Properties of Pure Substances and Phase Change: Boiling, Condensation, and Latent Heat

What “pure substance” means in practice

In thermodynamics, a pure substance is a material with a fixed chemical composition throughout. It can exist in more than one phase (solid, liquid, vapor) at the same time, but the composition is the same in each phase. Water is the classic example: liquid water and water vapor are still H₂O. Many maker-relevant working fluids are treated as pure substances: refrigerants (R134a, R1234yf), propane (R290), CO₂ (R744), and even metals near melting in casting processes.

What is not a pure substance? Most mixtures whose composition changes between phases, such as humid air (air + water vapor) or gasoline blends. Those can still be analyzed, but phase-change behavior becomes mixture-dependent (e.g., “glide” instead of a single boiling temperature).

Why makers should care

Pure-substance property data is the backbone of designing and troubleshooting devices that boil and condense: kettles, pressure cookers, espresso machines, distillers, heat pipes, dehumidifiers, air conditioners, and vapor-compression refrigeration loops. If you know how properties behave during phase change, you can predict heat transfer rates, size heat exchangers, estimate power needs, and avoid dangerous pressure conditions.

Phases and the phase-change idea: saturation and quality

For a pure substance, phase change at a given pressure happens at a specific temperature called the saturation temperature (often written T_sat). Likewise, at a given temperature, phase change happens at a specific saturation pressure P_sat. During boiling or condensation at constant pressure, the temperature stays essentially constant while energy is absorbed or released as the material changes phase.

Key terms you will use constantly:

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• Compressed (subcooled) liquid: liquid at a temperature below T_sat for the current pressure. Example: water at 20°C and 1 atm.
• Saturated liquid: liquid exactly at the verge of boiling at that pressure (T = T_sat).
• Saturated vapor: vapor exactly at the verge of condensing at that pressure (T = T_sat).
• Superheated vapor: vapor at a temperature above T_sat for the current pressure.
• Two-phase mixture: liquid and vapor coexist. The state is described by pressure (or temperature) plus quality.

Quality (x) is the mass fraction that is vapor in a saturated mixture: x = m_vapor / (m_liquid + m_vapor). It ranges from 0 (all saturated liquid) to 1 (all saturated vapor). Quality is a powerful concept because many properties in the two-phase region can be computed by linear mixing between saturated liquid and saturated vapor values.

For any specific property y (like specific volume v, internal energy u, enthalpy h, entropy s), in the saturated mixture region:

y = y_f + x (y_g - y_f) = y_f + x y_fg

Here, subscript f means saturated liquid, g means saturated vapor, and fg means the difference (g − f). This “lever rule” is one of the most practical tools in phase-change calculations.

Property tables and what to read from them

Pure-substance properties are commonly provided in saturated tables (organized by temperature or pressure) and superheated/compressed tables. For makers, you don’t need to memorize tables, but you should know what each column means and how to use it.

Saturated water table: typical columns

• T (or P): saturation temperature or pressure.
• v_f, v_g: specific volume of saturated liquid and saturated vapor (m³/kg). Note how v_g is huge compared to v_f—this drives expansion and pressure effects.
• u_f, u_g: internal energy (kJ/kg).
• h_f, h_g: enthalpy (kJ/kg).
• h_fg: latent heat of vaporization at that saturation condition (kJ/kg).

In many device calculations involving flow (boilers, condensers, throttling valves), enthalpy h is the most convenient property because it packages internal energy plus flow work effects. You will often compute heat transfer during phase change as a change in enthalpy.

Boiling: what actually happens in a real pot, boiler, or evaporator

Boiling is the phase change from liquid to vapor. In a pure substance at fixed pressure, boiling starts when the liquid reaches T_sat. From that point, additional heat input primarily goes into changing phase rather than raising temperature.

Step-by-step: heating water in an open pot (near 1 atm)

This is a simple but very instructive sequence:

• Step 1: Subcooled heating. Water starts below 100°C. Heat input raises temperature. No bulk vapor bubbles persist because the saturation temperature at 1 atm is about 100°C.
• Step 2: Onset of nucleate boiling. As the bottom surface reaches near 100°C, small vapor bubbles form at nucleation sites (scratches, dust, roughness). They may collapse if the bulk is still cooler.
• Step 3: Saturated boiling. Once the bulk reaches ~100°C, bubbles grow and rise; temperature stays close to 100°C while the liquid-to-vapor conversion consumes energy as latent heat.
• Step 4: Dry-out risk (if heating surface is too hot). If heat flux is extremely high (e.g., thin heater coil), a vapor film can form, insulating the surface and causing overheating. This is why immersion heaters and boiler elements need proper wetting and control.

