At ordinary room temperature of about 25 degrees Celsius, which of the following metals exists as a liquid rather than as a solid?

Introduction / Context:
Most metals are solids at ordinary room temperature, but a few metals have melting points near room temperature and can appear as liquids under certain conditions. Knowing which metals are liquid at about 25 degrees Celsius is a common general knowledge and chemistry question. This question asks which metal from the list is actually liquid at typical room temperature, making it stand out from the others.

Given Data / Assumptions:
- Room temperature is taken to be around 25 degrees Celsius under normal atmospheric pressure.
- Caesium, gallium, rubidium, mercury and sodium are metallic elements with different melting points.
- We focus on whether each metal is solid or liquid at about 25 degrees Celsius.
- Slight differences between exact melting points and typical classroom definitions of room temperature are considered in a simple way.

Concept / Approach:
Mercury is well known as a liquid metal at room temperature and is commonly used in thermometers, barometers and some electrical switches. Its melting point is well below 25 degrees Celsius, so it remains liquid under ordinary conditions. Caesium and gallium have melting points near or slightly above room temperature; they can melt in a warm environment or when handled, but they are usually considered solids at a standard room temperature. Rubidium and sodium have much higher melting points compared with 25 degrees Celsius and remain solid under normal conditions. Therefore, mercury is the only metal in the list that is definitely liquid at ordinary room temperature.

Step-by-Step Solution:
Step 1: Recall that mercury is a well known liquid metal used in traditional thermometers and other devices, which indicates that it is liquid at room temperature. Step 2: Consider caesium and gallium. Both have low melting points close to room temperature, but they are usually classified as solids that can melt when slightly warmed above typical laboratory room conditions. Step 3: Note that rubidium and sodium both have melting points well above 25 degrees Celsius, so they are solid at ordinary room temperature. Step 4: Compare all the options and recognise that only mercury is reliably liquid at about 25 degrees Celsius. Step 5: Conclude that mercury is the correct answer.

Verification / Alternative check:
Data tables confirm this reasoning. Mercury has a melting point of about minus 39 degrees Celsius, so it is liquid at any temperature above this value, including room temperature. Gallium melts at just under 30 degrees Celsius, which means a piece of gallium can melt in a warm hand, but in a cooler room it may remain solid. Caesium melts at around 28 to 29 degrees Celsius, again slightly above typical room temperature. Rubidium and sodium melt at much higher temperatures, so they stay solid under ordinary conditions. These melting point values support the identification of mercury as the only metal that is definitely liquid at room temperature in this list.

Why Other Options Are Wrong:
Caesium and gallium have low melting points but are still usually considered solid at a room temperature of about 25 degrees Celsius. Rubidium and sodium have much higher melting points and are clearly solid at room temperature. None of these metals shows the same obvious and consistent liquid behaviour at room temperature as mercury does. Therefore, options A, B, C and E are incorrect in the context of this question.

Common Pitfalls:
Students sometimes remember that gallium can melt in the hand and conclude that it must be liquid at room temperature, forgetting that the warmth of the hand raises the local temperature above 25 degrees Celsius. Others may confuse the metals with low melting points and assume that all of them are liquid under normal conditions. To avoid these mistakes, it is helpful to remember that mercury is the classic example of a liquid metal at room temperature and that most other metals, even those with low melting points, remain solid unless slightly warmed above typical room temperatures.

Final Answer:
At ordinary room temperature, the metal that exists as a liquid is Mercury.