Grapes and then... plasma!

You don’t always need a big laboratory for exciting experiments... Science LinX presents fascinating kitchen-table experiments. Do try this at home! But read the disclaimer first.

Although on earth we are surrounded by solids, liquids and gases, the majority of space consists of plasma – a fourth manifestation, or physical state, of matter. For example, the inner core of stars is made of extremely hot plasma, while interstellar clouds are formed by extremely cold plasma.

On earth you can find plasma in lightening flashes, the northern lights, some flatscreens, in burning fluorescent and neon lighting and in nuclear fusion research reactors. And if you want to see a plasma for yourself you can create one in your microwave oven! All you need is a bunch of grapes and a roll of paper towels.

But what exactly is a plasma? To put it simply, a plasma is a gas of which the atoms (or some of them) have collapsed into electrons and ions. Electrons are always negatively charged. The ions in the plasma - atoms minus one or more electrons - are positively charged.

Important notice: Read the disclaimer before you start experimenting. Carrying out scientific experiments at home can be dangerous. You could hurt, seriously injure or even kill yourself or others during an experiment. Property can get damaged, catch fire, explode, become electrically charged, or other unexpected and unwanted things may happen. You are responsible for any consequences. Science LinX, the UK university newspaper, the authors of this article and the University of Groningen cannot be held responsible for the consequences of your experiments, even if you follow the experiment instructions exactly. There is a short film of the experiment on the site if you would like to see how it is done.

Still keen on experimenting? Start by halving the grapes. Dry the cut faces carefully with a paper towel and then cut the halves in two again, only this time ensure that the halves are just held together by the skin. Dry the cut faces thoroughly again and leave them on a paper towel for half an hour.

Now take one of the halved grapes and lay it on a piece of paper towel on a microwave-proof dish and place it in the microwave oven at full power for about 20 seconds.If you did everything right, after a few seconds you should see plasma rays emitting from the grape.

How is that possible? An electric current is created between the two grape quarters in the microwave (potential difference). This causes sparks which cause part of the sugar in the grape to combust. A plasma is then formed from the combustion gases. And how does that work? According to Professor Ronnie Hoekstra of the KVI (Kernfysisch Versnellerinstituut) it’s possible that free electrons in the sparks cause extra electrons to be released from the gas under the influence of the microwaves. Hoekstra does have his reservations, though; this is the first time he’s heard of this grape experiment. ‘Maybe we’ll try it out here at the KVI during a lunch break,’ he remarks.

A couple of extra warnings: Do not try to scale up the experiment. One grape is more than enough. Really. It would appear that the microwave-proof dish and the paper towel absorb part of the microwave power and thus prevent damage to the oven. To prevent overloading, don’t leave the microwave on for much longer than 20 seconds.