What happens when citric acid is heated at 150 c ?

Start Melting beacause the melting point is 153 c 

At 175 c  start decompese 

Citric acid starts decomposing with the lose of carbondioxide which causes effervences. Water molecule is also lost during the process.

Decomposes by loss of CO2 and H2O.

It will from the anhydride of the acid loosing a molecule of water

Citric acid

Since the melting point of citric acid is C so by heating it atO C, it starts to melt. But by heating the Citric acid above C it decomposes producing carbon dioxide and water and a white ppt. 

It melts and dehydrates to give aconitic acid on heating at 175ºC. Further heating results in the formation of citraconic anhydride.