How to keep coffee hot longer: the physics

To keep coffee hot longer, preheat the mug with hot water, cover it with a lid, and pour it into a narrower or insulated mug. Each of those steps blocks one of the four routes heat takes out of your cup: conduction, convection, evaporation and radiation.

Heat always flows from a hotter thing to a cooler one. Your coffee is hotter than almost everything around it, so it is losing energy from the moment you pour it. You cannot stop that. You can slow it down, and you will do it far better once you can see the four routes clearly.

Route one: conduction

Conduction is heat passing directly through touching materials, molecule jostling molecule. When hot coffee meets a cold ceramic mug, a good share of its heat goes straight into warming the mug. The mug then passes heat on to the table and your hand.

What to do:

• Preheat the mug. Fill it with hot water from the kettle, let it sit for a minute, pour it out, then add the coffee. The mug has already taken its share of heat from the water instead of from your drink.
• Choose an insulated mug. A double-walled mug with a vacuum between the walls is the strongest defence, because a vacuum has almost nothing in it to conduct heat.
• Use a coaster or set it on wood or cork rather than stone or metal, which pull heat away faster.

Route two: convection

Convection is heat carried by a moving fluid. Air just above the coffee warms, rises and is replaced by cooler air, which warms in turn. Inside the cup, the same thing happens in the liquid: coffee cools at the surface and walls, sinks, and fresh hot coffee rises to take its place. A draught or a fan speeds the air side of this up considerably.

What to do:

• Put a lid on it. A lid traps a layer of still, warm air above the coffee and stops the stream of fresh cool air from passing over the surface. Even a saucer laid on top helps.
• Keep it out of draughts, away from fans, open windows and air vents.

Route three: evaporation

This is the route most people forget, and when the coffee is very hot it is one of the biggest. The fastest-moving water molecules at the surface escape into the air as vapour. Because they are the most energetic ones, they take more than their share of energy with them, and the liquid left behind is cooler. The steam rising from your cup is heat leaving.

Evaporation happens at the surface, so the size of the surface matters.

What to do:

• Use a lid, again. It is the single step that blocks two routes at once: convection and evaporation.
• Choose a narrower mug. For the same amount of coffee, a tall narrow mug has a smaller open surface than a wide shallow cup. Less surface, less evaporation and less convection from the top.
• Fill it fuller. A full mug holds more hot liquid for the same open surface.

Route four: radiation

Every warm object gives off energy as infrared light, which you cannot see. Hotter objects radiate more. Shiny metal surfaces radiate much less than dull or dark ones, which is why the inside of a classic vacuum flask is silvered. For an ordinary mug at room temperature, radiation is a smaller route than the other three, but it is never zero.

What to do:

• A vacuum flask or insulated mug with a reflective inner wall handles radiation as well as conduction and convection. That combination is why a good flask can keep a drink hot for hours.

The milk question

Here is a small puzzle worth solving yourself. If you take milk and will not drink the coffee for ten minutes, should you add the cold milk now or at the end?

A hot object loses heat faster when the gap between its temperature and the room's is larger. Black coffee left alone stays hotter for longer, so it loses heat faster the whole time. Add the milk at the start and the coffee is cooler during the wait, so it loses heat more slowly. In principle, adding milk early leaves you with a warmer drink at the end. Try it with two mugs and a kitchen thermometer, and you have done an experiment rather than taken my word.

Your checklist

• Preheat the mug with hot water.
• Use a lid, or at least a saucer.
• Pick a narrow or double-walled insulated mug, or a vacuum flask.
• Fill it close to the top.
• Keep it out of draughts.
• If you take milk and will wait, add it early.

A word on safety

The point is to keep coffee pleasantly hot, not dangerously so. Freshly made coffee can scald skin and mouth. Let it cool to a comfortable temperature before you sip, take extra care with lidded travel mugs, which keep heat in and can hide how hot the drink still is, and keep hot drinks well out of reach of children.

A reading path

If this made you curious, you can teach yourself the rest. Look up these topics, roughly in this order:

• Heat and temperature, and the difference between them.
• Thermal conductivity.
• Convection currents.
• Latent heat of vaporisation.
• Thermal radiation and emissivity.
• Newton's law of cooling.

Each one is a short read on its own, and together they explain every warm thing in your kitchen.