Chemistry Hougang: Refrigerants & Heat Pumps

A refrigerator keeps its contents cold while the back of the appliance can become warm. That observation contains an important Chemistry lesson: the refrigerator does not destroy heat. It transfers energy from one location to another with the assistance of an engineered system.

This Chemistry Hougang article explains refrigeration, heat pumps, refrigerants, evaporation and condensation, cooling efficiency and the climate effects of refrigerant gases. It connects Secondary Science ideas about states of matter to JC energetics, university thermodynamics and the practical questions that arise in a tropical country. Parents exploring Chemistry tuition in Hougang can use the topic to see how a learner progresses from a simple observation to a quantitative model.

I want students to recognise that a cold room, a supermarket freezer, a heat pump in a cool climate and a building air-conditioner share important physical principles. Their purposes and designs differ, but the core issue is moving heat across a temperature difference. We should also ask which working fluid is used, how energy is supplied, and what happens if refrigerant is released into the atmosphere.

What is the difference between a refrigerator and a heat pump?

A refrigerator is designed to remove heat from a space or product that we wish to keep cool, then reject that heat elsewhere. A heat pump often refers to a system designed to deliver useful heating to a space, although reversible systems can also provide cooling.

Both can use a vapour-compression cycle driven by a compressor. The US Department of Energy’s heat-pump explanation describes how electricity can be used to transfer heat from a colder region to a warmer one.

The phrase “heat pump” can sound puzzling. Heat normally transfers spontaneously from warmer to cooler bodies. A heat pump uses work input and a designed cycle to move heat in the desired direction, including transferring energy from a cooler outdoor environment to a warmer indoor space.

That is not a violation of thermodynamics. The compressor and the rest of the equipment consume energy and change the state of the refrigerant. The energy balance includes both the heat transferred and the supplied work.

In Singapore, the cooling function is immediately familiar because of air-conditioners, food refrigeration and chilled industrial processes. In colder climates, heat pumps are increasingly relevant for space heating.

What is a refrigerant?

A refrigerant is a working fluid chosen for a refrigeration or heat-pump system. Many vapour-compression designs exploit the refrigerant’s ability to evaporate and condense at pressures and temperatures appropriate to the application.

Different refrigerants have different thermodynamic properties, chemical stability, compatibility with equipment, safety classifications and environmental impacts. A fluid that is useful in one system is not automatically suitable as a replacement in another.

Some refrigerants are synthetic fluorinated compounds; other options include carbon dioxide, ammonia, hydrocarbons and further specialised substances. “Natural refrigerant” is a common industry phrase for some of these alternatives, but it does not mean the substance has no hazards.

A crucial boundary: refrigerants should not be released, heated, collected or transferred as a home Chemistry experiment. Refrigeration equipment may involve pressure, electrical systems, flammable or toxic working fluids and other hazards. Design, repairs and disposal belong to trained personnel operating under relevant requirements.

Evaporation is a phase change, not a fuel being burned

At the evaporator, the refrigerant can absorb heat from its surroundings while transitioning from a liquid-containing state toward vapour. This contributes to the cooling effect in a conventional vapour-compression refrigerator.

The physical change is different from combustion. No chemical fuel has to be consumed in the refrigerant’s ordinary phase-change loop. In a functioning cycle, the refrigerant circulates and changes pressure, temperature and phase.

The distinction matters in school Chemistry because students sometimes treat every visible or energetic change as a chemical reaction. Evaporation and condensation are changes in physical state, while reactions involve chemical transformations of substances.

For a student who knows heating and cooling curves, this becomes a natural application. Energy can enter or leave a system during a phase change even when temperature behaviour is not described adequately by only a simple change in sensible heat.

At university, the treatment becomes more exact and may use enthalpy, pressure–enthalpy diagrams and refrigerant property data.

Follow the four main stages of the cycle

A simplified cooling-mode vapour-compression cycle is often described using four components.

1. Evaporator: absorb heat from the cool region

Low-pressure refrigerant absorbs energy from the region being cooled. In many systems it evaporates as part of that heat-transfer process. The air inside a refrigerator or a room can lose heat to the refrigerant.

