![]() In a similar way, stalagmites begin to grow where the drips from the ceiling hit the cave floor. Over time, fingers of limestone grow down from the ceiling. Another drop can now run down the lump and it too leaves behind a bit of limestone. This is the exact reverse of the process by which the limestone was dissolved initially by the rain water (carbonic acid). As the drop hangs there, water and carbon dioxide gas slowly evaporate from it leaving a small lump of limestone behind. Stalagmites and stalactitesĭrops of water on the ceiling of a limestone cave contain dissolved limestone rock. The formation of stalactites demonstrates that the process of dissolving limestone is reversible. The action of this acid causes the limestone itself to dissolve slightly, releasing minerals into the water and causing a number geological features typical of limestone country such as caves, stalactites and stalagmites. Rain water is acidic because of the carbon dioxide gas in the atmosphere (yes, that again!) which dissolves slightly in the water to make a weak acid called carbonic acid. Like eggshell then, limestone is attacked by acids and the role of acid is played in this case by rain water. ![]() Compressed into rocks over millions of years and raised upwards, they became the familiar limestone landscapes of areas like the Yorkshire Dales and Cheddar Gorge. ![]() ![]() In fact, limestone rocks began their lives as the shells of marine organisms which accumulated to huge depths at the bottom of prehistoric oceans. In limestone regions, the rocks are made out of a form of calcium carbonate (mainly a mineral known as calcite), which is similar to egg shell. Eggshells, caves, kettle scale and acid rain You can find out more about acids here.īack to the top. This dissolves the egg shell more quickly, more completely and with less smell. You can make a concentrated solution of it in water that is stronger than the acetic acid in vinegar. This is the purified, crystalline acid from citrus fruits, such as oranges, lemons and limes. Each molecule of carbon dioxide has one carbon atom attached to two oxygen atoms, which is why it is sometimes referred to using the molecular formula, CO 2 (pronounced "see oh two").Īnother acid you can find in a supermarket is citric acid (chemists and home-brew suppliers also sell it). Vinegar is an acid (it's a dilute solution of acetic acid) and it converts the carbonate in the egg's shell to molecules of carbon dioxide gas. But where has this carbon dioxide come from? The egg shell itself the hard shell is made of a material called a carbonate, which contains both carbon and oxygen atoms.Ĭarbonates can be converted to carbon dioxide when they come into contact with materials known as acids. We breathe out carbon dioxide and the same gas is produced by burning fossil fuels, contributing to the greenhouse effect. What you can see are bubbles of the gas carbon dioxide. Time for some more work for you: how do you know that a gas is lighter than water? Overall, this makes the egg lighter and in fact less dense than water so that a stale egg floats. Anyway, as the egg gets older it ends up with more gas inside and less water, and the gas is lighter than the water. Many food products (vegetable, fruit, animal, etc.) give off gases when they go off, and they often smell! Eggs have such a distinctive, unpleasant smell when they go off that we talk about something "smelling like rotten eggs" (special precautions have to be taken at domestic waste tips to deal with the gases produced as rotting sets in). Can you think why a stale egg would be less dense than a fresh egg? Floating and sinking eggs the science bitĪs an egg ages, the water inside the shell slowly evaporates and is replaced by air, or by any gases that are produced as the egg rots (phew!). If a stale egg floats in pure water it must be less dense than the water itself, whilst a fresh egg that sinks must be more dense than the water. When a stale egg floats, it does so because the water is holding it up. Alternatively, stuff heavier than water is described as more dense than water and it sinks (for example, a coin). Such stuff is described as being less dense than water and it floats (for example, a piece of cork). Water can only hold up things if they are lighter than the same amount of water water can only hold up itself or any stuff lighter than itself.
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