Which of the following substances is a non metal that exists as a liquid at ordinary room temperature?

Introduction / Context:
Most elements are solids at room temperature, but a few are liquids or gases. Knowing which elements are metals and which are non metals, as well as their physical states, is a common general knowledge and chemistry exam topic. This question asks you to identify the non metal that is liquid at ordinary room temperature.

Given Data / Assumptions:
- Room temperature is around 25 degrees Celsius.
- Some metals, like mercury and a few others, are liquid near room temperature.
- The options list mercury, caesium, bromine, rubidium, and gallium.

Concept / Approach:
Mercury is a well known liquid metal at room temperature, and caesium and rubidium are alkali metals that melt near or slightly above room temperature. Gallium also melts just above room temperature. In contrast, bromine is a halogen and is the only non metal that is liquid at room temperature. It forms a reddish brown volatile liquid and is clearly classified as a non metal in the periodic table.

Step-by-Step Solution:
Step 1: Classify each option as metal or non metal using basic periodic table knowledge. Step 2: Recognise that mercury, caesium, rubidium, and gallium are all metals, whereas bromine is a halogen non metal. Step 3: Recall that bromine is a dark reddish brown liquid at room temperature and pressure. Step 4: Remember that many references highlight bromine as the only non metallic element that is liquid under these conditions. Step 5: Therefore, conclude that bromine is the correct choice.

Verification / Alternative check:
Standard periodic table summaries frequently state that there are two elements that are liquids at room temperature: mercury (a metal) and bromine (a non metal). Additionally, caesium and gallium have low melting points but are not normally liquid at 25 degrees Celsius. Rubidium also has a low melting point compared to most metals, yet it is a solid at room temperature. Only bromine among the non metals listed is genuinely liquid in ordinary conditions, supporting the answer.

Why Other Options Are Wrong:
- Mercury: It is indeed a liquid at room temperature but is a metal, not a non metal.
- Caesium: An alkali metal with a low melting point, but still classified as a metal and may not be liquid at standard room temperature.
- Rubidium: Another alkali metal that is solid at room temperature, not a non metal liquid.
- Gallium: A metal that melts just above room temperature and is used in low melting alloys; it is not a non metal.

Common Pitfalls:
A frequent error is to focus only on the physical state and forget to check whether the element is a metal or non metal. For example, students may choose mercury simply because they know it is liquid, overlooking the requirement that the answer must be a non metal. Always read the full wording of the question carefully, including qualifiers such as metal or non metal.

Final Answer:
The non metal that exists as a liquid at ordinary room temperature is Bromine.