Properties and Behaviour of Water

Introduction

Water is the principal extinguishing medium used in structural firefighting because it is widely available, comparatively inexpensive, non-toxic in ordinary use, and highly effective at removing heat. Its effectiveness is not explained simply by the fact that it is “wet.” Firefighters depend on several physical properties working together: high specific heat, high latent heat of vaporisation, expansion during vaporisation, practical incompressibility, surface tension, density, and the ability to flow through hoses, branches, pumps and nozzles.

Understanding these properties is fundamental to fire service hydraulics. The quantity of water delivered, the size of droplets, the point of application, and the pressure at the nozzle determine how effectively water absorbs energy and interrupts combustion. Poor understanding can lead to excessive water use, steam production, structural damage, inadequate reach, or failure to control the fire.

Water does not extinguish every fire safely. It may react dangerously with some burning metals, spread certain flammable liquids, and conduct electricity when contamination or equipment conditions permit current flow. Therefore, the physical properties of water must always be considered together with fuel type, fire development, ventilation, building construction and operational risk.

Learning objectives

By the end of this chapter, the learner should be able to:

1. Explain the molecular and thermal properties of water relevant to extinguishment.
2. Distinguish sensible heating from latent heating.
3. Calculate the approximate heat absorbed by water under stated conditions.
4. Explain why fine droplets and suitable application patterns improve cooling.
5. Relate water expansion, incompressibility, density and surface tension to hydraulic performance.
6. Identify situations in which water application is hazardous or ineffective.
7. Apply these principles to an operational fire-control decision.

1. Water as a fire-extinguishing medium

Combustion continues when heat, fuel and an oxidising agent are present in suitable proportions. This is commonly represented by the fire triangle. Water primarily attacks the heat side of this relationship, although steam may also dilute oxygen locally and cooling may reduce the rate at which fuel vapour is produced.

The most important operational function of water is heat removal. If the rate of heat removal is greater than the rate at which heat is released and transferred back to the fuel, the burning region cools and the fire may diminish. This is a dynamic balance, not a fixed quantity. A stream that appears large may be ineffective if it passes through the fire without vaporising or reaching heated surfaces.

Water can absorb energy in three principal stages:

• heating liquid water to its boiling temperature;
• changing liquid water into steam at approximately constant temperature; and
• superheating the steam after vaporisation, if hot gases continue to transfer energy to it.

For ordinary firefighting calculations, the first two stages are usually the most important.

2. Molecular structure and phase change

A water molecule consists of two hydrogen atoms bonded to one oxygen atom, represented as H₂O. Water molecules are polar: the oxygen side has a partial negative charge and the hydrogen side a partial positive charge. Attractions between neighbouring molecules, especially hydrogen bonding, help explain water’s relatively high boiling point, high specific heat, surface tension and latent heat of vaporisation.

Water exists as solid ice, liquid water and water vapour. At normal atmospheric pressure, liquid water boils at approximately 100 °C. The boiling temperature changes with pressure: it is lower at high altitude and higher in a pressurised system. Firefighting water is normally delivered as a liquid, but the energy absorbed during its conversion to vapour is central to extinguishment.

The phrase “water turns into steam” is operationally useful but requires care. Steam is invisible water vapour at temperatures above the boiling point; the white cloud commonly called steam is mainly condensed droplets suspended in air. This distinction matters because hot water vapour can cause severe burns, while visible mist may contain rapidly cooling droplets.

3. Specific heat and sensible heat absorption

Specific heat capacity is the heat required to raise the temperature of unit mass of a substance by one degree. Liquid water has a high specific heat capacity, approximately:

The sensible heat absorbed when water changes temperature without changing phase is:

where:

• Q_s = sensible heat absorbed, in joules or kilojoules;
• m = mass of water, in kilograms;
• c_p = specific heat capacity, in kJ/(kg·K) or J/(kg·K);
• ΔT = temperature increase, in kelvin or degrees Celsius.

