Which of the following reactions are exothermic?

 

$$\begin{gathered} 1) C + O2 → CO2 \\ 2) 2Mg + O2 → 2MgO \\ 3) CuSO4 + 5H2O → CuSO4.5H2O \\ 4) All the above \end{gathered}$$

Semiconductors are those materials that are neither good conductors nor the good insulators of heat and electricity. Silicon (Si), Germanium (Ge) and Cadmium Sulphide (CdS) are semiconductors which work as conductors in some special conditions only.

Alkanes are saturated hydrocarbons. Saturated means that the hydrocarbon has only single bonds and that the hydrocarbon contains the maximum number of hydrogen atoms for each carbon atom.

A chemical bond in which the electrons are not shared equally due to differences in the electronegativity of the atoms.

The polarity of a covalent bond is measured using its dipole moment.

 Large dipole moment = more polar

 Small dipole moment = more nonpolar.

Yes, Carbon disulfide is prepared by heating sulfur and charcoal. 

S2 + C ----> CS2

Burning gasoline is the only process that involving in a chemical change in the given options. A chemical change is a change in the starting substances that are called reactants from the formed new substances that are called products. 

Unlike other gas cycles, the phosphorus cycle differs in having atmospheric reservoir. No other gases are involved in it.

Monazite is an ore of cerium, thorium and lanthanum.