Instant coffee, or hot chocolate, doesn't come in big chunks, like the picture on the left. When you spoon it out of the jar, it's in tiny little grains. The reason should be obvious ... it will dissolve faster! A good question we might ask is why it dissolves faster if the coffee is in small grains, as in the picture on the right. Why should many little pieces dissolve faster in hot water than one big piece? The amount of coffee is exactly the same, isn't it? Keep reading, and we'll explain. First, it is obviously correct that the amount of coffee is the same if you take a big chunk and grind it up into little pieces. For simplicity, we'll use a cube of coffee that's one centimeter long on each side.
Since there are 1000 of these small cubes in the big cube, the total volume of all the 1 mm^{3} pieces will be 1000 mm^{3} ... the same total volume as the original big chunk. But that's not surprising, after all. What is surprising is what happens to the surface area of the big chunk when you split it into 1000 little pieces. Here's the big chunk again. Each of its faces is a square, 10 mm by 10 mm. The area of each square is 10 x 10 = 100 mm^{2}. There are six faces on the cube. So the total surface area of the big cube is 600 mm^{2}. Now let's look at a little cube. Each of its faces is a square, 1 mm by 1 mm. The area of each square is 1 x 1 = 1 mm^{2}. There are six faces on the cube. So the total surface area of one small cube is 6 mm^{2}. But there are 1000 of these little cubes altogether. So the total surface area of all the small cubes is 6000 mm^{2}. Here are the results summarized: pieces, the surface area becomes ten times larger! So how does this help the coffee dissolve faster? Well, in order for a solid like coffee to dissolve in a liquid, the water molecules must be able to get at the molecules of the coffee. The water can only get at the coffee from the outside. The surface area that's exposed to water is where it will start to dissolve. The large chunk of coffee has 600 mm^{2} of surface area for the water to get at, and dissolve. The powdered coffee, however, has a suface area of 6000 mm^{2} ... this is ten times as much area for the water to get at! So the water can dissolve it faster! What we've illustrated is the reason why solids dissolve faster when broken down into little pieces. Breaking them down increases their surface area. The finer the pieces, the more area is exposed. This is true for all solids in all liquids. It's also true for liquids that need to be absorbed. For example, inhalant sprays used by people with asthma squirt a fine mist of liquid particles into their lungs. The tiny particles of liquid in the spray are more quickly dissolved in the lungs because they have so much more surface area exposed.
For a similar reason, granaries are at risk of explosion from grain dust. Tiny particles of grain dust expose a very large surface area to the oxygen in the air, and can burn very easily if a spark occurs. Because the dust and oxygen are well mixed, the grain dust burns explosively fast ... just like the gasoline particles in an engine. This can result in an explosion and fire. The same principle is also involved when you cool a drink faster by putting crushed ice into it, instead of large ice cubes. Find out more here. |