A temperature difference of 1 K is numerically equal to a difference of 1 °C. The formula assumes that the specific heat is treated as constant over the temperature range. For normal fire-service estimates this is acceptable, although the actual value varies slightly with temperature and pressure.

Water’s high specific heat means that considerable energy can be absorbed before boiling begins. This cools burning solids, hot structural members and surrounding gases. Cooling the fuel is particularly important in compartment fires because it reduces pyrolysis—the thermal decomposition of solid fuel that produces combustible vapours.

Worked example 1: sensible heating

A branch delivers 10 kg of water initially at 20 °C onto a heated surface. Estimate the heat absorbed if the water reaches 100 °C but does not yet vaporise.

Using c_p = 4.18 kJ/(kg·K) and ΔT = 100 - 20 = 80 K:

Thus, approximately 3,344 kJ, or 3.34 MJ, is absorbed. The calculation describes only sensible heating; additional energy can be absorbed if the water vaporises.

4. Latent heat of vaporisation

Latent heat is energy absorbed during a change of state without an accompanying change in temperature. At normal atmospheric pressure, the latent heat of vaporisation of water is approximately:

The energy absorbed during vaporisation is:

where Q_v is latent heat absorbed and L_v is the specific latent heat of vaporisation. The equation assumes that the water has already reached its boiling temperature and that vaporisation occurs at approximately constant pressure.

The total heat absorbed when water is heated from an initial temperature to its boiling point and then vaporised is approximately:

where T_b is the boiling temperature at the relevant pressure and T_i is the initial water temperature. This is an idealised upper-bound estimate. In real fire attack, not all water reaches the boiling point, and some water runs off, drains away or remains liquid.

For 1 kg of water initially at 20 °C, the approximate energy absorbed on reaching 100 °C and vaporising is:

About 87% of this idealised energy absorption occurs during vaporisation. This explains why water that reaches hot surfaces and converts to vapour can remove much more energy than water that merely passes through a hot gas layer.

Limitations of the latent-heat calculation

The value 2,257 kJ/kg applies approximately to vaporisation at 100 °C and standard atmospheric pressure. It should not be treated as a universal constant for all conditions. Actual performance depends on:

• water temperature and pressure;
• droplet size and surface area;
• contact with hot gases or surfaces;
• ventilation and movement of steam;
• the proportion of water that vaporises; and
• heat losses to the environment.

A firefighter should therefore interpret calculated heat absorption as potential capacity, not guaranteed field performance.

5. Expansion during vaporisation

Liquid water expands enormously when converted to vapour. At 100 °C and standard atmospheric pressure, 1 kg of liquid water occupies approximately 0.001 m³, while the resulting dry steam occupies roughly 1.67 m³. The volume ratio is therefore about 1,670:1 under those reference conditions.

This expansion can be expressed generally as:

where R_V is the volume expansion ratio, V_v is vapour volume and V_l is liquid volume. The ratio varies with temperature and pressure. Steam at higher pressure occupies less volume per kilogram than steam at atmospheric pressure.

Expansion has two operational effects. First, rapidly expanding vapour can displace air and dilute oxygen near a fire. Second, it can create dangerous pressure, visibility and movement conditions in confined spaces. Steam is hot and can cause severe scalding; its movement may also transport heat to occupants and firefighters.

Expansion is not a reason to inject uncontrolled quantities of water into a compartment. Water must be applied in a controlled pattern and rate, with consideration of ventilation, escape routes and the location of occupants. The cooling benefit must exceed the hazards created by steam production.

6. Water density and mass flow

At ordinary fireground temperatures, water has a density close to 1,000 kg/m³, or approximately 1 kg/L. Density links volumetric flow to mass flow:

where:

• ṁ = mass flow rate, in kg/s;
• ρ = density, in kg/m³; and
• Q = volumetric flow rate, in m³/s.

