In the Hazardous Materials Handler Examination, understanding basic concepts related to heat, specific heat, and thermal capacity is essential. This article provides a detailed explanation of these concepts along with practice problems and explanations that can be used for exam preparation.
Basic Concepts of Heat and Temperature
When a substance heats up or cools down, the energy involved is called “heat.” To understand this heat, it is important to know about the relationship between calories (cal) and joules (J), specific heat, the relationship between absolute and Celsius temperatures, and thermal capacity. In particular, “specific heat” is crucial as it indicates how easily a substance heats up or cools down.
Relationship Between Calories (cal) and Joules (J)
The units used to measure heat are calories (cal) and joules (J). Their relationship is as follows:
Definition: 1 cal = 4.186 J
Reference: 1 cal is the amount of heat needed to raise the temperature of 1 gram of water from 14.5°C to 15.5°C. This definition is based on the temperature change of water.
Specific Heat Capacity
Specific heat is an important indicator that shows how easily a substance heats up or cools down.
Definition: Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1 K (or 1°C). It is expressed in units of J/(g·K) or J/(g·°C).
Characteristics: Substances with low specific heat experience large temperature changes with relatively small amounts of heat, making them “easy to heat and cool.” For example, metals have low specific heat and thus experience rapid temperature changes. Conversely, substances with high specific heat, like water, are harder to heat and cool down.
Relationship Between Absolute Temperature (Kelvin) and Celsius Temperature
There are two temperature scales, Celsius (°C) and absolute temperature (K), and they can be converted using the following equation:
Equation: K (Kelvin) = °C (Celsius) + 273
Absolute temperature is used as a direct indicator of the energy in a substance, with “absolute zero” (-273°C) as the baseline.
Thermal Capacity
Thermal capacity represents the amount of heat a substance can store overall.
Definition: Thermal capacity is the amount of heat required to raise the temperature of the entire substance by 1°C (or 1 K).
Formula: Thermal capacity = Specific heat × Mass
Characteristics: Substances with large thermal capacity resist temperature changes, making them slow to heat up and cool down. For instance, water has a high thermal capacity, which is why temperatures in oceans and lakes do not change rapidly.
Practice Problem 1
熱に関する一般的な説明について、次のうち誤っているものはどれか。
Which of the following general statements about heat is incorrect?
(1) Specific heat refers to the amount of heat required to raise the temperature of 1 g of a substance by 1 K (Kelvin).
(2) Substances with high thermal conductivity transfer heat easily.
(3) Substances with low heat absorb heat quickly and cool down quickly.
(4) Coefficient of thermal expansion is smallest in liquids and largest in gases.
(5) The volume of an ideal gas expands by about 1/273 of its volume at 0°C when its temperature increases by 1°C at constant pressure.
Explanation:(4) The coefficient of thermal expansion indicates how much a substance’s volume expands with increasing temperature. Typically, solids have the smallest expansion, while gases have the largest. Although liquids have a larger expansion coefficient than solids, it is smaller than that of gases. The correct statement is “solids have the smallest expansion, and gases have the largest.”
Answer: (4)
Practice Problem 2
50℃のある物質200gに2.5kJの熱量を与えたところ、温度が60℃に上昇しました。この物質の比熱はいくらか。
A 200g substance initially at 50°C is heated with 2.5 kJ of energy, raising its temperature to 60°C. What is the specific heat of this substance?
(1) 0.25 J/(g·K)
(2) 0.50 J/(g·K)
(3) 0.75 J/(g·K)
(4) 1.00 J/(g·K)
(5) 1.25 J/(g·K)
Explanation: Specific heat is calculated using the following formula:
Specific heat = Heat (Q) ÷ (Mass (m) × Temperature change (ΔT))
Here, Heat Q = 2500 J, Mass m = 200 g, Temperature change ΔT = 60°C – 50°C = 10°C.
Calculation:Specific heat = 2500 J ÷ (200 g × 10°C) = 2500 ÷ 2000 = 0.50 J/(g·K)
Answer: (5)
Conclusion
Specific heat is an important indicator of how easily a substance heats up or cools down and is a frequently tested topic in the Hazardous Materials Handler Examination. For exam preparation, ensure a thorough understanding of specific heat, thermal capacity, thermal conductivity, and other basic concepts, so you can tackle questions related to temperature changes.
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