What is antifreeze?

We’ve got the lowdown on antifreeze, what it does and how to top it up

]Antifreeze is important all year round, and is even more so in winter, so knowing what it is, how it works and how to check or replenish it is important. This guide will cover all these aspects.

What is antifreeze used for?

Petrol and diesel engines work by creating a rapid series of explosions inside the cylinders.

Given this, it’s little surprise that engines get hot: exhaust gases can be 700°C, while the explosions inside the cylinders are around 1,500°C, and can be as high as 3,000°C when the engine is being worked hard.

It is logical, therefore, that engines need a bit of help keeping cool, as an overheating engine will not last long.

And while air-cooled engines do exist, almost all car engines today are water cooled, and use liquid circulating around channels and passages in the engine block (known as the ‘water jacket’) to draw heat away from the engine, passing it to the radiator where it can be cooled down.

But water, as we all know, freezes at 0°C and, as the UK and many other countries regularly experience such temperatures, the water that cools the engine needs an additive to prevent it from freezing – both because this would prevent it from circulating around the engine (leading to overheating), and because it would expand as it freezes, damaging pipes and possible the engine itself.

How does antifreeze work?

Water freezes when the temperature drops to 0°C, and enough energy is removed from the molecules to reduce their motion enough so that they form a lattice structure – ice.

Antifreeze lowers the freezing point of water, preventing it from freezing. The molecules in antifreeze block the attractive forces between the water molecules, reducing their ability to align in a lattice.

This is highly effective: adding antifreeze to water in a 50:50 solution will reduce the freezing point of the liquid from 0°C, to -37°C. A 70:30 ratio drops the freezing point to -55°C.

Does antifreeze have any other benefits?

Yes. Water encourages rust, and antifreeze reduces this characteristic – a useful trait in a metal engine. It also increases the boiling point of water (to 112°C in a 50:50 ratio), reducing the chances of the coolant boiling off and the engine overheating. Another benefit of antifreeze is that it reduces the amount of mineral deposits from water forming, as they might do in your kettle, for example.

What is antifreeze made from?

Chemicals called ethylene glycol or propylene glycol. They’re very bad for you, so be sure to wash your hands after handling them or their containers.

Ethylene glycol has a slightly sweet taste, which can attract cats, and a tiny amount of ethylene glycol can be fatal for felines; be sure to sluice away any spills.

Antifreeze is widely available from garages and auto-parts stores, and can be bought in pure or ready-mixed formats, with the former requiring dilution with water to the ratio stipulated in your car’s handbook.

Is antifreeze the same as coolant?

No. A car’s coolant is the water: antifreeze is an important additive that is mixed with the coolant.

How do I know if I have enough antifreeze in my car?

Because antifreeze and coolant are often used synonymously, we’ll detail two meanings here.

To check your car’s coolant level, consult the handbook to find where the expansion tank is. This will typically be a translucent, often spherical tank about the size of a small bowling ball. There will be minimum and a maximum lines on this, and the level of the liquid inside the tank should be between these. If it is hard to see the level of the liquid, gently rock your car using your body, and the moving liquid should help you determine its level.

You will need a testing tool to check if you have the correct ratio of antifreeze in your coolant; these can be purchased online for around £10. They work by you squeezing a rubber bulb that draws coolant out of the tank via a flexible straw, and a gauge will display the concentration of antifreeze. A garage will also be able to test the concentration for you.

If your coolant level gets too low, perhaps because you have a leak somewhere in the system, your car should display a warning light.

Where does antifreeze go and can I add it myself?

Coolant is added to the expansion tank described above. Purchase the type of antifreeze recommended in your car’s handbook.

To top it up, make sure the engine is cold – if it’s hot you risk burning yourself from escaping steam, or the coolant could burst out of the expansion tank when you open it, posing significant risk of injury.

With the engine off and the handbrake on, remove the expansion tank lid slowly, letting any air from the pressurised system escape gently. Top up the expansion tank so the liquid is between the minimum and maximum lines, then replace the lid, being careful not to cross-thread it.

Coolant will also need replacing periodically – perhaps every 30,000 miles or so; this should be included in your car’s scheduled maintenance regime.

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