For water flowing at 400 L/min:

The heat-removal rate can be estimated by multiplying mass flow by heat absorbed per kilogram:

where P_abs is heat-absorption rate in kW when ṁ is in kg/s and q is in kJ/kg. This estimate is valid only for the fraction of water that actually absorbs the stated energy.

If all 6.67 kg/s in the example were heated from 20 °C to 100 °C and vaporised, the theoretical heat absorption would be:

This is approximately 17.3 MW, but it is an idealised value. Field heat absorption will be lower if water does not contact sufficiently hot surfaces or leaves the compartment as runoff.

7. Incompressibility and hydraulic performance

Liquids are commonly treated as incompressible in fire-service hydraulic calculations. This means that their density changes very little under the pressures normally encountered in hoses and appliances. Water does compress slightly under pressure, but the change is usually too small to affect routine flow calculations.

Practical incompressibility provides important advantages:

• pump displacement produces predictable flow;
• pressure is transmitted through connected water-filled systems;
• water hammer can occur when flow changes suddenly;
• hose lines do not store energy through large volume compression in the way gas-filled systems do.

Water hammer is a transient pressure surge caused by rapid changes in flow, such as abruptly closing a valve or nozzle. It can damage pumps, couplings, valves and hoses. Smooth valve operation and compliance with appliance procedures reduce this risk.

Incompressibility does not mean that pressure is identical everywhere in a flowing system. Pressure is consumed by elevation changes, friction and fittings, and some pressure is converted into velocity at the nozzle. A useful energy relationship for steady ideal flow is expressed by Bernoulli’s equation:

Here P is pressure, ρ is density, g is gravitational acceleration, v is flow velocity and z is elevation. Real hose systems require additional loss terms for friction and fittings, so the ideal equation is not sufficient by itself for appliance design or pump operation.

8. Surface tension, wetting and droplet behaviour

Surface tension is the tendency of a liquid surface to resist expansion because molecules at the surface experience unbalanced intermolecular forces. Water’s surface tension causes droplets to form and can reduce penetration into some porous or waxy materials.

Wetting describes how readily water spreads across and makes contact with a surface. A high-contact-angle droplet tends to bead; a low-contact-angle droplet spreads more effectively. Firefighting additives called wetting agents or surfactants can reduce surface tension and improve penetration into certain materials. Their use must follow product instructions, environmental requirements and appliance compatibility guidance.

Surface tension affects fire control in several ways:

• large droplets may resist breaking into smaller droplets;
• droplets may bounce from very hot or hydrophobic surfaces;
• improved wetting can increase contact with deep-seated or porous fuels;
• smaller droplets provide greater surface area for heat transfer;
• excessive atomisation may reduce reach and increase wind drift.

Droplet size is therefore a balance. Fine droplets have a high surface-area-to-volume ratio and evaporate rapidly, making them effective for gas cooling under suitable conditions. Larger droplets have greater momentum and reach, making them useful for penetrating smoke layers and contacting surfaces at distance. The nozzle pattern and operating pressure must match the tactical objective.

9. Water’s role in cooling gases and surfaces

Water can cool hot gases by direct heat transfer to droplets. A spray pattern exposes a larger total water surface area than a solid stream. However, a fine mist is not automatically superior: droplets may evaporate before reaching the target, be carried away by ventilation, or fail to penetrate the required distance.

Water applied to burning solids cools the fuel and the surrounding structure. Surface cooling is especially important in compartment fires, where hot walls and contents can continue to release combustible vapours after visible flames have reduced.

The effectiveness of application depends on:

1. Flow rate: sufficient mass must enter the hazard area.
2. Droplet size: droplets must be small enough for effective heat transfer but large enough to reach the target.
3. Pattern: the stream must cover the relevant gases, surfaces or fuel package.
4. Duration: cooling must continue until re-ignition risk is controlled.
5. Position: application from a safe and tactically suitable location is essential.
6. Ventilation: steam and hot gases must be managed rather than unintentionally directed toward occupants.

