What is the standard unit for measuring the quantity of heat in a substance?

The standard unit for measuring the quantity of heat in a substance is the British Thermal Unit (BTU). A BTU is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at a constant pressure. It is widely used in the heating and cooling industry, particularly in the context of air conditioning and refrigeration systems, to quantify energy transfer, capacity, and efficiency.

Other units such as Celsius (°C), Joule (J), and Calorie (cal) are also used to measure heat but have specific applications and contexts. Celsius is primarily a temperature scale, while the Joule is a unit of energy within the International System of Units, often used in physics and engineering. The calorie is related to the amount of heat needed to raise the temperature of water but is less common in industrial applications compared to the BTU. Therefore, in the context of typical HVAC (Heating, Ventilation, and Air Conditioning) practices, BTU is the most relevant and widely recognized measurement for heat quantity.