Why do some substances melt instantly from solid to liquid (like water/ice) and other substances gradually transition (like magma, or metals)?



How come there is no transition state in between something like ice and water like how there is between stone and lava. Is there actually some sort of viscous, in-between, water/ice that I just don’t know about?

In: Chemistry

Imagine LEGO. Each block represents an atom. Say you make LEGO wall brick by brick so that it’s strong. The blocks are each secured to each other making them stronger as a group and the way they’re arranged. Now make that same wall but add gaps and spaces. It still makes a wall but not a strong. Now lay the blocks side by side without snapping them. They make a wall but it could be knocked down no problem. This is like what you described. Some substances have atoms that are together and form a wall (a solid) but only a tiny bit of temperature (or change in energy, getting knocked over) is required to break that wall. While the strong wall represents substances that have strong bonds between atoms so they need a bigger change in temperature (energy) to get broken apart making the state change slower.

Magma is a mix of minerals with different melting points so when rock is heated, the minerals with the lowest melting points melt first and separate from those with higher melting points. This is what happens in the Upper Mantle when the basalt magma that creates the ocean floor separates from the higher temperature minerals in there Mantle itself. The liquid magma is less dense and pushes up towards the surface.

The same process happens in reverse when magma cools. The minerals with the highest melting point crystallise first leaving low temperature minerals in the magma. As it happens, the low melting point minerals tend to create stickier magma than high temperature minerals; so a body of magma under a volcano that has partially crystallised tends to create more explosive eruptions. Which is why some volcanoes are considered more dangerous if they have not erupted in a long time.

What you’re interested in is the glass transition temperature, or Tg. It’s modeled after (you guessed it) glass’s behavior. When polymeric materials heat up, their molecules are still tangled together even though they’re technically liquid. That makes it quite like spaghetti, all the individual molecules can twist and turn how they want, but the whole thing is just a rat’s nest. This behavior can be expanded to include most polymers (plastic) and some mixtures (like rock and magma)

Like another poster said, the components in magma all have different melting points, which contributes to its melt behavior too. At the temperature magma starts to flow, the entire thing won’t be fully melted. Like crystalized honey, it pours in globs and not as a smooth liquid…. because there’s chunks.

Other Fun facts about water. Water is non-compressible meaning no matter how much pressure you put on it you can’t significantly decrease it’s volume. It’s one of the few liquids that actually expand when it transitions from solid to liquid. It can also transition from solid to gaseous state without ever going through a liquid state.

Basically… I would never use water as an example of how liquids “should” behave.

There are two types of solids, crystalline and amorphous. In crystalline solids (salts, ice, napthalene etc) the structure is very regular. In amorphous solids (glass, many rocks etc) the structure is higglety-pigglety. A crystalline solid will have a sharp melting point as the key bonds that break are all the same. The amorphous material has many different strengths of bonds which break at different temperatures so they dont have a sharp transition.

I love these great questions……that took 7 years at university to understand…..and still dont truly understand lol. Thanks OP!