Write is quantum mechanical liquid i.e. helium (II) ? Give its two important characteristics.

Answer

**Step-by-Step Solution:**

1. **Understanding Quantum Mechanical Liquid:**
- Helium (specifically Helium-2) is referred to as a quantum mechanical liquid because it exhibits unique properties that are not typical of classical liquids. This behavior arises due to quantum effects that dominate at very low temperatures.

2. **Cooling of Helium:**
- When helium is cooled to a temperature of 4.2 Kelvin, it exists as Helium-1, which behaves like a normal liquid. However, upon further cooling to 2.2 Kelvin, it transitions into Helium-2, which is a superfluid.

3. **Defining Helium-2:**
- Helium-2 is characterized as a superfluid, meaning it can flow without viscosity and can exhibit behaviors such as climbing up the walls of containers.

4. **Characteristics of Helium-2:**
- **High Thermal Conductivity:** Helium-2 has an exceptionally high thermal conductivity, allowing it to transfer heat very efficiently.
- **Low Viscosity:** Helium-2 has very low viscosity, meaning it can flow with little resistance, which is a hallmark of superfluidity.

**Final Answer:**
Helium (II), or Helium-2, is a quantum mechanical liquid that exhibits unique properties at low temperatures. Its two important characteristics are:
1. High thermal conductivity.
2. Low viscosity.

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