10. Water and different fuel classes

Water is generally effective on Class A materials such as wood, paper, textiles and many plastics because it cools the fuel below its ignition or pyrolysis temperature and penetrates porous materials. Continued application may be required because deep-seated embers can retain heat.

Water is not universally suitable for Class B flammable liquids. If water is applied as a solid stream to a liquid lighter than water, it may spread the burning liquid. Some water-miscible liquids can also alter the behaviour of foam blankets. The product’s safety data and approved extinguishing method must be consulted.

Certain combustible metals can react violently with water, producing heat and sometimes flammable hydrogen. Water may also be dangerous around energised electrical equipment because of shock risk, although the hazard depends on voltage, water conductivity, equipment design and the conditions of application. Electrical isolation and appropriate extinguishing agents are required.

Cooking oils and fats present a special hazard. Water entering hot oil can flash into steam, expand rapidly and eject burning fuel, producing a fireball. A purpose-designed wet-chemical system or an approved method is required.

11. Hydraulic implications of water properties

Water properties influence the entire path from supply to fire. The pump must provide adequate flow and pressure; hose diameter and length influence friction loss; elevation changes influence required pump pressure; and the nozzle converts pressure into a discharge pattern.

A useful distinction is:

• Flow rate is the volume delivered per unit time, commonly L/min.
• Pressure is force per unit area, commonly kPa or bar.
• Velocity is the speed of water leaving the nozzle.
• Reach is the distance the stream can travel effectively.
• Pattern is the shape and distribution of discharge.

Increasing pressure may increase velocity and alter pattern, but it does not automatically improve fire control. Excessive pressure can reduce operator control, increase reaction force, create undesirable atomisation and damage equipment. Insufficient pressure may produce poor pattern formation and inadequate reach.

The fireground objective is not simply to deliver the maximum water quantity. It is to deliver the appropriate mass and pattern to the correct location while preserving firefighter safety, water supply and structural stability.

12. Applied case: compartment fire in a residential room

This is an illustrative case, not a report of an actual incident.

Background

A two-storey residential building contains a rapidly developing fire in a furnished living room. The door is closed, smoke is accumulating above the doorway, and occupants may still be elsewhere in the building. The fire involves upholstered furniture, timber components and synthetic materials.

Problem

The crew must cool the compartment, limit fire growth and avoid creating a dangerous steam flow path toward occupants. The water supply is adequate, but the crew must select a suitable application method and coordinate ventilation.

Analysis

The contents are primarily Class A materials. Water can absorb heat through sensible heating and vaporisation. Fine droplets can cool hot gases, while a controlled stream can cool burning surfaces and structural elements. However, indiscriminate high-flow application may generate steam faster than it can be safely controlled, disturb the thermal layer or obscure escape routes.

The crew assesses door position, likely fire location, ventilation conditions, hose-line pressure, nozzle reaction and escape paths. They recognise that calculated heat capacity is only potential capacity: water that misses the hot surfaces or exits through openings contributes little to cooling.

Approach and solution

The crew establishes a charged hose line, confirms pump and nozzle conditions, and applies a controlled spray from a protected position. Short, deliberate pulses cool the upper gas layer where appropriate, followed by application to the burning contents. Ventilation is coordinated rather than performed independently. After knockdown, the crew continues targeted cooling and checks concealed spaces for retained heat.

Outcome

The fire is controlled with limited unnecessary water damage. The compartment remains hot enough to require overhaul, but the fuel and surrounding surfaces are cooled below conditions that would support rapid re-ignition. The operation succeeds because water properties are matched to nozzle pattern, flow, access, ventilation and fuel.

Learning points

1. Heat absorption depends on actual contact and phase change, not merely litres carried.
2. Steam expansion creates both a cooling benefit and a life-safety hazard.
3. Surface cooling is necessary after visible flame reduction.
4. Hydraulic pressure and flow must support the tactical objective.
5. Water application must be coordinated with ventilation and occupant protection.

