LESSON 6

APPLYING THE HEAT

 

Matter expands when it is heated. There are also a variety of other changes that take place when substances are heated. These changes include changes in phase and chemical reaction.

 

Heat causes change.  These changes are then classified into two broad categories:

        Physical change                                 chemical change

 

Physical changes are reversible through physical means and do not involve the formation of new substances.

        Example: a change in phase and dissolving of a substance. 

 

Chemical changes are not readily reversible and do involve the formation of new substances.

                                                                                       

Problems with Chemical and Physical Changes:

·        Reversibility of phases of matter is often cited as the criterion for determining that a physical change has occurred.

·        Sometimes a process called sublimation occurs, a physical change in which a substance goes directly from the solid to the gas phase and then back to the solid phase. 

·        Carbon dioxide is a common example of a chemical compound that sublimes at atmospheric pressure—a block of solid CO2 (dry ice) at room temperature and at atmospheric pressure will turn into gas without becoming a liquid.

·       

·        Snow and other water ices also sublime, although more slowly, at below-freezing temperatures. This phenomenon, used in freeze drying, allows wet cloth to be hung outdoors in freezing weather and retrieved later in a dry state (although strong sunshine is required to drive the process.)

·        Snow Picture

·        However, ammonium chloride does not sublimate.  It separates into ammonia and hydrogen chloride – which then re-associates farther up the tube – in a chemical reaction.  This process occurs due to low temperatures.

·        Copper (II) sulfate, or CuSO4,  is heated, observations suggest that this process is a chemical reaction that yields two products, a white powder, anhydrous copper (II) sulfate, or CuSO4, and a clear liquid, water.  In fact, the copper sulfate remains intact during heating.  The water released is water of hydration, which is water incorporated into the copper sulfate crystals.

 

Phase Change and Chemical Reactions:

A chemical reaction is a change in which new substances are formed.

Chemicals that you will be working with are:

 

Potassium permanganate                            KMnO4

Ammonium chloride                                    NH4Cl

Copper  (ll) carbonate                                        CuCO3

Copper sulfate                                           CuSO4

Sodium chloride                                         NaCl

Zinc oxide                                                 ZnO

Sulfur                                                       S

 

Remember these facts:

1.    Matt is conserved in all chemical reactions.

2.   Invisible products in the form of gases are often produced in chemical reactions.

3.   The production of gases is not always a phase change during a chemical reaction. Sometimes carbon dioxide, or CO2,  is released as thermal decomposition, heat causing vapors.

4.   Changing phases is not a chemical reaction.

5.   Melting is a physical change.