The Disappearing Salt
A student adds 5 g of salt to 100 mL of water at 20 °C and stirs. The salt disappears and the water looks clear. The student says, "The salt has vanished - it no longer exists." A classmate says, "It is still there, just dissolved." To test this, they weigh the beaker and water before adding salt (145 g), then weigh it again after the salt dissolves and the outside is dry (150 g). Next they leave the beaker on a warm windowsill for a week. Most of the water evaporates, and white crystals form on the sides and bottom. The student tastes a tiny crystal (with teacher permission) and finds it salty.
1. Use the masses to decide whether the salt still exists after dissolving.
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The masses show the salt still exists. Before adding salt the beaker and water weighed 145 g. After adding 5 g of salt and stirring until it dissolved, the total was 150 g. If the salt had vanished, the mass would have dropped back toward 145 g, but it did not. Dissolving spreads the salt particles among the water particles so they are too small to see, but the particles are still in the beaker. The total mass staying the same is evidence that the salt was not destroyed. The white crystals that form later are further evidence that the salt was still present in the water.
2. Explain what the crystals on the windowsill show about the process.
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The crystals show that dissolved salt can be recovered when the water evaporates. As water leaves the beaker as vapour, the salt particles are left behind. When there is too little water to keep them spread out, they join together into solid salt crystals. The salty taste confirms the crystals are salt, not something new. This is a physical change: the salt dissolved and then came back as salt, with no new substance formed. It is not a chemical reaction, because the salt's identity did not change - it only changed from spread-out particles in water to solid crystals.