CHEMICAL ACTION 179 



to stand in air or in sunlight. Such compounds are 

 called unstable. Strong compounds, which do not easily 

 decompose, are said to be stable. Often when two or 

 more compounds are mixed together, they so act as to 

 decompose each other ; the atoms then unite with others 

 for which they have greater affinity, and form new 

 substances. 



205. Heat assists Chemical Action. Heat is a very 

 important aid to chemical action, both composition and 

 decomposition. Many changes which will not take 

 place at ordinary temperatures easily occur if the sub- 

 stances are heated. 



Experiment 111. Into a clean, dry test tube put a little 

 sugar ; heat gently. Notice what occurs, and when the mass 

 becomes solid examine it. Has a chem- 

 ical change occurred ? In a similar way, 

 treat some small bits of wood in a test 

 tube, and examine. Can you discover 

 whether heat has here caused composition 

 or decomposition ? 



Experiment 112. Put a small piece 

 of lead (a BB shot) into a test tube and . FlG - 128 

 add a little cold sulphuric acid (concentrated). Look for any 

 action. Carefully heat the acid and look again for signs of action 

 (Fig. 128). In this case the change includes decomposition and 

 composition. 



Combustion, or burning, is a very common sort of 

 chemical action; and we know that to burn any com- 

 mon substance it must first be heated. Gunpowder and 

 other explosives suffer rapid chemical change when 

 heat is applied ; and in many other sorts of chemical 

 action we find that heat plays an important part. 



