UNIT 3: CLASSIFYING MATTERContents:

Introduction Classifying pure substances and mixtures. Separation techniques Kinds of solution: concentration. Solubility

1. INTRODUCTION

As you already know, matter is anything around you, anything that has a mass and occupies a volume. Chemists can classify matter as solid, liquid or gas as we did in the previous unit, but matter can be classified in other ways as well. We´ll now discuss how all matter can be classified as either a pure substance or a mixture.

2. CLASSIFYING PURE SUBSTANCES AND MIXTURES

In the diagram above, matter is classifying depending on how can it be separated. Other way of classifying matter is attending on the composition: A pure substance has a definite and constant composition or make-up like salt or sugar. A pure substance can be either an element or a compound but the composition of a pure substance doesn´t vary. On the contrary, mixtures (physical combinations of pure substances) have no definite or constant composition(the composition of a mixture varies according to who prepares it).

Let´s see the next video to make the things clearer. Take notes in order to define the following concepts: Pure substance, mixture, element, compound, homogeneous mixture, heterogeneous mixture. Give examples.

http://youtu.be/J347VGJOnT0

You must remember that an atom, is the smallest particle of an element that still has its properties. Elements are the building blocks of matter. They are represented in the periodic table.

The elements are those represented in the periodic table

…as you can listen to in this song: http://youtu.be/U50qw3HTvc4

Now we are going to do the following exercises:

1. Tick with a circle the correct option:a) A mixture is/is not a chemical combining of substances.b) In a compound the atoms/molecules are chemically/physically

combined so that the elements that make up the compound retain/lose their identities and do/do not take on a new set of properties.

c) In a mixture, the individual substances lose/retain their identities.d) In a mixture the substances involved can/cannot be separated by a

simple physical process.

Mixtures

Variable composition

Pure substances

Fixed composition

e) In a compound the elements involved can/cannot be separated by a simple physical process because the elements are physically combined/chemically bonded.

2. Fill the blanks in the following sentence:a) The smallest identifiable unit of a compound is a __________________,

which is made up of _______________________ which are chemically bonded.

b) The smallest identifiable unit of an element is a(n) ________________.

3. From the following list of substances, circle the ones that are elements:Silver carbon dioxide wood alcohol chromiumWater hydrogen carbon nitrogenOxygen gold sugar saltAir sulfur magnesium nickel

4. Classify the following as pure substances or as mixtures:Air gasoline tap waterWater sugar goldMercury oxygen salt water

5. Classify the following as heterogeneous or as homogeneous:Sand and salt mixture hydrogen ironSalt water unfiltered air iron with rustPure water an apple nitric acidTossed salad granite wood

6. Classify the following as an element, a compound, a solution (homogeneous mixture), or a heterogeneous mixture:

7. Aluminium raisin bread:Carbon dioxide waterSugar and water sulfurSulfuric acid mercuryAn orange water and instant coffeeA pencil carbon particles and sugarNitrogen airGasoline grain alcohol

8. Place a check in the appropriate column:

Change Physical Chemical

Salt dissolves in waterHydrochloric acid reacts with magnesium to produce hydrogen gasA piece of cupper is cut in halfWater is heated and changed to steamIron rustsEthyl alcohol evaporatesIce meltsMilk sours (goes bad)Sodium and potassium reacts violently with waterPancakes cook in a griddleGrass grows on a lawnA tire is inflated with airFood is digested in the stomachWater is absorbed by a paper towelPaper burnsWater freezes at 0º CFireworks explodeClouds in the sky

3. SEPARATION TECHNIQUES

All mixtures can be separated by one or more of the following methods. All of them are based on the physical properties of substances.

Filtration: Used to separate a solid-liquid mixture in which the solid does not dissolve into de liquid. It´s a heterogeneous mixture of solid and liquid, for

example sand and water. It´s based on the size of the dissolved particles.

You need: Filter funnel, filter paper and a beaker.

Based in this same property, the size of the different particles that make up the mixture, we have the screening, to separate mixtures of different solids (sand and little stones), or liquid and solid (gold in water, as the gold seekers in EEUU).

Magnetic attraction: Used to separate magnetic materials from non-magnetic materials as iron and sand.

Decantation: It can be used to separate solid and liquid mixtures (as in filtration), or a mixture of liquids with different densities (water and oil). You´ll need a decantation funnel and a beaker.

Mixture of liquids Solid and liquid mixture

Distillation: We´ll see what distillation is in the next video. You have to take notes about: Which kind of mixtures can be separated by this method, physical property in which is based… http://youtu.be/mP4Hgui-g6U

Distillation is used to separate………………………………………………………………………………………

It´s based on the ………………………………………………………………………………………………………….

What´s the meaning of volatile?.......................................................................