2. Compressor: supply work

The compressor raises refrigerant pressure and generally increases its temperature. This work input helps make it possible for the fluid to reject heat in a warmer environment.

3. Condenser: release heat

High-pressure refrigerant gives up heat to a suitable sink, such as outdoor air or cooling water. It can condense to a liquid state under the chosen conditions.

4. Expansion device: lower pressure

The refrigerant passes through an expansion or throttling device that reduces pressure and prepares the fluid for another cooling stage. The details depend on equipment type.

The DOE heat-pump guide outlines the energy-transfer principle. The DOE’s heat-pump training materials illustrate the major components.

The school diagram is useful, but it is not every system. Real equipment may have heat exchangers, control systems, variable-speed compressors, supplementary components and special configurations.

Why does the condenser feel warm?

If heat is removed from a cooled region, the system must send it somewhere. In a conventional refrigerator, the rejected heat includes energy extracted from the cooled space plus the work supplied to the refrigeration system, within the defined energy balance.

That is why the warm side can reject more heat than was extracted from the cold side alone.

I ask students to imagine moving boxes from one room to another. The boxes do not vanish; energy supplied by the mover is also part of the activity. The analogy is imperfect but helps them remember that energy transfer and work input must both be accounted for.

A poor explanation says, “The fridge makes cold and releases extra heat.” A stronger one says, “The equipment transfers energy from its cooled compartment to the surroundings while consuming work.”

That sentence can be checked with an energy balance and is more scientifically useful.

Worked example: the refrigerator energy balance

Suppose an idealised refrigerator removes 3.0 kWh of heat from its cooled compartment over a period and receives 1.0 kWh of electrical work in the model.

Conservation of energy gives:

Heat rejected = heat removed + work supplied.

Therefore:

Qhot = 3.0 + 1.0 = 4.0 kWh.

The surroundings receive 4.0 kWh in this simplified account. Heat was not destroyed.

This model neglects complications such as changes in stored energy, heat leakage outside the chosen boundaries and detailed equipment losses. Yet it captures the most important relationship.

If the device had instead removed 2.0 kWh using 1.0 kWh of work over an otherwise comparable accounting period, the rejected heat would be 3.0 kWh. Students should use the stated quantities rather than assume a fixed ratio for all refrigerators.

What is the coefficient of performance?

A coefficient of performance (COP) compares a desired heating or cooling effect with the work input under stated conditions.

For a refrigerator, the cooling COP is approximately:

COPcooling = Qcold / W.

For a heat pump considered in heating mode:

COPheating = Qhot / W.

Using the example of 3.0 kWh removed from the cooled compartment and 1.0 kWh work input:

COPcooling = 3.0/1.0 = 3.0.

If the same abstract energy balance is viewed from the useful-heating side:

COPheating = 4.0/1.0 = 4.0.

A COP above 1 does not mean the device creates energy. It means the useful heat moved can exceed the work input because the system also transfers heat from its surroundings.

The precise COP depends on temperatures, equipment, loads and defined boundaries. A single quoted COP is not a guarantee of performance for every operating day.

COP and efficiency are not interchangeable percentages

Students often try to say that COP 3 means 300% energy efficiency in the same sense as the efficiency of a motor or heat engine. That comparison can confuse two different measures.

COP compares useful heat moved with work input, whereas conventional conversion efficiency might compare one energy output to a particular energy input in another kind of system.

It is usually better to say the cooling system moved three units of heat from the cold region per unit of work input under the given conditions. That wording is clear and does not suggest a violation of energy conservation.

At JC level, I would test understanding by reversing the question: if a device rejects 5 kWh of heat and consumes 1 kWh work, how much heat was absorbed under the simplified steady-state relation? The answer is 4 kWh.

This is an excellent example of using a formula to reason, not just to substitute numbers.

Why the boiling point of a refrigerant matters

The boiling temperature of a substance depends on pressure. By controlling refrigerant pressure, a system can use evaporation at a temperature appropriate to absorb heat, then condensation at a temperature appropriate to reject it.

