2DCHEM - PP#3 (Physical & Chemical Changes)

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SNC2DCHEMISTRY

CHEMICAL REACTIONS

L Physical & Chemical Changes

(P.~)

Physical & Chemical Changes

Some of the most useful and powerful properties of matter are thoserelated to how and why matter changes. There are countless changes inmatter that affect us every day: for example, applying heat to an egg,burning gasoline, freezing water, and mixing oil and vinegar, to name afew. Observing, understanding and categorizing kinds of change are animportant first step to making use of change.

February 9, 2013 2DCHEM - Physical & Chemical Changes 1

Physical & Chemical Changes

You can discover a great dealabout matter by observing acandle. The wax of thecandle is undergoing bothphysical and chemicalchanges.

PRACTICE

1. Which changes result innew substances? Whichdo not?


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Observations

RECALL!

An observation is information that you getthrough your senses. You observe that arose is red and has a sweet smell. You mayalso note that it has sharp thorns on itsstem. You may count the petals on theflower and the leaves on the stem andmeasure the length of the stem.


Observations

When people describe the qualities ofobjects and events, the observations arequalitative. The colour of the rose, theodour of the flower, and the sharpness ofthe thorns are all qualitative observations.

QUALITATIVE DATA

� information that you get through yoursenses


Observations

Observations based on measurements orcounting are quantitative since they dealwith the quantities of things. The length ofa rose’s stem, the number of petals, and thenumber of leaves are quantitativeobservations.

QUANTITATIVE DATA

� information based on measurements orcounting


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Observations

NOTE!

The evidence gathered in an experiment isoften described as the ultimate authority inscience. With this in mind you must gatherand communicate your qualitative andquantitative evidence clearly and accurately.Although the kind of observations are oftendictated by the question asked at thebeginning of the investigation, both qualitativeand quantitative evidence is often gathered.Do not make the mistake of forgetting torecord qualitative evidence when you aregathering quantitative evidence.


Physical Change

Changes of state - melting, freezing,evaporation, condensation, sublimation,deposition - are physical changes. Dissolvingis also a physical change. When you dissolvesugar in water, the sugar particles spread out,but they are still there, as sugar particles. Youcan reverse the process by evaporating thewater and collecting the sugar.

NOTE!

Most physical changes are easy to reverse.


Physical Change

PHYSICAL CHANGE

� substance remains the same but changesstate or form

� chemical properties do not change

� generally easy to reverse

� dissolving, melting, …


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Physical Change

PRACTICE

2. Copy and complete the diagram showing the physical changes thatwater undergoes as it changes states.


Chemical Change

Burning, cooking, and rusting are allexamples of chemical changes. Achemical change always involves theformation of a new substance. Forexample, if you have ever mixed vinegarand baking soda you have experienced achemical change. The bubbles that formare carbon dioxide gas – a newsubstance.


Chemical Change

CHEMICAL CHANGE

� produces new substances withdifferent chemical properties

� breaks compounds into atoms

� burning, cooking, rusting, ...


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Chemical Change

NOTE!

While some chemical changes are easy to categorize others are not soeasy. So how to do you distinguish between the two? While there areclues that can help you decide whether a chemical or physical change hasoccurred it is important that you do not come to a conclusion too quickly.All of these clues suggest that a new substance has been produced, butany one of them could also accompany a physical change. You mustconsider several clues in order to determine what type of change has takenplace.


Chemical Change


PRACTICE

3. There are 6 distinctive clues (either alone or together) that indicate achemical change has occurred. What are they?

CHEMICAL CHANGE CLUES

Î a new colour appears

Ï a new odour appears

Ð heat/light is given off/absorbed

Ñ bubbles of gas are formed

Ò a solid material (precipitate) forms

Ó the change is difficult to reverse

Physical & Chemical Changes – DYK?

Many chemical changes, such as those that occur when something burns,cannot be reversed. However, some chemical changes can be reversed.When you recharge a battery, you use electricity to reverse chemicalchanges that took place inside the battery to supply power.

PHYSICAL & CHEMICAL CHANGES

� some are reversible, while others are not


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Exothermic & Endothermic Reactions

The explosion of dynamite, the freezing ofwater, and the burning of wood are allprocesses in which thermal energy (heat) isgiven out to the surroundings. Physical andchemical changes that release heat to thesurroundings are said to be exothermic. Forexample, consider the combustion of gasoline:

gasoline + oxygen þ

carbon dioxide + water + heat



EXOTHERMIC REACTION

� chemical reaction that releases heat(energy) to the surroundings



On the other hand, other physical andchemical changes take place only whenenergy is continuously supplied to thereactants. Reactions that absorb heatfrom the surroundings are said to beendothermic. For example, consider themelting of ice:

ice + heat þ water


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ENDOTHERMIC REACTION

� chemical reaction that absorbs heat(energy) from the surroundings


U Check Your Learning

1. Classify each of the following as a physical or chemical change.

(a) Dry ice, solid carbon dioxide, sublimes at room temperature.

(b) Salt dissolves in water.

(c) Iron rusts in a damp environment.

(d) Gasoline burns in the presence of oxygen.

(e) A dead organism slowly decomposes.


P

P

C

C

C

U Check Your Learning

2. When sodium carbonate is added to water, the sodium carbonatedissolves. When hydrochloric acid is added to the solution, the solutionfizzes. What kinds of changes have occurred? Explain.

dissolves = change of state = physical

fizzes = bubbles of gas are formed = chemical


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COOL SCIENCE – The Mint-Cola Fountain

BACKGROUND

The mint-cola fountain experiment may be oneof the most popular science novelties of alltime. It begins by dropping mints into a bottleof carbonated cola drink. Within a fewseconds, a giant fountain of cola comeswhooshing out of the bottle.

QUESTION

How does it work? (3 reasons)

• carbon dioxide (dissolved in the cola)

• surface tension

• pits in the surface of the mint


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2DCHEM - PP#3 (Physical & Chemical Changes)