Difficulty: Introductory-intermediate | Prerequisites: Freezing point depression and colligative properties (see companion notes), basic thermodynamics (heat, temperature, energy conservation).
Big picture: This material picks up where freezing point depression leaves off. A good deicer needs to do more than just lower the freezing point: it should also release heat when it dissolves (exothermic dissolution), so it actively melts ice rather than just preventing refreezing. Calorimetry is the experimental technique used to measure that heat exchange. This topic ties together energy conservation, heat capacity, and practical decision-making about which salt to spread on a winter road.
TL;DR: Calorimetry measures heat flow by tracking temperature changes in an insulated container. The enthalpy of dissolution tells you whether a salt absorbs or releases heat when it dissolves. For deicers, exothermic dissolution is preferable. Comparing freezing point depression, enthalpy, cost, and environmental impact lets you evaluate which deicer is the best overall choice.
Calorimetry
The science of measuring heat changes during chemical or physical processes. In a teaching lab this is typically done with a simple insulated-cup calorimeter.
Calorimeter constant (Ccal)
The amount of heat the calorimeter itself absorbs per degree of temperature change (units: J/°C). Think of it as: a correction factor for the heat that goes into warming the cup and lid instead of the water.
Specific heat capacity (Cwater)
The amount of heat required to raise the temperature of 1 gram of a substance by 1 °C. For liquid water, Cwater = 4.184 J/(g·°C). This value is used in nearly every calorimetry calculation.
Enthalpy of dissolution (ΔHdiss)
The heat change when one mole of solute dissolves in a solvent at constant pressure, expressed in kJ/mol. A positive ΔH means the process is endothermic (absorbs heat); a negative ΔH means it is exothermic (releases heat).
Endothermic
A process that absorbs heat from the surroundings. The solution cools down when the solute dissolves. In simple terms, the solute "pulls" energy from the water, making the temperature drop.
Exothermic
A process that releases heat to the surroundings. The solution warms up when the solute dissolves. For deicing, exothermic dissolution is desirable because the released heat helps melt more ice.
Conservation of energy (in calorimetry)
The principle that the total heat in the system is zero: heat lost by one component equals heat gained by another. Written as qreaction + qwater + qcal = 0.
Before you can measure the enthalpy of dissolution, you need to know how much heat your calorimeter absorbs. This is the calorimeter constant, Ccal.
Method: mix a known mass of hot water with a known mass of cold water inside the calorimeter. Record the initial temperatures of each and the final equilibrium temperature.
Apply conservation of energy: qhot + qcold + qcal = 0. The water calculations use q = Cwater × m × ΔT for each portion. Whatever heat is "missing" (the difference between what the hot water lost and what the cold water gained) went into warming the calorimeter.
In the lab experiment, three trials gave an average Ccal of 12.203 J/°C.
Add a known mass of solute (e.g. KCl) to a known mass of water inside the calorimeter. Record the temperature before and after dissolving.
The temperature change tells you the total heat exchanged: qwater = Cwater × mwater × ΔT and qcal = Ccal × ΔT.
By conservation of energy: qreaction = -(qwater + qcal). This gives the heat released or absorbed by the dissolving process.
Divide qreaction by the number of moles of solute to get ΔH in J/mol or kJ/mol.
For KCl, the lab found ΔH = +20.96 kJ/mol, which is endothermic (the temperature dropped when KCl dissolved).
An exothermic deicer releases heat as it dissolves, actively melting surrounding ice. This is desirable.
An endothermic deicer absorbs heat from the surroundings, which can slow ice melting. KCl is endothermic, making it less effective as a deicer on this criterion.
CaCl₂ is notably exothermic on dissolution, which is one reason it is used in some commercial deicing products despite its higher cost.
The lab compared four salts: NaCl, KCl, MgCl₂, and CaCl₂.
Criteria for an effective deicer:
Large freezing point depression (more ions per formula unit helps)
Exothermic or only mildly endothermic enthalpy of dissolution
Low cost per unit
Minimal environmental damage (groundwater contamination, soil damage, concrete corrosion, heavy metal release)
KCl was found to be the most expensive ($202/unit at the time of the experiment), endothermic (+20.96 kJ/mol), and environmentally harmful (damages concrete, seeps into groundwater, disrupts soil mineral balance, releases heavy metals).
NaCl was identified as the best overall deicer: cheapest ($42/unit), only mildly endothermic, effective freezing point depression, and the least harmful to the environment among the four candidates.
Heat absorbed or released by water:
q_{\text{water}} = C_{\text{water}} \times m_{\text{water}} \times \Delta TWhere Cwater = 4.184 J/(g·°C), m is mass in grams, and ΔT = Tfinal - Tinitial.
Heat absorbed by the calorimeter:
q_{\text{cal}} = C_{\text{cal}} \times \Delta TConservation of energy (for finding Ccal):
q_{\text{cold}} + q_{\text{hot}} + q_{\text{cal}} = 0Conservation of energy (for finding enthalpy of dissolution):
q_{\text{reaction}} + q_{\text{water}} + q_{\text{cal}} = 0So qreaction = -(qwater + qcal).
Enthalpy of dissolution:
\Delta H = \frac{q_{\text{reaction}}}{\text{moles of solute}}Example from the lab: KCl gave ΔH = +20.96 kJ/mol (positive sign confirms endothermic).
