The mechanism that transfers most of the heat in between liquids and gases is called heat convection.

This means that when your hot tea in a cup cools down, the heat from your tea is transported by heat convection through the surrounding air away from the tea. Higher temperature means more motion of the molecules in the air. You can imagine this heat transport by an air molecule moving a lot and bouncing into others, making them move more and these ones then bounce into the next ones and so on. If the air temperature is higher than the temperature of your cold water, heat gets transferred from the air to the water and heats it up. This also means that your water is cooling down the air surrounding it.

A hydro flask is a double walled bottle with vacuum in between the two walls. Vacuum means that there are almost no air molecules in between the two walls that can bounce into each other and transfer heat. This prevents the heat from your hot tea getting transported out from the flask or heat getting transported to your cold drink. This mechanism therefore also prevents any solid food from cooling down or warming up in your Hydro Flask. So yes, you could keep your burrito warm in a Hydro Flask. However, I believe your water would taste weird after having a burrito in your flask, right?

To understand how a Hydroflask keeps a liquid at the intended temperature, we first need to think about what makes a hot liquid cool down or a cold liquid heat up over time.

If we leave a hot or cold liquid in a cup on our kitchen counter for several hours, when we come back to it, we find that the liquid has come to room temperature. If we leave a cup of hot or cold liquid in the car on a hot day, when we come back to it, we find that the liquid is warm. From these two observations, we might (correctly) hypothesize that a liquid tends to eventually reach the temperature of its environment. When this happens, we say the liquid has reached thermal equilibrium with its environment. But by the law of conservation of energy, we know that heat (thermal energy) doesn't just appear and disappear. Where does the heat from the hot liquid go? Where does the heat added to the cold liquid come from?

Heat transfer can happen by several mechanisms, including radiation (such as when your skin feels hot when you are outside in the sun), conduction (such as when a metal pan is placed directly on the hot metal coils of an electric stove), and convection (such as when a radiator is used to heat the air in a room: the warm air near the heat source rises, and the cool air sinks, only to be heated by the radiator, and soon the whole room is heated thanks to the flow of the air in response to the heat source).

In the case of the cup on the kitchen counter, the liquid is in physical contact with the cup (thermal conduction) and with the air (whose flow will cause most of the thermal transfer to occur by convection). The hot liquid loses heat through both the cup and the air, though you may notice that the cup itself will feel warm at first, and the air around it will not. This is basically because the cup is denser than the air, and denser objects usually have better thermal conductivity: the heat from the liquid conducts into the cup, and the heat from the warm cup dissipates into the air (note that the heat of the liquid also directly dissipates into the air at the liquid surface).

Air is a good dissipator of heat because air molecules are in motion, meaning that new molecules are continually coming into contact with the hot liquid or warm side of the cup. But in the cup itself, the same molecules feel the heat of the liquid the whole time. Stated another way, the air has more heat absorption capacity than the cup. The cold liquid similarly gains heat from the air directly at the surface, as well as from the air warming the sides of the cup, which at first are cold since we know that the cup is a good thermal conductor. The air in the kitchen does not heat up or cool down appreciably in reaching thermal equilibrium with the hot or cold liquid because the volume of air in the room is much greater than the volume of liquid in the cup.

Now we can finally consider how a Hydroflask works: it has an outer layer of metal in contact with the air around it, and it has an inner layer of metal in contact with the liquid inside it. Crucially, between those two layers of metal, the makers of Hydroflask have figured out a way to remove almost all the air. They have created a vacuum, which is a space in which there are no molecules of air or matter of any kind. We can think of this vacuum as a substance with zero density, which would make it a very poor thermal conductor. This makes sense since thermal conduction and convection require molecules contacting one another or moving around, and the vacuum has no such molecules in it.

Therefore, the liquid inside the Hydroflask comes to thermal equilibrium with the inner steel layer, but the inner steel layer is surrounded by vacuum, so its thermal energy has nowhere to go. The outside of the hydroflask comes to thermal equilibrium with the surrounding air, but it cannot transmit or absorb heat through the vacuum. The liquid on the inside is virtually thermally isolated and stays at the intended temperature for a long time, though it comes to thermal equilibrium with the environment eventually. The equilibration process goes faster with the Hydroflask lid off. Based on our discussion of heat transfer, can you think of why this might be?

If you put a burrito inside a Hydroflask, it will be surrounded by air inside the inner steel layer, so it has the opportunity to lose heat to that air. But if you keep the lid on, there will be a limit to the heat loss to air, so the burrito should cool more slowly than if you left it out on a plate. But because the air can surround it on all sides, a burrito will not stay as warm as, say, hot tea inside a Hydroflask. Generally, to keep solid foods warm or cold, bags or lunchboxes are lined with thermally reflective fabric whi

Hydro flasks keep their contents cold or hot by preventing transfer of heat in or out.

There are 3 ways that heat can be transferred:
convection (movement of the contents from one location to another),
radiation (emission/absorption of electromagnetic radiation, i.e., light), and
conduction (physical contact between matter at different temperatures).

