Can you answer this science question?

Question

Rita is waiting at a cafe for her taxi. She has ordered a turkish coffee and needs to drink it quickly. However the coffee is too hot. Should she add a little cold water and allow the coffee to cool or should she wait for the coffee to cool and then add add a little cold water?

Can you briefly or greatly explain how to

PLAN THE INVESTIGATON
Obtain evidence
analyse and consider evidence

thanks

Answer 1

Jaskaran S is correct; leave it to cool as long as possible, then add cold water.

This may be complicated if the cold water is also left standing - then the difference between coffee temp. and ambient must be greater than the difference between cold water and ambient, for this to be true. (And ignoring differences between the coffee cups & the water container in losing heat!)

PLAN Required to compare two situations - waiting & pouring and pouring & waiting. Need to measure temperatures between 0 to 100C to accuracy of 0.5C or better. Time measurement up to 30 or 60 minutes to accuracy of 1 second (though not strictly necessary if side-by-side comparison is made, time measurements will make the experiment more repeatable).

EVIDENCE Take 2 or more pairs of identical coffee cups, pour (stirred) coffee into each, add cold water to half the cups and stir; measure temperatures of all. Wait 30 minutes and add cold water to remaining cups and stir; measure temperatures of all.

ANALYSE Take average of final temperatures and compare. Check whether values show a definite trend or are too scattered. If no conclusion can be drawn, consider whether there is no difference or repeat the experiment for longer time period and/or more accurate temp. measurements.

With further intermediate temperature & time readings, graphs of temperature against time may be useful (- look for a relationship between time and temperature to the power of 4).

Answer 2

Which should she do? Well, is she more bothered about enjoying the coffee, or just getting it drunk? If it's the first she shouldn't add any water - she might not drink it all by the time the taxi arrives, but at least she hasn't spoilt it! Or, if she doesn't care how it tastes, she can cool it right down by adding a lot of cold water right away!

Seriously, though, you need to take into account how long she has to drink the coffee, how hot the coffee is now, how hot the coffee must be before she can drink it, and the temperature of the surroundings.

If she's pushed for time, and putting in some cold water will cool the coffee sufficiently for her to drink it she should do that straight away and not waste any time. Otherwise, you have to determine whether it's quicker to put the water in and wait, or wait and put the water in.

It's true that a hotter object will radiate faster to the surroundings, but also that an object will lose more heat to a cold object than to a hot one. So putting in the cold water *may* cool it faster - it will depend on the temperatures of coffee, water, and surrounding air.

But you have to determine which experimentally. So, decide the starting and finishing temperatures of your coffee. Also, decide the time at which the taxi will arrive and she has to leave - if neither are drinkable she needs to find another solution! Make it the same volume each time, and start at the same temperature. And, adding the same volume of water at the same temperature each time, time how long it takes to reach drinking temperature.

Answer 3

She should allow the coffee to cool , then add a little cold water
Reason: acc. to Newton's law of cooling: rate of cooling is proportional to difference in temperature of coffee and the surrounding.
initially difference is large(the coffee is hot)
so let it cool initially at a faster rate then add a little cold water when the difference decreases

Answer 4

she can add a little cold water and allow the coffee to cool

Answer 5

Rita should stir the coffee, stirring allows the coffee cool.

Answer 6

She should ask for it in a take-away cup with a lid, so she can drink it on the way. LOL!