I was watching a TV show where a criminal used a bullet made out of blood, so that after entering the body the "evidence" (the bullet) melted away. I was wondering if it were possible to do something like that. That is, is it possible to make a bullet out of blood, ice, or a similar meltable substance that could be fired without melting and that would be capable of piercing a target and that would subsequently melt away? Any supporting physics/chemistry equations would be appreciated. Thanks in advance!
[Physics] Ice/Melting Bullet
projectilewater
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I stumbled on this question rather late - and when the link to the image in @Georg's answer was no longer working I started a little digging of my own. I came upon the following plot (at http://www1.lsbu.ac.uk/water/microwave.html) which explains this very well:
It shows unambiguously that water has a strong absorption peak in the "low GHz" range (right around the microwave) while the absorption peak for solid ice happens at a much lower frequency - about 6 orders of magnitude lower.
The article goes on to explain this by stating that the dipole in the water molecule attempts to align with the changing electric field; when the phase difference of this alignment is at 90 degrees (resonance) the heat transfer is maximized. For liquid water you are near resonance - for ice, you are far away. Quoting from the page (I put key phrases in bold):
The water dipole attempts to continuously reorient in electromagnetic radiation's oscillating electric field (see external applet). Dependent on the frequency the dipole may move in time to the field, lag behind it or remain apparently unaffected. When the dipole lags behind the field then interactions between the dipole and the field leads to an energy loss by heating, the extent of which is dependent on the phase difference of these fields; heating being maximal twice each cycle. The ease of the movement depends on the viscosity and the mobility of the electron clouds. In water these, in turn, depend on the strength and extent of the hydrogen bonded network. In free liquid water this movement occurs at GHz frequencies (microwaves) whereas in more restricted 'bound' water it occurs at MHz frequencies (short radiowaves) and in ice at kHz frequencies (long radio waves).
Incidentally - and I admit, to my surprise - it seems that the resonance peak for liquid water shifts quite a bit with temperature; see this graph from the same source (I don't quite understand what the units are… but the general shape and direction with temperature are evident; note the 2.45 GHz line which corresponds to the typical frequency of the home microwave oven):
At 2.45 GHz, the dielectric absorption decreases as temperature goes up. This suggests that cold water heats more rapidly than hot water, but I haven't attempted to measure this myself. Might be a fun follow-up for somebody. I think that "microwave physics" is an underused topic for school science fair experiments…
It's certainly possible for ice to sink in water under the right conditions. The diagram this section of Wikipedia's ice page will show you the conditions under which the various types of ice can form. Most of the "exotic" ones such as XII will form only at pressures greater than around 200MPa. These high-pressure forms are all denser than water, so they would sink to the bottom. This means that they would displace less liquid than their weight, so melting them would result in an increase of the surface level.
Earth's oceans aren't deep enough for these types of ice to form. The pressure at the bottom of the Mariana trench is about 100MPa. Since pressure increases linearly with depth, the oceans would need to be around twice as deep in order for this to happen.
However, there is a solid form of water that does sink in Earth's oceans. This is methane clathrate, which is a crystalline solid consisting of methane molecules surrounded by water ones. It can form at pressures found in the ocean, and there's rather a lot of it in sediments below the sea floor. Although methane clathrates are denser than water, I don't know what the effect on the sea level would be if they melted. This is because when it melts the methane is released as gas and bubbles to the surface, and I don't know whether the volume increase due to the melting is bigger or smaller than the volume decrease due to the methane escaping into the atmosphere. (If large quantities of clathrate did melt then the direct effect on sea level would be the least of our worries, because methane is a very powerful greenhouse gas and there's an awful lot more of it locked up in clathrates than we currently release industrially. This could actually happen.)
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Best Answer
The reasons you are looking for (why it doesn't work) are twofold:
First, mass. Bullets are made out of lead, tungsten, depleted uranium etc. Heavy stuff. Aluminum and brass would make perfectly good ammunition if they weren't so light (or if the shootout was on the moon). The problem is lighter projectiles are greatly affected by air resistance and lose much of their accuracy and deliverable energy enroute to the target. The bullet is not so useful if it gets blown off course, or simply bounces off the target. Test this yourself - get a baseball, a baseball-sized ball of styrofoam, and a baseball-sized ball of lead. Have someone toss them from a 3rd-floor balcony and catch them bare-handed.
Second, heat. All that gunpowder igniting behind the bullet gets rather warm. And there's friction with the inside of the barrel. Plot that heat against the energy needed to boil 6 grams of water and we find that our ice projectile melts in the barrel, not in the target. And if it doesn't melt completely it certainly won't melt symmetrically, and a mis-shapen blob won't go where we want it to go.