Thaw seems to be used when thawing something, like food. Melt seems to be used with ice. Some dictionary definitions seem to conflict with these statements. When should each word be used?
As I understand it, "melt" implies a phase change from a solid (often a frozen) state to a liquid or near-liquid state.
- An ice cube melts when it warms and turns from a solid into a liquid.
- Cheese melts, turning from a firm solid, into a liquid or goo.
- Iron melts in a forge, turning into a (very hot) liquid.
"Thaw" is generally used to describe cases where a frozen item warms enough for ice to melt, but without a complete phase change. E.g.
- Frozen meat should thaw before cooking. The ice particles in it melt, but it does not liquify.
- Permafrost (frozen soil, usually found in the arctic) thaws, it does not melt. Individual ice crystals in the soil melt as the soil warms (becoming liquid), but the soil itself does not turn into a liquid.
Something frozen will THAW to a solid state.
Something frozen will MELT to a liquid state.
So ice would melt (being frozen water), a frozen sausage would thaw (to a sausage).
To be confusing, frozen cheese thaws, and then as it gets hotter, melts into a liquid.
This is an excellent question - as a native english speaker, I had to think about it for a moment.
The top current answers seem to focus on phase change; I think this is too narrow a view. Instead, as @Zenzizenzizenzic's answer mentions: lake thaw!
I would therefore explain the difference in a slightly more general way:
- Melting: the nature of the object changes. A ice cube becomes water, for example.
- Thawing: the nature of the object does not change. A frozen lake, after thawing, is still a lake.
If you refer to something in the same way before and after: it thawed. Otherwise, it melted.
In addition to the other answers, note that there are industry/regulatory implications when using specific verbiage such as "thawing" (as applied to industrial/commercial food processing sectors under federal inspection programs). In the USA, for example, these can fall under either USDA FSIS' or FDA's policy arenas depending on the product(s) in question. For example, under the FDA Food Code 2017, "thawing" is defined as and must meet the requirements of:
3-501.13 Thawing. Except as specified in (D) of this section, TIME/TEMPERATURE CONTROL FOR SAFETY FOOD shall be thawed:
(A) Under refrigeration that maintains the FOOD temperature at 5°C (41°F) or less; or
(B) Completely submerged under running water:
(1) At a water temperature of 21°C (70°F) or below,
(2) With sufficient water velocity to agitate and float off loose particles in an overflow, and
(3) For a period of time that does not allow thawed portions of READY-TO-EAT FOOD to rise above 5°C (41°F), or
(4) For a period of time that does not allow thawed portions of a raw animal FOOD requiring cooking as specified under ¶ 3-401.11(A) or (B) to be above 5°C (41°F), for more than 4 hours including:
(a) The time the FOOD is exposed to the running water and the time needed for preparation for cooking, or (b) The time it takes under refrigeration to lower the FOOD temperature to 5°C (41°F);
(C) As part of a cooking process if the FOOD that is frozen is:
(1) Cooked as specified under ¶¶3-401.11(A) or (B) or § 3-401.12, or
(2) Thawed in a microwave oven and immediately transferred to conventional cooking EQUIPMENT, with no interruption in the process; or
(D) Using any procedure if a portion of frozen READY-TO-EAT FOOD is thawed and prepared for immediate service in response to an individual CONSUMER'S order.
(E) REDUCED OXYGEN PACKAGED FISH that bears a label indicating that it is to be kept frozen until time of use shall be removed from the reduced oxygen environment:
(1) Prior to its thawing under refrigeration as specified in ¶(A) of this section; or (2) Prior to, or Immediately upon completion of, its thawing using procedures specified in ¶ (B) of this section.
A similar vernacular term would be "slacking", which is instead defined as:
"Slacking" means the process of moderating the temperature of a FOOD such as allowing a FOOD to gradually increase from a temperature of -23°C (-10°F) to -4°C (25°F) in preparation for deep-fat frying or to facilitate even heat penetration during the cooking of previously block-frozen FOOD such as shrimp.
And finally, the term "melting" is typically not applied upon finished or work-in-progress product (for that, we would use verbiage such as "temperature-abused" or "out-of-spec"), and is instead typically seen when describing product-external components that present a concern for cross-contamination hazards when temperature control is not properly enforced, e.g., fresh produce packed on ice formed with non-potable water source(s).
The main difference has been described by Chris Keefe already but here is some supplementary material.
There are figurative uses of both terms, thaw and melt, which are not interchangeable. In politics and elsewhere you might say that you were hoping for a thaw in relations between the US and China or between two people or factions. Melt would be wrong here. You might speak though of melting someone's heart in the sense of causing them an extreme positive emotional change of state.
There are two related terms which can be distinguished.
If a bank account is frozen by the bank because of suspected fraud then you would ask them to unfreeze it. This also applies metaphorically to mechanisms which have become stuck in some way like a hinge rusting solid. If you put oil on it and it now works you could say you managed to unfreeze it.
If you take an item from the freezer and put it in the microwave oven you would say you are going to defrost it and the microwave may even have a defrost setting. This is different from scarping the frost off the car windscreen in winter. I am not sure this is universal in all dialects but it is common in the one I speak - south east England. I am not sure why we do not say we are going to thaw it which would be logical although we may or may not be going to melt it as well as defrost it.
Thawing means to take something that was previously frozen and warm it until it is no longer frozen.
Melting can happen at any temperature and implies a loss of shape. When butter melts, it stops being a block shape and becomes a puddle of liquid butter. You can also melt iron at very high temperatures. Neither of these could be described as "thawing"
Since a frozen chicken keeps its shape when warmed to room temperature, that would be described as "thawing". But an ice cube, when warmed, becomes a puddle of water, so it should be described as "melting". However, an object encased in ice would be thawed by melting the ice.
I believe you want a generally understood definition, without scientific specificity, or literary license (such as the thawing of relations between countries after war, or the melting of one's resolve).
Frozen liquids such as snow and ice melt. But so do normally-solid materials such as plastic and cheese and even rocks, at high enough temperatures. Anything that melts goes from solid to liquid or semi-liquid state. Becoming liquid is central to the definition.
"Thaw" means coming out of ice or a frozen state. So a frozen solid (like a steak) thaws and remains solid. A forest (trees, terrain) can thaw after winter, as the snow on it melts.
Thaw and defrost mean the same thing: "return from frozen to its expected state."
Melt means "change state from a solid into a liquid (due to heat)"
Here, "expected state" means exactly what you think it means—typically the state it's in at room temp or fridge temp. That's all there is to it. The other answers make it seem more complicated than it is.
The distinction is also about the direction of the action.
We have a thaw in the spring as the weather warms up, so it's a slower action with the temperature change generally applied by the environment.
We can melt a piece of ice, or soup, directly on a cooker. It's faster and directed at the item.
You melt a thing (that can melt), while the thaw comes to that thing.
Many solid objects (esp. frozen food) do not conduct heat very well, so the direct application of heat on a cooker just isn't satisfactory - the outside can be burnt, while the centre is still frozen (see the 'Baked Alaska' cake/pudding/treat), so such frozen foods need to be thawed.
It should be noted that the microwaves in a microwave oven are not readily absorbed by ice, so are not really suitable for 'thawing' food. Also microwaves don't reach very far into the thawed food, so may need 'standing time' to allow the heat to reach all parts. Microwave cooking has helped the misuse of many phrases used in cooking (as had the availability of freezers).