State departments of transportation (like Minnesota's) use exactly this kind of analysis when choosing road deicers. The chemistry is straightforward, but the decision involves balancing multiple factors: a salt that is chemically excellent might be too expensive or too damaging to the local ecosystem.
Calorimetry itself appears across science and industry. Food scientists measure the caloric content of foods using bomb calorimeters. Materials engineers use differential scanning calorimetry (DSC) to study phase transitions in polymers and metals. The same conservation-of-energy principle underpins all of it.
Students often forget to account for the calorimeter constant. If you skip Ccal and assume all heat goes into the water, your calculated enthalpy will be too low because the calorimeter absorbed some of that energy.
A temperature drop when salt dissolves does not mean the salt is "cold." It means the dissolution process is endothermic: the salt is drawing heat from the water, cooling the solution.
Students sometimes confuse enthalpy of dissolution with freezing point depression. They measure different things. A salt can produce a large freezing point depression (many ions, high i) while still being endothermic. Both matter for deicing, but they are separate properties.
The sign convention trips people up. Positive ΔH = endothermic (heat absorbed by the reaction, temperature of surroundings drops). Negative ΔH = exothermic (heat released by the reaction, temperature of surroundings rises).
⚠️ Be able to calculate the calorimeter constant from hot/cold water mixing data. This is a standard multi-step problem: compute qhot and qcold, then solve for qcal and divide by ΔT.
⚠️ Know how to calculate enthalpy of dissolution from calorimetry data: find qwater, find qcal, apply conservation of energy to get qreaction, divide by moles.
⚠️ Understand the sign convention. If the temperature drops when solute dissolves, ΔH is positive (endothermic). If it rises, ΔH is negative (exothermic).
⚠️ Be ready to compare deicers on multiple criteria (freezing point depression, enthalpy, cost, environmental impact) and justify which is "best" given the trade-offs.
⚠️ Percent error for enthalpy measurements may appear. The lab found 6.3% error for the enthalpy of KCl, and you should be able to explain sources of error (heat loss to surroundings, imprecise temperature readings, impure water).
True or false: An endothermic dissolution causes the solution temperature to rise. (False, it causes it to drop.)
Fill in the blank: The specific heat capacity of liquid water is ______ J/(g·°C). (4.184)
True or false: The calorimeter constant accounts for heat absorbed by the calorimeter itself. (True.)
Fill in the blank: qreaction + qwater + qcal = ______. (0)
True or false: For a deicer, exothermic dissolution is better because it helps melt ice. (True.)
Q: You mix 20 g of hot water (48 °C) with 20 g of cold water (20 °C) in a calorimeter. The final temperature is 32 °C. Calculate the calorimeter constant.
A: qhot = 4.184 × 20 × (32 - 48) = -1338.9 J. qcold = 4.184 × 20 × (32 - 20) = 1004.2 J. qcal = -(qhot + qcold) = -(-1338.9 + 1004.2) = 334.7 J. ΔT for the calorimeter = 32 - 20 = 12 °C. Ccal = 334.7 / 12 = 27.9 J/°C.
Q: You dissolve 0.8 g of KCl (molar mass 74.55 g/mol) in 15 g of water inside a calorimeter with Ccal = 12.2 J/°C. The temperature drops from 21 °C to 18 °C. Calculate ΔH.
A: qwater = 4.184 × 15 × (18 - 21) = -188.3 J. qcal = 12.2 × (18 - 21) = -36.6 J. qreaction = -(qwater + qcal) = -(-188.3 + -36.6) = +224.9 J. Moles of KCl = 0.8 / 74.55 = 0.01073 mol. ΔH = 224.9 / 0.01073 = +20,960 J/mol = +20.96 kJ/mol (endothermic).
Q: Why is NaCl generally considered a better road deicer than KCl?
A: NaCl is much cheaper, only mildly endothermic (closer to neutral than KCl), provides effective freezing point depression, and is the least environmentally harmful of the four salts compared in the experiment. KCl is the most expensive, strongly endothermic, and damages concrete, groundwater, and soil.
Q: What does a positive ΔH for a dissolution process tell you about the temperature change of the solution?
A: The solution temperature decreases. A positive ΔH means the process is endothermic, so it absorbs heat from the surrounding water.
Q: Name three sources of error in a calorimetry experiment.
A: Heat loss to the surroundings through the calorimeter walls or lid, imprecise temperature readings (lag in the probe response), and using impure water or inaccurate mass measurements.
Calorimetry connects to Hess's law and thermochemistry more broadly. The same conservation-of-energy logic used here to find ΔH for dissolution is used to find ΔH for any reaction measured in a calorimeter, including combustion and neutralisation.
The deicer evaluation brings together colligative properties (freezing point depression, Van't Hoff factor) with thermodynamics (enthalpy of dissolution) and practical engineering considerations (cost, environmental impact). This kind of multi-criteria analysis appears throughout applied chemistry and chemical engineering.
Solubility and temperature dependence of solubility (not tested in this lab but mentioned as a suggested extension) connect to equilibrium and Le Chatelier's principle: how the solubility of a salt changes with temperature affects how well it performs as a deicer in extreme cold.
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