Water is dimagnetic, which means that it exerts a weak magnetic field, and repels other magnetic fields. If a magnet is suspended over water, the water's dimagnetism will repel the magnet. This weakens the magnet's effect on other objects. When salt is added to water, it weakens the water's magnetic field further, so that it ceases to have any significant effect on other magnetic fields. However, salt water conducts electricity better than non-salt water, so magnets placed near it can cause significant turbulence in the water.
Dimagnetism refers to an object's tendency to generate a weak magnetic field in opposition to a magnetic field applied to it. Dimagnetic objects repel magnets. Water is dimagnetic, although it is not as strongly dimagnetic as carbon-graphite. The presence of a strong magnetic field can cause a dimagnetic object to appear to levitate. According to wondermagnet.com, powerful magnets are able to make a frog levitate due to the dimagnetism of the water in its body. There are no magnets strong enough to make water itself levitate, but strong magnets can make a few drops of oil rise to the top of a glass of water.
Salt's Effects on Water
The addition of salt lessens water's dimagnetic properties. In addition, salt raises the freezing point and lowers the boiling point of water. Salt also strengthens the water's ability to conduct electricity. Due to these effects, magnets do not affect salt water the same way that they do regular water.
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Dimagnetism, Levitation & Salt Water
A strong magnet placed near a dimagnetic object can cause the object to levitate; the dimagnetic object repels the magnetic field, causing the object to move in the opposite direction of the external magnet. However, salt water lessens this effect because the salt lessens the water's dimagnetic properties. The addition of salt to water weakens the opposing magnetic field so that the water no longer repels an external magnetic field. Thus, it would be impossible to levitate objects by placing a strong magnet near salt water.
Magnet Neutralization & Salt Water
Water's dimagnetism will neutralize the effect of magnets on objects on or near the water. A magnet suspended or immersed in water will lose some or all of its effect until removed from the water. Salt water has less of an effect on magnets placed near it than regular water because the salt lowers the water's dimagnetism. A magnet placed near or in salt water will continue to attract magnetic objects in the water.
Salt Water & Electromagnets
Salt strengthens water's ability to conduct electricity. When a electromagnet is placed near salt water, it creates a moving magnetic field in the water due to salt water's conductive properties. The salt water then creates an opposing magnetic field. This creates water turbulence.