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As you say, physical changes are changes in phase of matter while chemical changes fundamentally change the substance. Given that definition, the right answers are:
A. Physical -- it's going from solid to liquid but could go back again.
B. Physical -- same idea as above
C. Chemical -- clearly, your digested food is never going to turn back into what it was that you ate.
D. Chemical -- can't make it back into flour, butter, etc.
E. Chemical -- same as C and D
F. Physical -- you can evaporate the solvent and get the sugar back
G. Chemical -- same as the others.
The definitions you provide should be able to guide you very well in this exploration. If a physical change is when there is a phase change, ask yourself what helps to facilitate a phase change. For example, if you heated an ice cube, you would be adding heat energy to the substance causing it to change from a solid to a liquid. Continue to add heat and it becomes a gas. In all three phases, the substance is still the same and this is why a physical change is present. As you correctly identify, phase changes are physical changes. Ask yourself this in looking at your examples. Which ones involve the changing of phases through heat energy and do not actually change the chemical composition of a substance? In doing this, you might be able to answer some of these on your own just by using the analysis you provide.
a. Melting butter: In this the physical property of the butter changes. That is the butter changes from solid to liquid state. This is a physical change.
b. Boiling of water: In this temperature of water changes. Simultaneously, some of the water changes from liquid state to gaseous (steam or water vapour). Both these changes are physical changes.
c. Digesting food: Digesting of food is a very complex process including changes in chemical composition of food, dissolving of nutrients in water and other fluids, and absorption of food by blood. Thus digesting of food involves combination of physical as well as chemical change.
d. Baking brownie: This process, like digesting of food is a complex process involving physical as well as chemical change. For example, some of the water in the dough gets evaporated. This is a physical process. Also some of the ingredients in the dough change their chemical composition. This is a chemical change.
e. Exploding TNT: In this process, what happens to TNT is a pure chemical change. But the accompanying impact of explosion on surroundings is a physical change.
f. Dissolving sugar: This is a purely physical change.Pleas note that dissolving of sugar is not same as converting sugar from solid to liquid state. Dissolving is a different type of physical change.
g. Burning fuel oil: This is purely a chemical change. This is primarily a result of chemical process called oxidation.
The chemical change pertains to the structural change in the molecules.The number of atoms that combined before and after is different. The substances as reactants and products are different in achemical change. A physical change is cheracterised by a physically different form for look in which molecules does not undergo any internal structural change but only a change in their relative distances.The physical change is mostly reversible but not the chemical change. With this in mind,the classification could be done:
Chemical change: (g)
Partly physical and chemical:(c)digesting (d) baking brownies (e): In all these making substances into small pieces is a physical change.Process of addtion of chemicals which converts into other substances is chemical. Act of just explosion is physical or adiabatic change, but before that there is chemical chemical change which generates an enormous pressures.
Physical changes: melting butter,boiling water,dissolving sugar
Chemical changes: digesting food,baking brownies,exploding TNT, burning fuel
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