I just bought a microwave from Goodwill, but I don’t know if it works the way it is suppose to or not…
I put 16 oz of water in a glass measuring cup, how long should a microwave take to boil it?
For some strange reason, water doesn’t seem to “boil” in a microwave. Oh, it will reach 212, or 100 C, but the familiar boiling action doesn’t appear.
Typically, in a newer 1000W microwave, a cup of water should reach boiling temperatures in probably 1:00 to 2:00 minutes.
It will be hot, but it won’t bubble, burp, and spit like you’d expect. I have no idea why.
could be about 8 minutes if older lower power. depends on its power (W) and setting (high, medium, defrost) too.
16 oz will take more than a minute or two. 3-5 would be more likely for the standard larger 1000 watt machine. Add a couple more for a 600 watt smaller model.
bubbles form at a nucleation site, a sharp edge or item (spoon), this often occurs in a metal pan used on a stove top. in a glass container used in microwave this might not occur. this is a potential hazard because the water can be at the boiling point and the violent bubbling starts when you move the container or put a spoon or something into it.
Be sure you don’t use a perfectly smooth glass. If there’s no where for bubbles to form it won’t ever boil, but will continue heating, leading to a steam explosion.
As others have said, about 1-2 minutes is enough for heating room-temperature water to boiling. If it’s like 15+ years old, it might be more like 2:30-3:30 minutes for that much water.
I don’t know the wattage of the microwave, since I bought it at goodwill, I don’t have any sort of manal, and don’t know the model… But it was $6 so I couldn’t resist trying it. I have a few days to return it so I to know If it works or not and I need to return it…
For the record, it does have the manufacture date, it was made in 1987.
I know this is an issue, so when I put the water in it and didn’t see it boil, I put a knife in while in the microwave just to make sure. After 2 minutes at full power, it didn’t look like it was boiling, before or after the knife. The glass was hot to the touch though.
So, how do I tell if it works correctly?
Well, if it gets stuff hot, then it works, doesn’t it?
ETA - I bet it looks just like the one we got in like 1986 or so. They were still new, so my dad got up early one day when we had just gotten it and put a sausage biscuit in for fifteen minutes. The smell is burned onto my brain.
16 ounces of hot water after 2 minutes is a working microwave.
If it was made in 1987, you’ve probably got a 500-800 watt model. Most instructions on frozen foods are designed for like 900-1200 watt microwaves, so when you’re Hungry-Man dinner says 3:30-5 minutes, you’ll probably wanna go 5:30-6:30 minutes.
Ok, I just looked at the stickers on the back of the microwave. It says it has an output of 500W (1000ml Water Equivalent). Now… Since this is a used microwave, the real question I have, is if it is working correctly, as it is suppose to. So, since it was built in 1987, and has an output of 500W (1000ml Water Equivalent), how long should it take to boil approx 16oz of water? (since I have a measuring cup, the amount is variable if need be, however I don’t have a thermometer).
I don’t see it taking any more than 10 minutes to microwave that amount of water, and probably closer to 5-7 for a lower wattage model. My last microwave was manufactured in '86 and I knew it had given out once it seemed incapable of boiling water anymore.
IMHO, if it’s heating stuff it’s still working. That’s all a microwave is supposed to do. If it turns water from 75 degrees Fahrenheit to 180 degrees fahrenheit, be it 2 minutes or 8 minutes, it’s definitely working. You just need to know the amount of heat your applying to a food over a period of time.
If I were strapped for cash, and my microwave took 11 minutes to heat a TV dinner, I’d still consider it working. As far as efficiency… I’ll leave that to the mathematicians and their formulas. 
It’s just microwaves, not like you’re actually “nuking” your food and up’ping your chance for cancer from overexposure from using a microwave too long. Some work quicker (use more power), but they’ll all get the job done as long as you’re food isn’t the same temperature as when you put it in.
The specific heat capacity of water is 4.186 joules per g for each kelvin raised.
So you want to go from 25 [sup]o[/sup]C to 100[sup]o[/sup]C, which is 75 Kelvin rise in temperature.
For 16 oz of water (16 ounces = 453.6 grams) = 142 000 J = 142 kJ
1 Watt = 1 joule per second
So, 500 W = 30 kJ/minute
Which makes 142/30 = 4.7 minutes (4 minutes and 44 seconds)
The water will be at 100[sup]o[/sup]C but may not be boiling if there are no nucleation points to facilitate formation of bubbles.
Hope this is not a school question. If it is my answer may be wrong.
The numbers/formulas were enough to blow us passer-by’ers away.
(Hence my reason to pass the buck to the mathematicians in my earlier post.
Fill the oven with slices of buttered bread laid flat, with a cheese slice on top of each. Then turn on the oven, a minute or two at a time. You will quickly see if it works or not, and how quickly, and will also notice if there are hot or cold spots in the oven.
This was exactly the type of answer I was looking for. I just tried it out in the microwave, at 4min 50sec (slight margin of error for the water temp, and not exactly 16oz), and it didn’t boil… My curiosity made me do 2 minutes more, then 2 more minutes. The water started to boil at 8min 20sec (of actual microwave, not including the time to open and stick a knife in to see if it boiled). So, that means the microwave isn’t working as it should, so I will go ahead and return it.
And no, not a school question, I’m in pharmacy school, so the physics stuff is long gone. I guess that’s one reason I’m good at pharmacy, the ability to understand, and know why, not necessarily the ability to figure out when it comes to stuff like this.
Thanks!
that calculation is if all the microwaves get absorbed by the water.
the microwave doesn’t apply all the energy to the water effectively. the warm water vaporizes and fills the oven cavity, this transfers some heat to the metal oven walls. so heat has to be supplied to the water in the container until that water reaches the boiling point. so accounting for this energy flow in the microwave then 8 minutes sounds fine for a 500W unit (i recall having that size and that is what it took).
I agree with johnpost, there is no need to return it. While antechinus gave a nice highly ideal analysis, he assumed that all the power of the microwave went into the water (unlikely) and neglected all the heat that will inevitably be lost to the vessel walls, evaporation, and convection at the water’s surface.
For a 500W microwave ~8 mins for reach boiling seems pretty reasonable to me. My 1000W microwave at home takes about 2 minutes to heat 8 oz of tap water to boiling. Popcorn and other prepackaged items all heat within the times provided in the instructions so I assume it is working correctly.