Heating up water versus water with an iron brick

[weirdoguy]

Hi guys, girls, and non-binaries. I'm a private high school teacher and I'm preparing myself to prepare others for polish physics olimpiad. So I delved into books which have a lot of tricky questions and from time to time I stumble upon one I have trouble with. And that's exactly what happened today. And since I lack self-confidence today, I needed to ask you all

I know it's simple, because it's not even high school level, it's from middle school task book, but I really don't see why the answer is the way it is.

So, here we go:

Two identical burners heat beakers, the first of which contains 400g of water, the second 200g of water and a 200g of cast iron. In which of these beakers will water reach the boiling point first?

There is also a drawing which I will attach but I don't think it's important. So, the answer is that it will reach the boiling point in both beakers simultaneously. And I don't see why. Specific heat of iron is almost 10 times lower than that of water so it needs less energy to heat up to 100 degrees. So overall I would say that beaker with water and iron will need less energy and (assuming both burners have the same power) it will reach the boiling point faster. What in the world am I missing?

 

[DaveE]

Their answer is wrong. You're not missing anything.

 

[weirdoguy]

Thank you!

 

[gmax137]

Are you sure you read/translated the question correctly? If the first beaker contains 400 ml water, and the second contains 200 ml water plus 200 ml iron, then the times will be close. The volumetric heat capacity of iron and water are similar.

 

[weirdoguy]

Are you sure you read/translated the question correctly?

Yes, you can see grams (g) in the photo. But good to know about the volumetric heat capacity.

 

[jbriggs444]

One can also see that the volume in the beaker on the right is about half the volume in the beaker on the left, just as it should be if grams are meant.

 

[Mister T]

What in the world am I missing?

A few years ago I taught an MCAT physics prep class. This is the test you take in the USA to gain entrance into medical school, and it contains a section on physics. I was astonished at the poorly-worded questions and wrong answers in the book we used that claimed to contain actual MCAT test questions.

I taught the class three years in a row. After learning my lesson from the first year of teaching it, I avoided those confusing questions the next two years. It was the best paying teaching gig I ever had. Our federal tax dollars at work!

 

[weirdoguy]

I was astonished at the poorly-worded questions and wrong answers in the book we used that claimed to contain actual MCAT test questions.

I know how task books look like in general, I've been teaching physics for 12 years (I started when I was in high school) and I've been through a lot of them and usually I'm quite certain that the wrong answer is wrong. But there are areas where I don't feel that much confident. And those pesky, tricky little questions from thermodynamics are part of those areas. Since I also stay in touch with more advanced parts of physics (QFT, mathematical physics) and devote a lot of my time to them, I have less time to get through all of my high school books.