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Welcome to the latest post from the Science Geek. I must first start with an apology, as when I started this blog last month I intended to post something weekly. However in the last two weeks I have moved house and also been on holiday to Norway, out of the reach of the internet, so I haven’t quite managed it!

In my blog I intend to post something from time to time about the scientific origin of some everyday phrases. The one I have picked for this post is “the acid test”. I hope you enjoy reading it and, as always, let me have any comments.


The term “the acid test”, to test the real worth of something, is widely used. The definition and example given in the Cambridge On-line English dictionary are succinct:

The true test of the value of something. It looks good, but will people buy it? That’s the acid test. 

The origin of this phase is based upon some interesting chemistry.

Nitric Acid

Nitric acid is both a strong acid and a powerful oxidising agent, which means that it will corrode materials such as wood, animal matter and nearly all metals. Indeed, metals such as copper, silver, lead and tin – which are resistant to corrosion and are not attacked by other strong acids – will dissolve in nitric acid.

Nitric Acid


Gold is a noble metal which will never corrode or tarnish when exposed to air or water even for a very long time. It has been a valuable and highly sought-after precious metal for coinage, jewelry, decoration and other arts since the beginning of recorded history.

gold jewellery

Image provided by Marie-Lan Nguyen

Gold can be beaten into a very thin sheet known as gold leaf. In fact a gramme of gold can be beaten into a sheet which is one square metre in area but only 0.00000005 metres thick. Gold leaf has many applications, such as to give woods and metal a golden colour which will not fade with age. Interestingly, gold leaf is edible and is sometimes used to decorate food or drink, typically to promote a perception of luxury and high value.

gold leaf

 A sheet of gold leaf photo courtesy of World Images


Although it is highly prized and has a wide number of uses, a key fact about gold is its rarity. Indeed only 180 000 tonnes of gold have been mined in recorded human history. This is equivalent to a cube 21.0 m on each side. The combination of widespread uses and scarcity means that gold has always been very expensive. The price has been volatile recently and the world gold council website (www.gold.org) gives a figure around $45 000 per kilo. At that price a life-size pure gold human statue would be worth around 70 million dollars!

Gold and Nitric Acid: The Acid Test

As stated above, gold does not tarnish or corrode when exposed to air or water. In addition it is one of only very few metals that are not attacked by nitric acid.  The acid test therefore consists of applying nitric acid to a gold coloured metal and seeing if it dissolves in the acid. If it does the material does not contain gold.