Cold is not a substance, so it can't sink. The air around the dry ice, and the gaseous carbon dioxide being formed will be cold and therefore dense, so they will sink through warmer air.
Dry ice decreases in size, because it is sublimating. This means it is turning from a solid in to a gas. Where as regular ice melts in to a puddle of liquid water, dry ice evaporates in to CO2 gas.
melting and from a solid to gas ,as in dry ice, it is sublimating
The process of dry ice sublimating, or turning directly from a solid to a gas, is a physical change. No new substances are formed, just a change in the physical state of the dry ice from solid to gas.
Cold,dry climate with snow and ice cover
The best way to keep dry ice from sublimating quickly is by packing them as close to each other as possible to avoid empty space. Empty space will cause the dry ice to sublimate faster. Fill in these empty space with new bubble packs, newspaper or Styrofoam peanuts. It is also advisable to purchase dry ice as close to the time of usage as possible. Check link below for more information on dry ice and dry ice makers.
Dry ice should be stored at a temperature of -109.3°F (-78.5°C) to keep it in its solid form. It should be kept in a well-insulated container to prevent it from sublimating too quickly.
With dry ice.
ice...dry ice...
a place where there is nothing but ice
Things freeze in dry ice because dry ice is solid carbon dioxide that is extremely cold at -78.5°C (-109.3°F). When an object comes into contact with dry ice, heat is transferred from the object to the dry ice, causing the object's temperature to drop rapidly and freeze.
The melting of ice is a physical change. The physical change include physical change include sublimating dry ice, boiling water, crushing an aluminum can or breaking a glass bottle.
The dry ice is the solid form of the Carbon Dioxide (CO2) so you can name it: Carbonic anhydride, Carbonic acid gas, Carbon oxide, Carbonic oxide, Carbon(IV) oxide, and also as you called it, Dry ice (solid phase).