A substance’s normal atmospheric boiling point alone is not enough to select it. Engineers need refrigerant property information over the intended operating range, alongside compatibility, safety, cost and environmental considerations.

This is why refrigerators are not simply bottles of a “liquid that is naturally cold.” The cycle creates and maintains pressure and temperature conditions that allow useful heat exchange.

The student who understands this is ready to connect vapour pressure, phase equilibrium and heat transfer. These concepts also relate to distillation and evaporative processes, though the equipment and purpose differ.

We revisit molecular volatility in our perfume Chemistry article, where evaporation supports scent release rather than mechanical cooling.

Worked example: energy involved in a phase change

Imagine a fictional refrigerant in a highly simplified problem with a stated latent heat of vaporisation of 200 kJ kg⁻¹ under specified conditions. Suppose 0.10 kg undergoes vaporisation while absorbing heat.

Using:

Q = mL,

we obtain:

Q = 0.10 kg × 200 kJ kg⁻¹ = 20 kJ.

This predicts the energy associated with the stated mass and phase change under the hypothetical model. It does not describe the complete cooling supplied by a real device, because refrigerant enthalpy changes, sensible heating and the cycle details must also be considered.

A student should not treat the fictitious latent heat as a specification for any commercial refrigerant.

The key insight is that phase change can carry a substantial energy transfer without requiring the fluid to undergo a new chemical identity.

Why choose one refrigerant rather than another?

Refrigerant selection involves trade-offs among pressure, temperature range, heat-transfer performance, chemical compatibility, flammability, toxicity, cost and environmental impact.

A fluid with a favourable climate metric may require specific safety controls or redesigned equipment. A refrigerant suitable for a domestic refrigerator may not be suitable for a large industrial chiller.

It is therefore unsafe to advise replacing one refrigerant with another based only on the chemical name or global warming potential. Refrigerant substitution is not a do-it-yourself chemical experiment. Equipment approvals, engineering compatibility, servicing competence and applicable regulations matter.

The US EPA’s HFC transition programme illustrates how refrigerant choices are handled within sector-specific rules rather than one universal ban or instruction.

A serious Chemistry learner asks what the material must do in the actual system before deciding whether it is a suitable choice.

Ozone depletion is different from global warming potential

Some older refrigerants and related substances have been regulated because they can contribute to stratospheric ozone depletion. Other substances may have little or no direct ozone-depleting potential while still being powerful greenhouse gases.

These are different environmental effects.

For example, many hydrofluorocarbons (HFCs) are not significant direct ozone-depleting chemicals in the way that certain older chlorine-containing substances were, but several have substantial global warming potentials (GWPs).

The US EPA’s overview of fluorinated gases explains their contribution to warming. The Singapore NEA’s Montreal Protocol information documents Singapore’s participation in international controls.

I do not want students saying that every high-GWP refrigerant damages the ozone layer. This is a common confusion between two environmental mechanisms.

What does GWP mean?

Global Warming Potential (GWP) is a measure comparing the integrated warming effect of a greenhouse gas with the effect of an equal mass of carbon dioxide over a specified time horizon, commonly 100 years.

Carbon dioxide serves as the reference with GWP 1. Other gases can have much higher or lower values depending on the metric and time period.

The US EPA’s explanation of GWP describes its use for comparing emissions. GWP is not the same as the immediate temperature produced in a room, and it does not tell us the toxicity, flammability or energy efficiency of a refrigerant.

A gas may have a relatively low GWP while presenting other design constraints. Likewise, a high-GWP refrigerant held securely in a well-managed system presents a different direct-emissions situation from the same mass leaked to air.

This is why climate management involves both refrigerant selection and leakage prevention.

Worked example: how a small leak can matter

Suppose a fictional refrigerant has a 100-year GWP of 2,000 and 0.20 kg is released to the atmosphere.

The simplified carbon-dioxide-equivalent mass is:

0.20 kg × 2,000 = 400 kg CO₂-equivalent.

This expresses a warming-effect comparison over the selected time horizon under the stated GWP definition. It does not mean the leak produces 400 kg of actual CO₂ molecules; the substance released is still the refrigerant gas.

