Question

Lowest temperature is reached by using (a) \(\mathrm{CaCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) (b) Acetone \(+\) dry ice (c) \(\mathrm{NH}_{4} \mathrm{Cl}\) (d) Ether + dry ice

Short answer

The answer is (b) Acetone + dry ice. When combined, these two substances complete an endothermic reaction that absorbs heat from the surroundings, reducing the temperature, and has a low freezing point, allowing it to remain in liquid form at lower temperatures.

Step-by-step solution

Identify the Nature of Substances

First, identify whether the given substances are endothermic or exothermic. Endothermic substances absorb heat, thus lowering the temperature. Exothermic substances release heat, therefore increasing temperature.

Understand the Freezing Point

Next, research the freezing points of the mentioned substances. Lower freezing points mean that the substance can achieve a lower temperature without becoming solid.

Compare Temperatures

After gathering this information, compare the temperatures of the substances. The one with the most endothermic nature and lowest freezing point will reach the lowest temperature.

Key concepts

Endothermic and exothermic reactions

In thermodynamics, understanding the distinction between endothermic and exothermic reactions is crucial for predicting temperature changes during chemical interactions.
Endothermic reactions are processes where a substance absorbs heat from its surroundings. This absorption lowers the temperature of the immediate environment. Reactants in an endothermic reaction have higher enthalpy than products, requiring energy input to proceed.
Examples include the melting of ice and evaporation of water. On the other hand, exothermic reactions release heat energy into the surroundings, consequently raising the surrounding temperature. In these reactions, the products have lower enthalpy than the reactants, allowing them to liberate energy as they form. Combustion and the reaction of acids with bases are classic examples of exothermic processes.
Knowing these differences helps in determining which chemical reactions could be used in scenarios like lowering temperatures, as seen in the given exercise.

Freezing point

The freezing point of a substance is the temperature at which it changes state from liquid to solid. This property is key for identifying how low a temperature a substance can reach without freezing.
Substances with lower freezing points can stay liquid at lower temperatures, allowing them to be utilized effectively in cooling processes.
For example, acetone has a low freezing point of around -94 °C, making it suitable for creating extremely cold environments when combined with substances like dry ice. Understanding freezing points is essential in processes that aim to reach minimum temperatures. Knowing these values further aids in choosing the right materials for different cooling or heating applications. In our scenario, selecting a combination with the lowest freezing point could provide a practical solution for achieving lower temperatures efficiently.

Temperature lowering processes

Temperature lowering processes involve techniques and substances that facilitate the reduction of temperature in a given space or object. Various methods include using endothermic reactions or utilizing substances that absorb heat as they change state.
One of the most effective temperature lowering processes is the use of dry ice, which sublimates, or transitions from solid to gas, at -78.5 °C while absorbing large amounts of heat.
When coupled with acetone, which has a very low freezing point, even lower temperatures can be achieved, enhancing the cooling effect dramatically. Such combinations are utilized in laboratory settings and industrial applications where precise temperature control is critical. Recognizing the capacity of specific substances to absorb heat and lower temperatures is essential for designing systems that require efficient cooling.