This is a bit of a classic endothermic reaction demonstration. Made impressive because of a few different things. First of all, it's two solids coming together to reduce a liquid. So that's impressive by itself. This reaction of two solids that is an endothermic reaction, as an endothermic process. So sometimes this reaction is known as the ice block. Okay, in the demonstration, the endothermic reaction between two solids. We're reacting two solids, barium hydroxide, octahydrate and ammonium chloride. We have sort of a wetted piece of something underneath. So, wetted piece of cardboard, wetted piece of plywood, wet safety mat, something like that. We've just sprayed a little bit of water. I don't need much to pass to prepare the square of water on top of the wood. And the reason why we've become able to do this later. We then have a little plastic cup or a beaker, or we've added one of the powders. We'll place it down in the area of the board that's wet. We're pouring the other powder and then gently mix them with a glass stirring rod. Going to add some ammonium chloride solid. Again, the amount's trying to hand out. But you're going to add that and then you're going to keep stirring, stirring with two solids together. You'll start, it'll look like a bit of white snow starting to form. The beaker will start to feel quite cold. Now again, you're noticing this for everyone else. It just looks like you're mixing two white solids together and it's still a white solid, so it doesn't appear much as happening. And the temperature drop from this reaction will actually freeze the water beneath it and fuse the beaker and whatever you've got underneath it together. And then right at the end after the reactions occurred, you can actually lift the whole thing up and you'll be connected between the two of them. And that shows the temperature drop really, really well. When you try and lift the beaker off the safety mat, you'll see that it's frozen the water underneath and you're actually able to lift up the whole safety mat. And that often really impresses the students, really blown away with it. You could involve the students and having the students do the stirring and then they tell them to lift it up and start to the class and they'll lift the whole thing up and they'll be quite amazed. Why is that happened? Well, the reaction is so endothermic. It's a very endothermic reaction is that the reaction itself has drawn energy from its environment. And it's environment in this case with that liquid water sitting on the wooden block. It's frozen that water and it's essentially, it's the beaker to the wooden block and then you lift the whole thing up. I'm only a gas, I could give it off and it's quite a strong smell. I mean, it just bursts in a meter square radius. So if the students are gathered around, they'll notice that quite distinctly. So it's not that stinky to be honest, I smell worse, but you could do it in a few of them and then you wouldn't necessarily need to worry about the ammonia vapor. In terms of teaching, this is essentially an acid based reaction that we haven't seen a lot of. So a lot of the time when we do acid based reactions, we focus on strong acids, strong bases, usually water as the only byproduct. This is a weak acid with a strong base. We get ammonia gas as one of the products. It's still just a hydrogen transfer. So a fairly easy one to work out, but it's in a pretty different context to what we've seen, usually up to this point in chemistry. You can also use it at junior level science and junior level chemistry when you start introducing it. What is a chemical reaction? Because there's a number of things that are observable from this particular reaction. It could drop all the way down to like negative 20, negative 30. It can be incredibly cold. The one we did for the video earlier about negative 15, but it was, it can get really, really quite cold in a fraction of time. One to two minutes will be and you've got a change of 40, 50 degrees, which is quite impressive.