Question

When the substances $[Si]$, $[KCl]$, $[C{H_3}OH]$ and $[{C_2}{H_6}]$ are arranged in order of increasing melting point, what is the correct order?
A. $[Si]$, $[KCl]$, $[C{H_3}OH]$ , $[{C_2}{H_6}]$
B. $[C{H_3}OH]$, $[{C_2}{H_6}]$, $[Si]$, $[KCl]$
C. $[KCl]$, $[Si]$, $[{C_2}{H_6}]$, $[C{H_3}OH]$
D. $[{C_2}{H_6}]$, $[C{H_3}OH]$, $[KCl]$, $[Si]$

Answer 1

The melting point of a substance is the temperature at which a solid changes into a liquid. The melting point of a substance depends on the attractive forces between the particles in the solid. All the substances have a specific melting point, as all the substances are solid at a particular temperature. At extremely low temperature, gases and liquids do exist as a solid.

Complete step by step answer:
The list of substances given here contain three compounds, $[{C_2}{H_6}]$, $[C{H_3}OH]$, $[KCl]$ and an element $[Si]$. Not all these substances exist as solid at room temperature.
The crystalline solids are formed when liquids are cooled and freeze. The crystalline solids are categorised into different categories, covalent network solids, ionic solids, molecular solids and metallic solids. The extent to which a particular solid is crystalline affects the physical properties of that solid such as density, melting point and boiling point.
Let us understand the structure of each substance.
Silicon is a metalloid, possessing the properties of both metals and nonmetals. It is solid at room temperature. The silicon has a giant covalent structure, in which silicon atoms are bonded together forming a uniform structure. The four valence electrons of each silicon atom are bonded with other four silicon atoms by the formation of covalent bonds, forming a network. Therefore, a perfect single crystal of silicon is a single giant molecule. Also, there is so no free electron left in the structure of silicon. In order to break strong covalent bonds and its network in silicon, a high amount of energy is required. Hence, silicon has a melting point as high as $1414^\circ C$.
The potassium chloride, $[KCl]$ is an ionic compound. It is formed by ions of opposite charges held together by strong electrostatic force of attraction. In $[KCl]$, cation $[{K^ + }]$ and $[C{l^ - }]$ are bonded together by the strong force of attraction that is ionic bonds. The charged particles in $[KCl]$ are arranged in a giant structure in which opposite charged ions are held together tightly. The breaking of ionic bonds in the giant structure of $[KCl]$ will require a high amount of energy, because of which $[KCl]$ has a high melting point equal to $770^\circ C$.
Methanol, $[C{H_3}OH]$ is solid at very low temperature and is a covalent compound, it is categorised as a molecular solid. The methanol has strong intramolecular forces of attraction between the constituent elements $[C,{\text{ }}H{\text{ }}]$ and $[O{\text{ }}]$. However, the intermolecular forces of attraction between methanol molecules are very weak. Hence, $[C{H_3}OH]$ have a very low melting point of $- 97.6^\circ C$.
Similarly, $[{C_2}{H_6}]$ is a covalent compound and is classified as a molecular solid. The intermolecular forces of attraction between $[{C_2}{H_6}]$ molecules are weak which results in its low melting point. The melting point of $[{C_2}{H_6}]$ is $- 182.8^\circ C$.

If $[Si]$, $[KCl]$, $[C{H_3}OH]$ and $[{C_2}{H_6}]$ are arranged in the increasing order of their melting points, the correct order is $[{C_2}{H_6}]$, $[C{H_3}OH]$, $[KCl]$, $[Si]$.

So, the correct option is D.

Additional information:
The temperature at which a liquid is converted into a solid is the freezing point of that liquid. Ideally, the melting point and the freezing point of a substance should be the same, but practically there is always a small difference between these two properties of a substance.
Also, it is extremely difficult to heat a substance above its melting point but it is possible to cool a liquid below its freezing point. When a liquid is cooled below its freezing point, the liquid is said to be supercooled.

Note: Though ethane, $[{C_2}{H_6}]$ occur as a colourless and odourless gas at standard conditions, $0^\circ C$ and 1 bar pressure, at specific conditions it occurs as a solid or liquid. Above temperature of $- 88.6^\circ C$ and at ambient pressure, ethane is a liquid whereas, at $- 182.8^\circ C$ ethane exists as a solid.
Methanol is a liquid at room temperature, but it can be solid at very low temperature and high pressure.