Identifying A Substance's State Liquid Between -100°C And 78°C And Gaseous Above 78°C

Introduction

Understanding the states of matter is a fundamental concept in chemistry. Substances can exist in three primary states: solid, liquid, and gas, depending on temperature and pressure conditions. The transition between these states is governed by the substance's physical properties, such as its melting point and boiling point. In this article, we will explore the behavior of different substances across a range of temperatures, specifically focusing on identifying which substance remains liquid between -100°C and 78°C but transitions to a gaseous state above 78°C. This involves examining the melting and boiling points of common substances like mercury, alcohol, acid, and water to determine their phase behavior within the specified temperature range.

Understanding States of Matter

Before diving into the specifics of each substance, let's briefly revisit the concept of states of matter and phase transitions. A solid has a fixed shape and volume due to strong intermolecular forces holding its constituent particles in place. When heat is applied, the particles gain kinetic energy, and at the melting point, the solid transforms into a liquid. Liquids have a definite volume but take the shape of their container, indicating weaker intermolecular forces compared to solids. Further heating increases the kinetic energy of the particles, and at the boiling point, the liquid transitions into a gas. Gases have neither a fixed shape nor volume, as the particles move freely with minimal intermolecular interactions.

The melting point is the temperature at which a substance changes from a solid to a liquid, while the boiling point is the temperature at which a substance changes from a liquid to a gas. These transition temperatures are crucial in determining a substance's state at a given temperature. For a substance to exist as a liquid within a specific temperature range, its melting point must be below the lower bound of the range, and its boiling point must be above the upper bound. Therefore, to answer the question of which substance remains liquid between -100°C and 78°C and becomes a gas above 78°C, we need to compare the melting and boiling points of the given substances with this temperature range.

Analyzing the Answer Options

A. Mercury

• Mercury, a metallic element, is well-known for its unique property of being liquid at room temperature. Its melting point is approximately -38.83°C, and its boiling point is 356.73°C. Considering the temperature range of -100°C to 78°C, mercury remains in the liquid state since its melting point is significantly below -100°C and its boiling point is far above 78°C. This makes mercury a viable candidate for being in a liquid state within the specified range. However, to fully answer the question, we need to check if mercury transitions to a gaseous state above 78°C. Given its high boiling point, mercury remains liquid well above 78°C, meaning it does not transition to a gas within the context of the question's parameters.

B. Alcohol

• Alcohol is a broad term encompassing various organic compounds with a hydroxyl (-OH) group. For the purpose of this question, we'll consider ethanol (C2H5OH), a common type of alcohol. Ethanol has a melting point of -114°C and a boiling point of 78.37°C. The melting point of ethanol is below -100°C, meaning it exists as a liquid at -100°C. Its boiling point, 78.37°C, is very close to the upper limit of the temperature range specified in the question. This indicates that ethanol is in a liquid state within the temperature range of -100°C to 78°C and transitions to a gaseous state just above 78°C. Therefore, alcohol, specifically ethanol, fits the criteria described in the question.

C. Acid

• Acids are a diverse group of chemical compounds, so it's essential to specify which acid we are considering. For this analysis, we'll use hydrochloric acid (HCl) as a representative example. Hydrochloric acid has a melting point of -114.2°C and a boiling point of -85°C. The melting point is below -100°C, which means it can exist as a liquid at the lower end of the temperature range. However, its boiling point of -85°C is well below 78°C. This means that hydrochloric acid would be in a gaseous state at temperatures above -85°C, well within the specified range of -100°C to 78°C. Therefore, hydrochloric acid does not remain in a liquid state throughout the entire temperature range, nor does it transition to a gas above 78°C.

D. Water

• Water is a ubiquitous substance with a melting point of 0°C and a boiling point of 100°C. Within the temperature range of -100°C to 78°C, water undergoes a phase transition from solid (ice) to liquid at 0°C. Thus, below 0°C, water exists as ice, and above 0°C, it exists as liquid water. The boiling point of water is 100°C, which is higher than the 78°C threshold mentioned in the question. Therefore, water remains a liquid up to 100°C and does not transition to a gas at 78°C. This eliminates water as the correct answer.

Conclusion

After analyzing the melting and boiling points of the given substances, it's evident that alcohol (specifically ethanol) is the substance that exists in a liquid state between -100°C and 78°C and transitions to a gaseous state at temperatures above 78°C. Mercury remains liquid beyond this range, acid is already in a gaseous state, and water boils at a much higher temperature. Thus, the correct answer is B. Alcohol.

Understanding the physical properties of substances, such as their melting and boiling points, is crucial in predicting their behavior under different temperature conditions. This knowledge is fundamental in various fields, including chemistry, physics, and engineering, allowing for the design of processes and applications that leverage the unique characteristics of different materials. The ability to identify a substance's state at a specific temperature is a basic yet essential skill in scientific inquiry and problem-solving.