In maker builds, the key practical point is: after reaching the boiling point, the temperature of the liquid doesn’t climb much, but the energy rate can still be large because phase change is energy-hungry. That’s why kettles draw 1–2 kW and still take minutes to boil off a small amount of water.

Boiling point depends strongly on pressure

Boiling is not “at 100°C” universally; it is “at T_sat(P).” Lower pressure lowers the boiling temperature (useful for vacuum distillation), higher pressure raises it (pressure cookers). This matters for safety: a sealed vessel with boiling liquid can build pressure quickly if heated.

Latent heat during boiling

The energy required to convert saturated liquid to saturated vapor at the same pressure is the latent heat of vaporization, often approximated as h_fg. For water near 100°C, h_fg is on the order of 2250 kJ/kg. That means evaporating 0.1 kg (100 g) of water requires roughly 225 kJ just for phase change, not counting heating from room temperature.

Condensation: turning vapor back into liquid

Condensation is the reverse phase change: vapor to liquid. In a pure substance at fixed pressure, condensation occurs at the same saturation temperature as boiling. During condensation, the substance releases latent heat to its surroundings while staying near T_sat.

Where makers meet condensation

• Distillers: steam condenses on a cooled coil, releasing heat to cooling water or air.
• Dehumidifiers and air conditioners: water vapor in air condenses on cold evaporator fins (note: the air is a mixture, but the condensed water itself is a pure substance).
• Heat pipes: vapor condenses at the cold end, returning liquid via wick action.
• Refrigeration condensers: refrigerant vapor condenses to liquid, rejecting heat to ambient air.

Condensation is often easier to manage than boiling because the condensing surface can be designed to stay wetted by liquid, maintaining good heat transfer. However, non-condensable gases (air leaks in a condenser) can dramatically reduce performance by forming a diffusion barrier near the surface.

Latent heat vs “sensible” heating in real builds

When you heat a liquid without changing phase, you are adding energy that shows up as a temperature rise (often called sensible heating). During phase change, you add or remove energy with little temperature change (latent heat). In practical systems, both happen in sequence:

• Boiler/evaporator: subcooled liquid warms to T_sat (sensible), then boils (latent), then may superheat (sensible again).
• Condenser: superheated vapor cools to T_sat (sensible), then condenses (latent), then may subcool (sensible).

This segmentation is extremely useful for sizing heat exchangers: the latent section often dominates energy transfer, while the sensible sections set temperature approaches and control stability.

Using quality to compute properties in the two-phase region

Quality lets you compute mixture properties without tracking separate liquid and vapor states. Example workflow:

Step-by-step: find mixture enthalpy at a given quality

• Step 1: Identify saturation condition (either T or P is given). Use a saturated table to read h_f and h_g (or h_fg).
• Step 2: Use the quality relation: h = h_f + x(h_g − h_f).
• Step 3: Use h for energy calculations (heat transfer, throttling outcomes, etc.).

Practical note: quality is a mass fraction, not a volume fraction. Even a small mass fraction of vapor can occupy a large volume because v_g is so large. This is why two-phase flow can cause surging, noise, and unstable pumping in DIY loops.

Critical point and why it matters for design limits

As pressure increases, the saturation temperature rises and the difference between liquid and vapor properties shrinks. At the critical point, the distinction between liquid and vapor disappears; there is no latent heat and no clear phase boundary. For water, the critical point is at high temperature and pressure; for CO₂, it is near room temperature compared to water, which is why CO₂ refrigeration often operates in transcritical regimes where “condensation” is replaced by gas cooling.

For makers, the key takeaway is that property behavior changes drastically near the critical region, and “boiling/condensing at constant temperature” assumptions stop working there.

Practical calculations you can do for real machines

1) Estimating boil-off rate from heater power

If you have an electric heater supplying power Q̇ into a boiling liquid at steady conditions, a first estimate of mass boil-off rate ṁ is:

ṁ ≈ Q̇ / h_fg

Example: a 1500 W kettle boiling water near 1 atm. Using h_fg ≈ 2250 kJ/kg = 2.25×10⁶ J/kg:

ṁ ≈ 1500 / (2.25e6) ≈ 6.7e-4 kg/s ≈ 0.67 g/s

That is about 40 g/min of steam production once fully boiling, ignoring losses. In practice, some power is lost to heating the kettle body and to convection/radiation, so actual boil-off is lower.

2) Estimating condenser heat load in a distiller

If your distiller produces ṁ of vapor that fully condenses, the condenser must remove approximately:

Q̇_condense ≈ ṁ h_fg + ṁ c_p,liq (T_sat - T_out,liq)

The first term is latent heat; the second is subcooling the condensate (often small unless you intentionally cool it a lot). This helps you size a cooling coil and decide whether air cooling is enough or you need a water loop.

