You ask how long it takes for ice cream to melt when left out at room temperature. I answer, "It depends."

The amount of time it takes for ice cream to melt depends on the amount, the shape, and the ingredients of the ice cream. But before we delve into why the melting time of ice cream depends on these factors, let's take a long, hard look at melting.

What is happening when something melts? Say I have a ball of solid stuff. When we say it melts, we're saying it changes from being solid stuff to liquid stuff; it changes its physical phase. If our stuff is solid, we have increase its temperature, add heat, to get it to melt. Why do we have to add heat? Heat is energy in the form of vibrations of atoms, which are the most basic building block of the elements and all the materials we know and love. Stuff that is relatively hot has atoms that are vibrating more rapidly than the atoms in stuff that is relatively cold.

When our stuff melts, the air heats up our stuff. Vibrating air molecules bombard our stuff and pass on their energy by making the atoms of our stuff vibrate more rapidly. Imagine throwing thousands of bouncy balls at a pool full of still bouncy balls. At the surface of the pool the bouncy balls will start moving around. As air molecules keep bouncing off of our stuff, more of the atoms of our stuff will vibrate with the same thermal energy (heat) as the air molecules.

What about the atoms deep in the ball of solid stuff? They aren't being hit by rapidly vibrating air molecules, so how do they heat up? Atoms on the surface of our ball of stuff start vibrating faster once in contact with the air, and as the air continues to share its heat the vibrations are shared between neighboring atoms of stuff. That's why the outside of our ball of stuff melts before the inside.

As more heat is added to solid stuff, the energy of the stuff atoms becomes so great that they break away from each other while staying loosely associated with each other. This state is known as the liquid phase. Solids are rigid and resistance to having their shape or volume changed. Add enough energy, and solids become liquids, which are not rigid or resistance to having their shape changed, but have a fixed volume.

Earlier I mentioned that three factors are important to consider when wondering how long it takes for something to melt. First I said the amount of time it takes for ice cream to melt depends on the amount; the more ice cream you have, the longer it takes to melt because the heat energy has to spread from the outside to the inside of your ice cream shape. The second factor I mentioned can make melting faster or slower.

The shape of the ice cream will change the melting time depending how it changes the ratio of the area of the surface to the volume of the ice cream. The more ice cream that is in immediate contact with air, the faster it will melt. A lower surface area to volume ratio speeds melting because the air can share its heat energy with more of the ice cream at one time. Imagine how much easier it is to for an ice cream flake to melt than a ball of ice cream. For the ice cream to completely melt, the air has to share its heat energy with the ice cream on the surface and the ice cream on the surface has to share its heat energy with the ice cream in the interior of the flake or ball. It's much easier to get to the center of a flake than a ball, is it not? The flake has a higher surface area to volume ratio than the ball.

The ingredients of the ice cream determine the melting temperature of the ice cream, or how much heat energy is required to change the phase of the ice cream. Every material or ingredient has a different melting temperature or amount of heat energy required for a change in phase. Water freezes and melts at 32 degrees Fahrenheit (0 degrees Celsius), while milk melts at about 31 degrees Fahrenheit (-0.5 degrees Celsius). Other ingredients like sugar, cream, salt, Red #3, and so forth will change the overall melting temperature of the ice cream. The melting temperature is different between ingredients because different kinds of atoms and molecules require different amounts of energy to change the way they associate with one another and change from a solid to a liquid.

Melting (and freezing) is all about changing the frequency of vibration of the atoms and molecules that make up a certain stuff. The higher the frequency of vibration, the more heat energy the stuff has, making it more likely to melt. Sharing heat energy between the stuff and its surroundings can cause a change in the phase of the stuff from solid to liquid, or the other way around. The amount and shape of the stuff affects how quickly the heat energy spreads, while the type of stuff determines how much heat energy is needed for a change in phase.

The time it will take for your ice cream to melt will depend on many things! For example, it will depend on the moisture in the air, the size of the ice cream container, the material of the ice cream container, how windy it is, just to name a few. Instead of give you a time, let me describe how these factors will influence the time it takes to melt.

If the air has more moisture, it will likely transfer heat more readily, and your ice cream will likely melt faster. However, if a layer of ice forms on the outside of the container, this might slow the melting process.

If you have a big gallon tub of ice cream, it will take more time to melt than a cone of ice cream because it requires more heat to melt a larger amount of ice cream.

If the ice cream container is made of plastic, it will conduct heat less well than a metal container. For this reason, you would expect ice cream inside a plastic container to melt more slowly.

If it is a windy day, the air around the ice cream container will constantly be replaced with warm air. This will melt your ice cream more quickly. For the same reason, keeping the lid on the container will help make your ice cream last longer.

So how can you make your ice cream last longer if you don't have a cooler? Keep your ice cream in a plastic container, and put a blanket around the ice cream bucket. The blanket will slow the exchange of air from around the ice cream container, and make it melt more slowly. This is the same reason why you wear thick clothing to stay warm, except you're using it in reverse!