If instead 0.05 kg leaks under the same hypothetical metric:

0.05 × 2,000 = 100 kg CO₂-equivalent.

The result illustrates why managing leaks can matter even if the mass of gas involved seems small. These numbers are hypothetical; actual calculations require the proper GWP value and the correct substance or blend.

A Chemistry student who can explain the difference between physical mass and CO₂-equivalent is ready for more advanced environmental data.

Singapore’s refrigerant transition: a useful current case study

Singapore has already introduced global-warming-potential limits for several classes of refrigeration and air-conditioning equipment. On 28 May 2026, the National Environment Agency announced additional measures beginning 1 July 2027 for certain commercial refrigeration and equipment-related activities.

According to the NEA’s May 2026 announcement, new relevant centralised commercial refrigeration equipment from July 2027 will face a prescribed GWP limit of 150, and new passenger-car and light-goods-vehicle air-conditioning follows a related limit from July 2028.

The announcement distinguishes new equipment from existing installed systems, which are not simply required to stop functioning on those dates. It also includes requirements for proper recovery and disposal in stated equipment categories.

These are dated regulatory facts, not a universal promise that all refrigerants will be replaced at once. Readers making purchasing or compliance decisions should check the latest NEA refrigerant requirements.

For a Chemistry student, the lesson is how one material property becomes relevant to technology policy. The policy cannot be understood by an equation alone, but the Chemistry helps explain why it exists.

Why servicing and end-of-life recovery matter

A refrigerator or air-conditioner may run for many years. If refrigerant escapes during normal operation, poor servicing or disposal, its environmental effects depend on the identity and mass released.

Managing refrigerant-containing equipment involves trained maintenance, suitable recovery procedures and safe disposal. Some fluids can be flammable or present other hazards, and pressurised systems cannot be treated as ordinary open containers.

NEA’s guidance describes obligations for regulated systems and spent refrigerants. International approaches differ, so professional users should follow the rules and standards that apply to their location and equipment.

Never cut open refrigeration tubing, remove gas or attempt to recharge equipment for a school investigation. A diagram and an energy-balance problem are safer and more educational than an improvised experiment involving pressure and unknown chemicals.

Chemistry education includes learning which tasks belong to licensed or suitably trained specialists.

Refrigeration, food safety and the global cold chain

The cold chain supports transport and storage of temperature-sensitive foods, medicines and other products. Reliable cooling can help reduce spoilage or preserve product quality under suitable conditions, but temperature control is only one part of safe handling and product management.

A cold room in Singapore, a vaccine transport container, and a warehouse in a cold country share the need to maintain controlled conditions while using equipment and energy. Their requirements and risks differ.

Refrigeration therefore connects Chemistry with food security, health systems, logistics and climate action. A low-GWP fluid is valuable, but a system must also be reliable, maintainable and safe.

The DOE heat-pump information explains thermal energy transfer, while NEA’s standards show how material choice can influence environmental policy. We need both perspectives.

An excellent student should be able to say why energy efficiency, refrigerant leakage and equipment service life are separate but related dimensions of performance.

What makes a cooling claim scientifically meaningful?

Suppose a company says that its new system is “50% greener”. I would ask which measure improved.

Did electricity use fall by 50% under comparable loads? Was the refrigerant GWP reduced? Did leakage decline? Were manufacturing emissions counted? Is the system expected to last longer? What was the baseline?

These are not synonyms. One property may improve while another remains unchanged or even worsens.

A proper comparison identifies the functional job—such as keeping a given space within a specified temperature range for a specified time—and accounts for relevant energy and material impacts.

A chemically literate student learns to resist a number that has no clearly stated denominator, boundary or test condition.

Five misconceptions that slow Chemistry learning

“The refrigerator creates cold energy.” The system removes heat from one region and rejects it elsewhere with work input.
“Evaporation is a chemical reaction.” In a normal refrigeration cycle, evaporation is a physical phase change.
“COP above 1 violates energy conservation.” It compares heat moved with work input and includes heat taken from the surroundings.
“Every refrigerant is an ozone-depleting gas.” Ozone depletion and GWP are different properties.
“Lower GWP means any refrigerant can be safely swapped into existing equipment.” Compatibility and safety require professional assessment.