3) Recognizing flashing after a pressure drop (throttling)

In many maker systems, a liquid passes through a restriction (capillary tube, needle valve, or orifice). A sudden pressure drop can cause part of the liquid to boil instantly, producing a two-phase mixture. This is called flashing. You will notice it as a temperature drop, hissing, and sometimes vibration.

A common simplified model for a throttling device is that enthalpy remains approximately constant across it. If you know the upstream state (often subcooled liquid) and the downstream pressure, you can compare the upstream enthalpy to saturated values downstream to estimate quality:

x_down ≈ (h_in - h_f,down) / h_fg,down

This is a powerful diagnostic: if x becomes too high, your evaporator may be starved of liquid, and heat transfer can suffer.

Hands-on demonstrations (safe, maker-friendly)

Demo A: Boiling point vs pressure using a syringe (low-risk vacuum)

This demonstrates that lowering pressure lowers the boiling temperature.

• Materials: large plastic syringe (30–60 mL) with a tight plunger, a small amount of warm water (around 50–70°C), optional thermometer.
• Step 1: Draw a small amount of warm water into the syringe (5–10 mL). Seal the tip with a cap or your finger (ensure it is clean).
• Step 2: Pull the plunger back to increase volume and lower pressure. Hold it steady.
• Step 3: Watch for bubbling. The water can boil at temperatures far below 100°C because the pressure is reduced.
• Step 4: Release the plunger slightly; bubbling reduces or stops as pressure rises.

Practical insight: vacuum chambers for resin degassing can make liquids boil unexpectedly; you must account for this when degassing solvents or water-containing mixtures.

Demo B: Latent heat is “big” compared to sensible heating

This compares energy to heat water vs energy to evaporate it.

• Materials: kitchen scale, kettle or hot plate, measuring cup, timer.
• Step 1: Measure 500 g of water into a pot. Heat it to a steady boil.
• Step 2: Once boiling steadily, start a timer for 5 minutes and keep power constant.
• Step 3: After 5 minutes, turn off heat and re-weigh remaining water (careful of steam and hot surfaces).
• Step 4: The mass lost is the evaporated mass. Multiply by h_fg (roughly) to estimate energy used for phase change and compare to the heater’s electrical energy input (power × time).

Even with rough measurements, you will see that most energy during steady boiling goes into evaporation, not raising temperature.

Common pitfalls in phase-change reasoning

Confusing “steam” with “visible vapor”

True water vapor is invisible. The white cloud above a kettle is mostly tiny liquid droplets formed when vapor cools and condenses in air. This matters when you try to “see” whether a system is producing vapor; you may be seeing condensation, not vapor itself.

Assuming temperature must rise when heating continues

During boiling at fixed pressure, temperature stays near T_sat. If your sensor is on a hot surface (heater plate) rather than in the fluid, it can read much higher than the boiling temperature. This is a sensor placement issue, not a violation of phase-change behavior.

Ignoring volume expansion and pressure hazards

Because v_g is so large compared to v_f, a small amount of liquid flashing to vapor can create large volumes of gas. In sealed or poorly vented systems, pressure can rise rapidly. Any DIY boiler, still, or heat pipe must include pressure-rated components and relief paths.

Using quality outside the saturated mixture region

Quality is defined only when liquid and vapor coexist at saturation. It does not apply to superheated vapor or subcooled liquid. If your state is superheated, you must use superheated property data rather than x-based mixing.

How phase change shows up in real machine components

Evaporators: keeping surfaces wet and controlling superheat

In an evaporator, you want efficient boiling without drying out the surface. Many systems aim for a small amount of superheat at the outlet to ensure no liquid reaches the compressor (in refrigeration) while still keeping most of the evaporator in the two-phase region where heat transfer is strong.

Condensers: removing latent heat and managing subcooling

In a condenser, most heat rejection happens during the phase change. Designers often add a small subcooling section to ensure the outlet is fully liquid, which improves stability through expansion devices and prevents flash gas in the liquid line.

Heat pipes: phase change as a passive pump

A heat pipe uses boiling at the hot end and condensation at the cold end. The latent heat transport allows very high effective thermal conductivity. The wick returns liquid by capillary action; if the wick cannot supply enough liquid, the evaporator dries out and performance collapses.

Pressure cookers and sealed boilers: saturation sets operating temperature

In a sealed vessel, pressure rises with heating, which raises T_sat. That is why pressure cookers cook hotter than 100°C. It also means temperature is a proxy for pressure only if you know the fluid is saturated and the vessel contains both phases; otherwise, the relationship can be different.