albertlee Posted June 10, 2004 Posted June 10, 2004 Why would ice melt more slowly when adding salt or sugar in it? Albert
Bryn Posted June 10, 2004 Posted June 10, 2004 I don't think so. The sugar or salt ions in the water would disrupt the formation of a perfect ice structure and so weaken it. So if anything it would melt faster. As it happens i did freeze some water which was totally saturated with sugar the other day (don't ask why). I didn't notice any significant change in the time it took to melt, but it did make a cool looking ice structure. Looked like something out of alien. Also if you think about it ice lollies melt faster than you'd expect a comparable lump of ice to, and they've got rather large quantities of sugar in it. Also the addition of salt lowers the freezing temperature of water, so it will melt sooner than normal water.
albertlee Posted June 10, 2004 Author Posted June 10, 2004 But this what my chemistry teacher tells me.... He says that the salts would absorb the heat given from the ice...... Any other can explain? Albert
swansont Posted June 10, 2004 Posted June 10, 2004 Why would ice melt more slowly when adding salt or sugar in it? Albert It doesn't - it tends to help it melt more quickly. When you dissolve something in water' date=' the freezing point decreases. So the liquid can absorb more energy from the solid that allows it to melt. Under conditions where the ambient temperature is a little below 0 C, you can still melt the ice. The freezing point decrease is related to the concentration of solute. Salt does a better job because it's ionic, and you get both Na and Cl contributing. More on freezing point depression
aommaster Posted June 10, 2004 Posted June 10, 2004 Great link Swansont. Does that mean that anything dissolved in water would reduce a the freezing point? IF so, does that mean the distilled water would freeze before mineral water or hard water?
albertlee Posted June 10, 2004 Author Posted June 10, 2004 So, Swansont what thing to you add to ice when you want them to keep as solid as possible? like the ice around the wine bottle? Albert
swansont Posted June 11, 2004 Posted June 11, 2004 Great link Swansont. Does that mean that anything dissolved in water would reduce a the freezing point? IF so, does that mean the distilled water would freeze before mineral water or hard water? As far as I know, yes. It's simply a function of the molar concentration of the solute. So pure water freezes at the nominal temperature of 0 C, and everything else is a some point colder than that.
swansont Posted June 11, 2004 Posted June 11, 2004 So' date=' Swansont what thing to you add to ice when you want them to keep as solid as possible? like the ice around the wine bottle? Albert[/quote'] If you want to get or keep something cold, you don't want the ice to be as "solid" as possible. In making homemade ice cream*, for example, you add salt to the ice so that it stays colder than 0 C but you want some liquid so there is always contact between the ice/water and the container - it conducts heat better than air, and with just ice you have air gaps. I'm not sure if this make the ice harder or more solid. It's possible, though, since the ice is surrounded by a colder reservoir of water. Colder ice should be harder, but I'm not sure if it's just a function of absolute temperature or temperature below the melting point. *homemade ice cream is more fun to make with liquid nitrogen
albertlee Posted June 11, 2004 Author Posted June 11, 2004 So adding salt would make the ice colder.......but why? secondly, if it is below 0c, then why it still melts? Albert
aommaster Posted June 11, 2004 Posted June 11, 2004 So adding salt would make the ice colder.......but why? It doesn't make it colder DIRECTLY. It makes the water freeze at a cooler temperature (maybe -3 Degrees) compared to pure water that freezes at 0. That is what I meant by indirectly, as when you freeze it, it is cooler! secondly, if it is below 0c, then why it still melts? That may already answer this question. If no, Why wouldn't it melt?
Sayonara Posted June 11, 2004 Posted June 11, 2004 It melts below 0c because the salt has lowered its melting (or rather 'freezing' in this case) and boiling points, as Swansont already stated.
albertlee Posted June 11, 2004 Author Posted June 11, 2004 Then how about sugar? secondly, I want to know how does salt lowers the freezing point of ice? (I know the answer has been given already, but it is still a bit unclear to me...sorry for asking again) Will be apreciated for knowing the answer.. Albert
aommaster Posted June 11, 2004 Posted June 11, 2004 Then how about sugar? Do you mean when it is dissolved in the water? It again, lowers the freezing point.
Bryn Posted June 11, 2004 Posted June 11, 2004 is the reason we add salt to water when cooking pasta/courgettes etc to lower the boiling temperature because they cook at less than 100 degrees?
Sayonara Posted June 11, 2004 Posted June 11, 2004 I fry courgettes - much nicer But other than that, "yes".
Skye Posted June 11, 2004 Posted June 11, 2004 Salt doesn't have as much of an effect on the boiling point of water, it raises it a small amount IIRC.
Skye Posted June 11, 2004 Posted June 11, 2004 Well from the equations on that page swansont put up the boiling point elevation and the freezing point depression are proportionate to the inverse of the heat of vapor and the heat of fusion respectively. The heats of vapor for Na and Cl are higher than their heats of fusion, so this will mean the bp elevation is smaller than the freezing point depression. If that makes any sense.
Sayonara Posted June 11, 2004 Posted June 11, 2004 I thought it was just something to do with changing the SHC of the water?
Skye Posted June 12, 2004 Posted June 12, 2004 Yeah it is more or less, but I think the specific heat capacity is a standard at 4 C and 1 atm. So we have to use different standards for calculating te changes that occur at other temperatures, especially during phase changes.
albertlee Posted June 12, 2004 Author Posted June 12, 2004 Then how about sugar? secondly' date=' I want to know [b']how does salt lowers the freezing point of ice? [/b] (I know the answer has been given already, but it is still a bit unclear to me...sorry for asking again) Will be apreciated for knowing the answer.. Albert again, how does salt lowers the freezing point of ice? Alberty
albertlee Posted June 13, 2004 Author Posted June 13, 2004 Can any body expain the question as mentioned above? I know Swansont has already expained it, but it is still too vague to me........because I dont get the concept of that.....like why ionic substance like salt, would help decreasing the freezing point? and what is the concentration of solute? Again,the question still is how does salt lowers the freezing point of ice? ps, please explain it scientifically, as you are explaining this to a student who has only 1 year chemistry explerience........or explaining on your labreport, which requires exact reasoning and preciseness.... Apreciate so in advance Albert
wolfson Posted June 13, 2004 Posted June 13, 2004 To enable the salt solution to reach the dynamic equilibrium, where the number of freezing particles equals the number of frozen particles we must lower the temperature so that the particles attach to the ice molecules. When the particles/molecules equal each other the are said to have reached “dynamic equilibrium” The higher the molarity of salt the lower the freezing point. The solution (NaCl·2H2O) crystals and ice crystals. This heterogeneous is an eutectic mixture.
dryan Posted June 13, 2004 Posted June 13, 2004 It makes the water freeze at a cooler temperature (maybe -3 Degrees) compared to pure water that freezes at 0. On a side note, we had a competition in chemistry class to see who could get water the coldest with salt and ice. The coldest we got was close to -20C. (And a guy held his finger in there for one minute for a $5 bet, and had to be treated for frostbite...)
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