I would rather correct these misconceptions carefully than have a student recite every component name without knowing why it belongs in the cycle.

Secondary-to-university learning progression

Lower Secondary: states of matter and energy transfer

Learn evaporation, condensation, thermal energy and the distinction between physical and chemical changes. Trace where energy enters and leaves an everyday device.

Upper Secondary: calculations and particle explanations

Connect changes of state with latent heat and calculate energy from suitable given data. Keep units consistent and distinguish work from heat transferred.

JC H1/H2: thermodynamics and quantitative reasoning

Where relevant to the course, extend into energetics, phase equilibria, energy balances and environmental data. Explain why a model has limitations rather than presenting every ideal result as a product specification.

Polytechnic, university and professional work

Study thermodynamics, refrigerants, heat transfer, mechanical and chemical engineering, process safety, product testing, leakage management and climate assessment.

The progression is from recognising a cold surface to explaining an engineered thermal system responsibly.

Four-week paper-based project

Week 1 — Draw the cycle. Sketch evaporator, compressor, condenser and expansion device. Mark the heat input, work input and heat output in the cooling-mode diagram.

Week 2 — Energy accounting. Repeat the 3 kWh plus 1 kWh example, then design an example with COPcooling 2.5 and 2 kWh work input. The useful cooling is 5 kWh, and the rejected heat is 7 kWh under the simplified balance.

Week 3 — Climate metrics. Calculate CO₂-equivalent mass for a hypothetical 0.10 kg refrigerant leak with GWP 1,500. The result is 150 kg CO₂-equivalent under the selected metric.

Week 4 — Current rules. Read NEA’s 2026 announcement. Write 250 words distinguishing existing equipment, new-equipment requirements, safe refrigerant recovery and the environmental reasoning for policy changes.

No fridge or air-conditioner needs to be opened, modified or operated outside its ordinary use.

Self-test questions and answers

What is the main job of a refrigerant in a vapour-compression system?

To circulate as a working fluid and enable heat transfer through controlled thermodynamic states, often including evaporation and condensation.

Why is the outside of a refrigerator sometimes warm?

It rejects heat removed from the cooled compartment along with the work supplied to the system under the energy balance.

If a system removes 6 kWh of heat using 2 kWh of work, what is COPcooling?

6/2 = 3.0.

Does GWP measure whether a gas causes skin irritation?

No. GWP is a climate metric, not a toxicological or skin-safety measure.

Is the rule for new commercial refrigeration systems in Singapore from July 2027 a requirement to dismantle all existing equipment that day?

No. NEA distinguishes restrictions for new relevant equipment and separate recovery or disposal requirements. Check the exact current scope before making an operational decision.

Can students safely release refrigerant gas to demonstrate cooling?

No. Refrigeration systems involve environmental and safety hazards and must not be opened for informal experiments.

Questions parents and students ask

Is refrigerant Chemistry useful for Secondary Chemistry examinations?

It is a practical application of phase changes, energy transfer and environmental Science. The detailed engineering belongs to enrichment unless required by the specific course.

Are heat pumps useful outside cold countries?

Yes. Cooling applications such as air-conditioning use related thermodynamic principles; systems also vary by local climatic and heating needs.

Do all refrigerants have high GWP?

No. They vary. Their safety and engineering suitability also differ, so material choice requires a multi-criteria evaluation.

What careers connect to this topic?

Chemical Engineering, mechanical engineering, refrigeration systems, materials science, energy auditing, equipment safety and environmental assessment all contribute.

Sources and further reading

Continue the Chemistry Hougang library

If a student’s notes say “the fridge makes cold”, ask them to draw the energy flow and identify where the work enters. A correct explanation is far more useful than memorising the equipment’s names. Families interested in learning support can arrange a consultation with eduKate Singapore.

Chemistry Hougang | Refrigeration and Heat Pumps Explained: Refrigerants, Phase Changes and Climate