Convection is avoided by the flask not having holes (apart from the one to drink) which would let anything in or out. Radiation is precluded by making the flask opaque so light cannot pass through. Both of these hold for any container though. The key to Hydro flasks is in preventing conduction. To do this, Hydro flasks are a type of vacuum flask.

The basic design of a vacuum flask is essentially a container inside of another container, with the walls of two separated by "nothing", i.e., by vacuum. Since a vacuum is the absence of physical "stuff", there is nothing to carry heat between the outer wall, which is exposed to the environment and will change temperature depending on the surroundings, and the inner wall, which is at the temperature of the flask's contents. Thus, a cold drink cannot gain heat from the surroundings and a hot drink cannot lose heat.

None of these methods of heat transfer depend on the contents being liquid. (Convection kind of does, as it is technically for fluids only, but solid objects can still move.) This means that a warm or cold solid, such as a burrito, would also be kept at that temperature.

Hydro flasks are not perfectly insulating containers though. The inner and outer walls need to be connected, which they are at the top under the cap. This allows for some heat transfer. Thus, eventually the contents will reach the same temperature as the ambient environment.

Hi Allegra, another great question. A hydro flask prevents heat from leaving or entering the contents of the flask by creating a vacuum barrier between the exterior shell of the bottle and the internal canister that contains all of the hot/warm liquid. Metal is a very good heat conductor but vacuum is not. A simple metallic bottle would be bad at insulating the heat and would allow the liquid to quickly become the same temperature as the exterior air.

On the other hand, the vacuum between the interior canister and the shell insulates the heat inside the flask from the exterior environment and keeps your drink the same temperature it was earlier. The same principles still apply to a solid food and therefore, if you could fit the whole burrito in the flask, then you can keep your burrito warm.

A flask keeps water cold or hot by stopping heat from leaving the flask, or coming in from the outside.

The flask has some thermal insulation material in the walls that stops the heat from coming through, or just air or even vacuum - air and vacuum are very good at stopping heat from moving about!

It doesn't matter if you have water inside the flask, or a burrito - the flask will prevent it from cooling down, or heating up just the same, because the walls of the flask stop thermal energy from coming through.

To answer your question, we have to think about what the concept of temperature means. For most situations, we say that temperature is how hot or cold an object feels when we touch it. But what is happening at the microscopic level? What are the object's atoms and molecules doing?

At any temperature above absolute zero, atoms and molecules vibrate, jostle, rotate, and collide with each other. While we could hypothetically use laws of classical physics like Newton's Laws and the Conservation of Momentum to figure out what each and every atom is doing, we only want to know what the whole collection of atoms is doing. With that in mind, it's more helpful to use statistics and think about what the atoms are doing on average_ instead of what they do individually. Temperature is one of these important average quantities (others include volume and pressure). Specifically, temperature is an average of the energy the atoms have as a result of their movement, which from a classical point of view we would call "kinetic" (from Greek, meaning "movement") but from our new statistical perspective would call "thermal."

When two objects of different temperatures touch (like a hot mug and your hand, for example), the thermal energy of the hotter object flows into the colder one. On a microscopic level, the faster-moving, faster-vibrating atoms in the hot object collide with the slower-moving ones in the cold object. Eventually, the two objects reach thermal equilibrium, when they become the same temperature. This also happens with air, which is how you can know a stove is hot without having to touch it and burn yourself: the stove touches the air and makes it hot (but not immediately as hot as the stove), and the air is in contact with your hand and makes it hot (but not immediately as hot as the air).

Hydro Flasks and other similar containers slow down the thermal energy transfer between their contents and the environment using a technology called a vacuum seal. A Hydro Flask bottle is actually two bottles, one inside the other. The air between them is pumped out, and then the tops of the bottles are sealed together. This means that the only things touching (and thus able to move thermal energy around) are the tops of the bottles, which are insulated with a lid that also slows the heat transfer. This is how hot liquids stay hot inside a Hydro Flask, and cold liquids stay cold.

The transfer of energy to an environment that's a different temperature is slowed down by the fact that there's no air between the two layers, and that the cap is insulating. The question "Why does an insulating cap change temperature more slowly?" has a more complicated answer that involves something called "heat capacity" or "specific heat."

You'll also sometimes see that Hydro Flasks have different ratings for how long they can keep hot things hot versus keep cold things cold. That's because cold things are typically not that much colder than the outside - ice water is 32 Fahrenheit, but maybe the surroundings are 70. Hot coffee brewed with near-boiling water is closer to 200 Fahrenheit. The larger temperature difference between hot liquid and room temperature means the heat transfers more quickly. This is why most bottles say they can only keep hot things hot for 12 hours but can keep cold things cold for 24 hours.

A hydro flask is a kind of a thermos, a container that has a layer of vacuum between the inside of it and the outside world. Air conducts and convects heat, so this vacuum acts as a barrier to the transfer of heat between the inside of the thermos and the outside world. The process of the transfer of heat works the same whether the medium is liquid, solid, or gas, so yes, if you had a thermos that were large enough, you could keep a burrito warm inside of it.