13. Limitations, safety and environmental considerations

Water damage can weaken ceilings, damage electrical and electronic equipment, destroy documents and contaminate property. Firefighters should use sufficient water for control, but avoid wasteful application after the heat hazard has been reduced. Runoff may carry soot, chemicals, fuels and hazardous combustion products into drains or soil.

Operational decisions should comply with applicable fire-service standards, occupational safety requirements, building regulations, water authority procedures and environmental controls. Risk assessment should consider flashover potential, backdraft indicators, steam burns, collapse, electrical hazards, contaminated runoff and the possibility of hazardous material involvement.

Technology can support judgement but cannot replace it. Thermal imaging cameras help identify hot surfaces and concealed heat; flow meters and pump displays help verify delivery; computer-based hydraulic calculations assist planning; and digital incident records support post-incident learning. These tools have limitations, including calibration errors, obstructed views, incorrect assumptions and overreliance on displayed values.

Best-practice principles

• Identify the fuel and confirm that water is an appropriate agent.
• Use the minimum effective flow, not automatically the maximum flow.
• Match droplet size, pattern and reach to the hazard.
• Maintain adequate nozzle pressure and avoid abrupt valve closure.
• Anticipate steam movement and protect occupants and escape routes.
• Cool both the flame zone and heated fuel or structural surfaces.
• Coordinate water application with ventilation.
• Monitor for re-ignition and concealed heat.
• Control runoff where hazardous contamination is possible.
• Record unusual hydraulic or thermal behaviour for training and improvement.

Chapter Summary

Water extinguishes fire principally by absorbing heat. Its high specific heat allows it to absorb substantial energy while its temperature rises, and its high latent heat of vaporisation allows it to absorb still more energy during conversion to steam. The total idealised heat absorbed can be estimated from sensible and latent components, but actual field performance depends on contact, droplet size, flow, ventilation and the fraction of water that vaporises.

Vaporisation causes major volume expansion, which can dilute oxygen but also produce dangerous steam and pressure conditions. Water is treated as practically incompressible in routine hydraulic calculations, enabling predictable flow but allowing damaging water hammer during sudden flow changes. Surface tension affects droplet formation and wetting, while density links volumetric flow to mass flow and therefore to potential heat-removal rate.

Effective fire control requires more than selecting water. Firefighters must match water properties and hydraulic delivery to the fuel, fire development, compartment geometry, ventilation, nozzle pattern and safety conditions. Water is highly effective on many Class A fires but may be dangerous or ineffective for flammable liquids, combustible metals, cooking oils and energised electrical equipment. Professional application is therefore based on both science and disciplined operational judgement.

Key Terms

Specific heat: Heat required to raise the temperature of unit mass of a substance by one degree.

Sensible heat: Heat that changes temperature without changing physical state.

Latent heat: Heat absorbed or released during a change of state without a temperature change.

Latent heat of vaporisation: Energy required to convert liquid water into vapour at a specified pressure and temperature.

Vaporisation: Change from liquid water to water vapour.

Steam: Water vapour, often used operationally to describe hot vapour and visible condensed droplets.

Heat absorption: Removal of thermal energy by a substance.

Specific heat capacity: The heat capacity per unit mass of a material.

Density: Mass per unit volume.

Mass flow rate: Mass of water delivered per unit time.

Volumetric flow rate: Volume of water delivered per unit time.

Incompressibility: The practical resistance of a liquid to volume change under pressure.

Surface tension: Resistance of a liquid surface to expansion caused by intermolecular forces.

Wetting: The spreading and contact of a liquid over a surface.

Droplet size: Characteristic dimension of water particles in a spray or mist.

Water hammer: A pressure surge caused by a rapid change in liquid flow.

Pyrolysis: Thermal decomposition of a material producing gases or vapours.

Sensible heating: Addition of heat that raises temperature without phase change.

Heat-removal rate: Thermal energy absorbed per unit time.

Runoff: Water leaving the fire area without contributing effectively to extinguishment.

