Which of the following will lower the melting point of a sample of water? A) Dissolving table salt in the water, B) Cooling the water, C) Decreasing the pressure acting on the water, D) Stirring the water, or E) Mixing gold powder into the water.
Let’s have a quick recap of what melting point means. Melting point is a property of a substance. A melting point is simply the temperature where the substance will turn from a solid to a liquid. The melting point of pure water is zero degrees Celsius. What this means is that if we take pure water, we understand that at below zero degrees Celsius, it’ll be a solid. And when we go above zero degrees Celsius, it’ll transform into a liquid. What this means is when we look at a sample of liquid water, we can still talk about its melting point because we’re talking about the melting point of the water not the sample. So for a liquid sample, we can think of the melting point as the temperature where if we turned it into a solid, it would turn back into a liquid.
Now, let’s have a look at the five options. If we dissolve table salt in water, we’ll produce a solution. Table salt is sodium chloride with the symbol NaCl. When we take sodium chloride in its solid form and add it to water, it will quickly dissolve, producing sodium ions and chloride ions. Aqueous solutions, solutions where water is the solvent, have lower melting points and higher boiling points than pure water. And the more table salt we add to the water, the lower the melting point will be.
So what this means is that if we take, for instance, sea water or salt water and try and cool it down, it will get to a lower temperature before it turns into ice. Part of the reason for this is that the individual ions interfere with water’s ability to stick to itself. So we end up having to extract more energy from it to turn it into a solid. So dissolving table salt in the water will lower the melting point of a sample of water. However, let’s look through the other four options to see what we can discover.
The next proposed action will be cooling the water down. Cooling the water down does get the sample closer to the melting point. But it doesn’t actually change the value. You can think of it like driving faster towards your destination. The faster you drive, the quicker you get there. But it doesn’t actually change the destination. So cooling the water won’t lower the melting point.
The third option is decreasing the pressure acting on the water. When we have a sample of water at normal room conditions, there will be lots of air particles bombarding the surface, keeping it in the liquid form. If we decrease the pressure, it’s going to be easier for the liquid to turn into a gas. So it will affect the boiling point. But broadly speaking, we’re not going to impact the melting point. That isn’t to say that changing the pressure on a solid or a liquid is not going to impact the melting point to some degree under certain conditions. But generally, we’re talking about very extreme conditions. So for a sample of water at room temperature and pressure, decreasing the pressure is not going to affect the melting point.
The next option is stirring the water. Stirring the water may interfere with the water’s ability to form the solid in the first place. So it will slow freezing down. But we’ll still have the same melting point. The temperature required to turn from a liquid to a solid or back will be the same.
The last option, mixing gold powder into the water, will have no effect either. Gold does not dissolve in water. So it will simply fall to the bottom. Although it’ll look very pretty. Since the gold doesn’t dissolve, we don’t form a solution. So we don’t get a reduction in the melting point. This gives us one answer, which is: dissolving table salt in the water will lower the melting point of a sample of water.