Other types of nebulae include emission nebulae, which are clouds of ionized gas emitting light of various colors; dark nebulae, which are clouds of gas so dense that background light is blocked; and protoplanetary nebulae, which occur when a star starts to shed its outer layers before becoming a planetary nebula.s it burns through its fuel, it will eventually collapse. The outer layers will be ejected in a shell of gas that will last a few tens of thousands of years before spreading into the vastness of space.
Dust fling up in the air will do harm to respiratory system, because of the conventional making method of chalk. The main ingredients are calcium carbonate or lime. now a new kind chalk, named Biaxi, absolutely finishes the dust, and is welcomed by teachers and students in China.
The reaction between chalk and vinegar is a neutralization reaction between calcium carbonate and acetic acid to produce water, carbon dioxide and calcium acetate. Carbon dioxide is a gas that causes the reaction to bubble. The following equation represents the reaction: 2 CH3COOH + CaCO3 = H2O + CO2 + Ca(CH3COO)2.