Skip to content
Who's in the Video

Dr. Kate Biberdorf

Dr. Kate Biberdorf is a scientist, a science entertainer, and a professor at the University of Texas. Through her theatrical and hands-on approach to teaching, Dr. Biberdorf is breaking down[…]
  • University of Texas professor and science entertainer Kate the Chemist joined Big Think to talk about water molecules and to answer two interesting and important questions: Why does boiling water make it safe to drink, and what happens to water when you boil or freeze it?
  • According to Kate, when water is heated to a certain temperature (100°C/ 212°F) the hydrogen bonds break and it goes from a liquid to a gas state. Boiling for a minimum of 5 minutes kills any viruses and bacteria that were in the water.
  • “Water is a freak and so it is one of my favorite molecules ever,” Kate says. “It has these unique properties and we are surrounded by it constantly. We also are made of water. We have to drink water to survive…It’s a really, really fun molecule to investigate.”

DR. KATE BIBERDORF: When you boil water what you are doing is making sure that the temperature of your water is so hot, basically as hot as it possibly can be before all the hydrogen bonds break and the water itself goes from the liquid state to the gas state. And so when we're looking at water that happens at 100 degrees Celsius which is around 212 degrees Fahrenheit. When we do that we bring the thermal energy of the water up to a high temperature that any viruses or bacteria that are inside that water actually are killed if and only if you boil that water for at least five minutes. So the recommendation is five minutes. When I am boiling my water because we had an issue here in Austin about a year ago where we had to make sure we had very safe water. We had to protect it. When I boiled my water for that I boiled it for ten minutes just because I'm a little bit of a freak or I like to overkill things. But what you want to do is you're basically making sure that there's enough thermal energy inside that water that it makes sure that the viruses and the bacteria that are currently in there can no longer survive. And so it's about getting the temperature hot enough and keeping it there for a long enough period of time to make sure all those viruses and bacteria are killed.

Water is a freak and so it is one of my favorite molecules ever because it has these unique properties and we are surrounded by it constantly. We also are made of water. We have to drink water to survive. Some of us like to swim so we're always inside of water. So it's a really, really fun molecule to investigate. What's cool about water is it has these things called hydrogen bonds. And so what that means is it forms an intermolecular force between one water molecule and another water molecule. So the oxygen on one water molecule is partially negatively charged and so that oxygen is somewhat attracted to the partially positively charged hydrogens on another water molecule. So oxygen from this one, hydrogen from this one are attracted to each other. And so that certain thing is called a hydrogen bond and the distance or the length of the hydrogen bond or the distance between the molecules is what sets how much space those water molecules together occupy.
So a glass of water contains a ton of different water molecules and they all have different hydrogen bonds between them. So when you take a chunk of water like and put it in an ice cube tray and then you put it into your freezer we're going to see that the water actually expands. So water is super weird. This is not a normal thing but when it goes from the liquid state to the solid state the distance and the length of those hydrogen bonds actually increases. So water is actually more stable in the liquid state which his super rare. That's just uncommon. But it is what it is. That's what water does and we're around it all the time so there's your answer.