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Heating up agar? Just add a cup of water and avoid the glitter and crumbs

Posted in: Basic Lab Skills and Know-how
Heating up agar? Just add a cup of water and avoid the glitter and crumbs

It’s ironic how much folklore and superstition comes with being in science. “That’s a lucky pipette”, “playing Bach for your cells will help them grow”, “always make your own solutions”; we all have our own tips. Some of them might be well-founded others not so much…

Tips from trusted colleagues can be very helpful though. Whether it’s understanding PCR , picking the best strain of E. coli, or what to do when your gal promoter is letting you down, BitesizeBio has some wonderful tips and so too do our readers.

One such tip we received was from Claus Jensen and he also had a question for us:

Hi A technician once told me to put in an additional cup of water when boiling LB agar in the microwave oven to avoid “explosive” boiling… and it works (strange???)”

Like glitter and crumbs, agar can make a huge mess. Previously, Vicki covered how to heat your agar without it costing a microwave and beaker after a reader requested a step-by-step guide to heating up agar without it coating the microwave and this tip of adding a cup of water adds another great tip to this guide.

Now, a lot of you may use water when microwaving leftover pizza (ta-dah! No soggy crust issues!), but how does it work to prevent your agar from exploding?

Let’s talk about Claus’ tip and how it works.


If you’ve ever eaten microwaved food, you’ve probably noticed that sometimes part of the food is boiling hot and the other part is stone cold. This is because of a lack of uniformity in the absorption of the microwaves. To help avoid this, microwaves have a rotating glass plate. By rotating the sample, you increase the probability that the dinner you’re trying to heat is hit by and absorbs the microwaves. The walls of the microwave do not absorb the microwaves but reflect them until they’re absorbed by your food. If you turn your microwave onto a high setting, you addi more microwaves and heat your dinner more quickly. Unfortunately, some parts of the sample absorb the microwaves better, resulting in a half hot/half cold lasagna.

Making your agar solution

You add your agar to pure water, mix it up and then pop it into the microwave with the lid screwed on loosely. You watch it carefully to see if it bubbles but it doesn’t seem to be doing so. It has been a few minutes but still nothing. You open the microwave to stir your solution and BAM! molten agar goes everywhere. This can also happen while your solution is still in the microwave and both scenarios are extremely dangerous.

Why does this happen?


Superheating occurs when a liquid is heated above its boiling point without boiling. This sounds like an oxymoron but lets take a closer look at some definitions.

Boiling: Water is said to be boiling when you see bubbles rising to the surface of the liquid. For this to happen, the temperature must be sufficiently high that the vapor pressure exceeds the ambient pressure (the steam would win an arm wrestling match against atmospheric pressure).

Superheating: This is the exception to the rule of when a solution is technically boiling. Even though the vapor pressure has exceeded the ambient pressure, the bubbles are not rising to the surface. This is due to the surface tension of the water, which suppresses the growth of bubbles. For the bubbles to counteract this and grow, the water must be superheated, or raised above its boiling point.

Superheating can lead to explosions because once the bubbles begin to grow, the pressure due to surface tension reduces, and the bubbles grow so quickly and abruptly that the liquid and even the container are suddenly under a large amount of pressure; it has to go somewhere – resulting in an explosion.

It is so forceful that heating one liter of water just 1°C above 100°C produces a highly unstable superheated state that can produce about 3 liters of steam that force the liquid to boil rapidly and even push it out of the container. Glass vessels may also shatter.

Think of it like blowing up a balloon. At first it’s very hard to get the balloon to inflate, but once it expands a little, it’s a lot easier. Now imagine if you continued to blow into the balloon with the same amount of force as you did to get it started. POP! An exploded balloon.

So why does Claus’ tip work?

Imagine standing in a room full of extremely bouncy tennis balls bouncing off the walls. The more people in the room, the less likely you are to be hit. If it’s just you, balls will bounce off the walls until they eventually hit you. It’s the same with a microwave. By having a second vessel of water (called a “dummy load”), your sample absorbs less waves and so it boils more slowly.

Microwaving at home

Before you decide to throw out this handy kitchen appliance, please read on!

It is very unlikely that superheating will happen at home.

At home, most containers have scratches or other imperfections which trap pockets of air that provide starting bubbles. This may not be to case with labware, so there is a tendency for solutions to end up everywhere but where you want them.

Another reason for superheating to be more likely to happen in a lab is because the water used to make agar solution is extremely pure and has a tendency to superheat. Tap water is full of impurities that make it less likely to superheat and explode. With microwaved distilled or deionized water however, a small disturbance and the water may explode (as demonstrated on Myth Busters).

Avoiding the burst

In addition to pure water and perfect glass-ware, other factors that contribute to superheating include:

  • using glazed containers
  • extended periods of heating
  • adding substances (like agar)
  • capping or sealing the container (loosening the lids of containers might not be enough to relieve the pressure buildup – microwave ovens heat materials so rapidly that the lid can seat upwards against the threads allowing no more room for expansion and leading to an explosion).

But the simplest solutions to avoiding sudden boiling are turning down the power of your microwave by choosing a different setting and using Claus’ tip: add a vessel of water to absorb excess waves.

So there is your answer! By adding a glass of water, you give the microwaves something else to hit so less of them hit your agar.

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