i. How does a supercritical state look like?

A supercritical state is a phase of matter that occurs when a substance is heated and pressurized beyond its critical temperature and critical pressure.

• In this state, the substance does not have a distinct boundary between liquid and gas.

• It has gas-like ability to expand and fill a container, and liquid-like ability to dissolve materials.

• It looks like a dense vapor or fog because it is more compressed than a gas but less dense than a liquid.

• Supercritical fluids are used in industrial processes like decaffeination of coffee and supercritical CO₂ extraction.

Example: Carbon dioxide becomes supercritical above 31.1°C and 73 atm.

ii. In what way is plasma created in a fluorescent?

In a fluorescent lamp, plasma is created using the following steps:

1. The tube contains a low-pressure gas, such as mercury vapor, and a phosphor coating on the inside.

2. When electric current passes through the gas, electrons collide with mercury atoms, exciting them.

3. The excited atoms emit ultraviolet (UV) radiation as they return to their ground state.

4. This UV light hits the phosphor coating, which converts it into visible light.

5. During this process, the gas becomes ionized, forming plasma, which contains free-moving electrons and ions.

Conclusion: Plasma in a fluorescent tube is responsible for sustaining the light-producing process.

iii. Where does the difference exist in Organic and Biochemistry branches of Chemistry?

Though both branches deal with carbon-based compounds, there are key differences:

• Organic Chemistry is the study of all carbon compounds, including both natural and synthetic substances.
  It includes hydrocarbons, alcohols, acids, plastics, dyes, and pharmaceuticals.

• Biochemistry is the branch of chemistry that studies chemical processes in living organisms, including the structure and function of biomolecules like carbohydrates, proteins, lipids, enzymes, and DNA.

iv. Give the reason of brilliance shown by diamond. Can you improve it?

Brilliance in diamond is due to two main factors:

1. Crystal Structure: Diamond has a tetrahedral arrangement of carbon atoms, forming a strong 3D network. This high symmetry and hardness allow it to reflect light internally multiple times.

2. Refractive Index: Diamond has a very high refractive index (2.42), meaning it bends light significantly and disperses it into various colors (fire).

Can we improve it?
Yes, by precise cutting (called faceting), we can maximize internal reflection. The angles must be designed so that most of the light entering the diamond reflects back out, making it more brilliant.

v. Explain the dissolution of sodium chloride in water.

When NaCl (table salt) is added to water:

1. The ionic bonds between Na⁺ and Cl⁻ are broken due to the polar nature of water.

2. Water molecules surround each ion:

   • Na⁺ attracts oxygen (negative end) of water.

   • Cl⁻ attracts hydrogen (positive end) of water.

3. This process is called hydration, where the ions are stabilized in solution.

4. As a result, NaCl dissolves completely, forming a true (homogeneous) solution.

Chemical Equation:
NaCl (s) → Na⁺ (aq) + Cl⁻ (aq)

Conclusion: The dissolution is a physical change but involves strong ion-dipole interactions.

vi. Why do different compounds have different solubilities in water at a particular temperature?
Answer:
Different compounds have different solubilities in water at a particular temperature due to the nature of their chemical bonding and molecular structure. Solubility depends on how well the solute particles interact with the solvent particles. Polar compounds dissolve better in polar solvents like water because of strong dipole-dipole or hydrogen bonding interactions. On the other hand, non-polar or less polar substances have weak interactions with water, making them less soluble. Temperature also affects the kinetic energy of particles, which can either increase or decrease solubility depending on the nature of the compound (endothermic or exothermic dissolution process).

vii. Why NaCl can not be crystallized from water just like KNO₃?
Answer:
NaCl (sodium chloride) cannot be easily crystallized from water like KNO₃ (potassium nitrate) because its solubility does not increase significantly with temperature. KNO₃ is highly temperature-dependent — its solubility increases rapidly with rising temperature, so when a hot saturated solution is cooled, it forms crystals easily. However, NaCl has almost constant solubility across a range of temperatures, so even when the hot solution is cooled, the difference is not enough to cause crystallization like KNO₃.

viii. Why is graphite slippery to touch? Which property of graphite enables it to be used as a lubricant?
Answer:
Graphite is slippery to touch because it has a layered structure. In graphite, each carbon atom is bonded to three other carbon atoms in flat sheets of hexagonal arrays. These layers are held together by weak van der Waals forces, allowing them to slide over one another easily. This sliding nature gives graphite its slippery feel and makes it an excellent solid lubricant, especially in high-temperature environments where liquid lubricants may not work.