why does the dry ice form clouds of white vapor, especially when you blow on it? what is the vapor composed of?
The vapor is water vapor, condensed, as a fog.
When dry ice is exposed to air, it undergoes a process called sublimation, where it changes directly from a solid to a gas without becoming a liquid first. Dry ice is composed of solid carbon dioxide (CO2), so as it sublimes, it releases cold CO2 gas.
The reason you see clouds of white vapor, especially when you blow on dry ice, is due to the rapid temperature difference between the dry ice and the surrounding air. When you blow on it, you're adding moisture from your breath which contains water vapor. The cold CO2 gas from the sublimating dry ice causes the water vapor in the air to rapidly condense, forming tiny water droplets or ice crystals that create a fog-like effect.
In this case, the vapor is primarily composed of water droplets or ice crystals suspended in the air. The temperature difference causes the water vapor to quickly condense and become visible, forming the white clouds that you observe.