Review Questions

Short Answer Questions

1. What is the primary extinguishing action of water in most structural fires?
2. Distinguish between sensible heat and latent heat.
3. Why can steam production be both beneficial and dangerous?
4. What is meant by treating water as incompressible in fire hydraulics?
5. How does surface tension affect wetting and droplet behaviour?

Long Answer Questions

1. Explain how specific heat and latent heat work together during the extinguishment of a compartment fire.
2. Discuss the relationship between droplet size, surface area, reach and heat transfer.
3. Explain the hydraulic significance of water density, mass flow and practical incompressibility.
4. Describe the operational hazards associated with vaporisation and steam expansion.
5. Evaluate the suitability and limitations of water for Class A, Class B, cooking-oil, electrical and combustible-metal fires.

Analytical / Application Questions

1. A hose delivers 300 L/min of water initially at 20 °C. Assuming all water is heated to 100 °C and vaporised, estimate the theoretical heat-absorption rate. State why the actual rate would normally be lower.
2. A nozzle operator increases pressure but observes reduced reach and poor control because the pattern becomes excessively fine. Using water properties and hydraulic principles, explain possible causes and recommend corrective actions.
3. During a closed-compartment fire, a crew plans to apply a large continuous flow without coordinating ventilation. Analyse the potential benefits and hazards, then propose a safer water-application strategy.

Multiple Choice Questions with Answers

Question 1

Which property allows water to absorb substantial energy while its temperature rises?

A. High specific heat capacity

B. Low density

C. Low latent heat

D. High compressibility

Correct Answer: A. High specific heat capacity

Question 2

At approximately standard atmospheric pressure, the latent heat of vaporisation of water is closest to:

A. 4.18 kJ/kg

B. 80 kJ/kg

C. 1000 kJ/kg

D. 2257 kJ/kg

Correct Answer: D. 2257 kJ/kg

Question 3

What is the principal operational danger associated with rapid steam expansion in a confined compartment?

A. Loss of water density

B. Scalding and dangerous movement of hot vapour

C. Permanent reduction in atmospheric pressure

D. Elimination of all combustion gases

Correct Answer: B. Scalding and dangerous movement of hot vapour

Question 4

Which equation represents sensible heat absorption?

A. Q_s = mc_pΔT

B. Q_s = m/L_v

C. Q_s = ρ/Q

D. Q_s = P/(ρg)

Correct Answer: A. Q_s = mc_pΔT

Question 5

Why are fine droplets often effective for gas cooling?

A. They have a lower total surface area

B. They cannot evaporate

C. They provide a high surface-area-to-volume ratio

D. They always have greater reach than large droplets

Correct Answer: C. They provide a high surface-area-to-volume ratio

Question 6

Water is treated as practically incompressible in routine fire hydraulics because it:

A. has no mass

B. changes volume very little under normal system pressures

C. cannot transmit pressure

D. always flows without friction

Correct Answer: B. changes volume very little under normal system pressures

Question 7

A flow of 400 L/min is approximately equal to what mass flow rate for water?

A. 0.067 kg/s

B. 0.667 kg/s

C. 6.67 kg/s

D. 66.7 kg/s

Correct Answer: C. 6.67 kg/s

Question 8

Which fire presents a particular hazard because water can be converted rapidly to steam and eject burning fuel?

A. Timber furniture

B. Paper storage

C. Ordinary textiles

D. Hot cooking oil

Correct Answer: D. Hot cooking oil

Question 9

What does surface tension primarily influence in a firefighting spray?

A. The nuclear composition of water

B. Droplet formation and wetting of surfaces

C. The gravitational acceleration of the building

D. The boiling point of all fuels equally

Correct Answer: B. Droplet formation and wetting of surfaces

Question 10

Water hammer is most likely to result from:

A. A gradual opening of a nozzle

B. A rapid change in water flow

C. Low surface tension alone

D. Water reaching its boiling point in an open bucket

Correct Answer: B. A rapid change in water flow