the molten lead(II) bromide, PbBr₂.  At the anode, bromide ions, Br^- are discharged by donating electrons to form bromine molecules, Br₂. At the cathode, each lead(II) ion, Pb²⁺ is discharged by accepting two electrons to form a lead atom.

Formation of hydrogen molecules

Hydrogen atom has one valency. To become stable with hydrogen atom, it needs one more electron, When 2 hydrogen atoms join, they share their electrons, on which, the share becomes 2 electrons, which is now a noble gas configuration, being shared between these 2 atoms.

H – H single bond [they share an electron pair (2 electrons)].

Summary of Laws of Thermodynamics

• First Law: In any process, the total energy of the universe remains constant.

Universe = system + surroundings.

• Second Law: In any spontaneous process, the overall entropy of the universe increases.
• Third Law: As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.

Rules for figuring out oxidation numbers

• Atoms in elemental form have an oxidation number of zero.
• Single-atom (monoatomic) ions have an oxidation number equal to their charge.
• In a neutral compound, oxidation numbers add up to zero. In a charged compound, oxidation numbers add up to the compound’s charge.
• In compounds, oxygen usually has an oxidation number of –2, in which its oxidation number is –1.
• In compounds, hydrogen has an oxidation number of +1 when it bonds to nonmetals and an oxidation number of –1 when it bonds to metals.
• In compounds

Group 1 atoms (alkali metals) have oxidation number +1.

Group 2 atoms (alkaline earth metals) have oxidation number +2.

Group 3 atoms have oxidation number +3.

Group 17 atoms (halogens) usually have oxidation number –1.

Explain why the melting and boiling points of argon are higher than helium?

The atomic size of argon is bigger than helium. Thus, the forces of attraction between argon atoms are stronger than the forces of attraction between helium atoms. As a result, more heat energy is required to overcome the stronger forces of attraction between argon atoms. Hence, the melting and boiling points of argon are higher than helium.  

Why is helium gas used in airships while argon gas used in electric bulbs?

• Helium gas is suitable to fill airships because it is very light and non-flammable.
• Argon gas is suitable to use in electric bulbs because it is chemically inert. Hence, the hot tungsten filament in the electric bulb does not react with it.