Other techniques to separate mixtures are: evaporation, crystallization, extraction, chromatography, centrifugation, dissolving or dissolution…

Exercise: Explain all the right methods to separate these mixtures:a) water + sand; b) water + alcohol; c) water + gasoline; d) oil + gasolinee) oil + salt; f) water + sugar; g) iron + sand; h) water + oxygeni) sulphur + iron; j) salt + sand; k) water + chlorinel) iron powder + aluminium powder

4. KINDS OF SOLUTIONS: CONCENTRATIONThe homogeneous mixtures are sometimes called solutions. The air you breathe is a solution, your tap water is a solution. Remember that homogeneous means that its properties are the same throughout. A solution is composed of a solvent and one or more solutes.

The solvent is the substance doesn´t change state and is present in the largest amount. The solute is the substance that changes state and is present in the lesser amount. However water is always considered the solvent.

When I talk about solutions, most people think of liquids. But there can also be solutions of gases. Our atmosphere is a solution. Air is almost 79 percent nitrogen, nitrogen is considered the solvent, and the oxygen, carbon dioxide, and other gases are solutes. Solids can also make solutions. Alloys, for example, are solutions of one metal in another metal. Brass is a solution of zinc and copper.

Why do some things dissolve in one solvent and not in another? For example, oil and water don´t mix to form a solution. Two substances that don´t mix are inmiscible. Oil dissolves in gasoline, they are miscibles.

You know from your own experiences, that only so much solute can be dissolved in a given amount of solvent. The amount of solute you can dissolve in an amount of solvent depends on temperature. Based on the amount of solute for a given amount of solvent we can classify solutions as:

Saturated solutions, that contain the maximum amount of dissolved solute at a given temperature.

Unsaturated solutions that have less than the maximum amount possible. Supersaturated solutions: Sometimes, under unusual circumstances, the

solvent may actually dissolve more than its maximum amount possible for a given temperature and a given amount of solvent. This supersaturated solution is unstable, and sooner or later, solute will precipitate (form a solid, usually in the form of crystals), until the saturation point has been reached.

If the solution is unsaturated, then the amount of solute that is dissolved may vary over a wide range (remember: the composition of a mixture is always variable). A couple of rather indefinite terms describe the relative amount of solute and solvent that you can use:

A solution is dilute when it contains very little solute per given amount of solvent.

A solution may be concentrated when it contains a large amount of solute per the given amount of solvent.

These terms don´t are very accurate and sometimes you need to know the precise amount of solute in them (think of a solution to use in the hospital). So you must have a quantitative method to

describe the relative amount of solute and solvent in a solution. Such quantitative a method is the solution concentration units.

There are a lot of different concentration units: molarity, ppm(parts per million), percentages…we are only to study three:

solute percent by weight = weight of soluteweight of solution

×100

1. Determine the percent by weight of a solution of NaCl if there are 30 grams of NaCl dissolved in water to yield a total solution weight of 200 grams.

2. Twenty-five grams of NaOH (solute) are added to 175 grams of water (solvent). What is the % by weight of NaOH in the solution?

3. In 100 grams of an aqueous solution that is 15% NaCl by weight, there are how many grams of NaCl? How many grams of water?

4. Concentrated HCl is actually an aqueous solution. Concentrated HCl is 37,9% by weight HCl. How many grams of concentrated 37,9% HCl solution does it take to yield 10 grams of HCl.

Instead of using weight percentage of solute in the solution and converting to volume if needed, it is possible to determine volume percentage of the solute in the solution. Volume percentage is most often used when dealing with a solution involving two liquids, such as alcohol and water. Finding the volume percentage of the solute involves the following formula:

solute percent by volume= solute volumetotal volume

×100

5. A certain brand of homogenized milk contains 35 milliliters of butterfat in a total volume of 1 liter. What is the percent by volume of butterfat?

6. The drugstore sells a solution of 3% hydrogen peroxide (H2O2) by volume. In a 150 milliliter quantity of this solution, there are how many milliliters of H2O2?

7. What weight of dilute nitric acid solution contains 20 grams of HNO3? The solution is 19% HNO3 by weight. The density of 19% HNO3 solution is 1,11 grams per milliliter. Which is the volume of the solution.

Other concentration unit is g/l (mass in grams of solute divided by volume of solution in liters). You have to be careful with this last unit. It has the same units than a density, but remember, in density the mass and volume you divide correspond to the same substance.

5. SOLUBILITY

The solubility is related to the temperature of the solvent. For solids dissolving in liquids, solubility normally increases with increasing temperature as you can see in the picture bellow.

However, for gases dissolving in liquids, such as oxygen in water, the solubility goes down as the temperature increases. This is the basis of the thermal pollution, the addition of heat to water that decreases the solubility of the oxygen and affects the aquatic life.

Solubility is the máximum amount ot solute that will dissolve in a given amount of solvent at a specified temperatura. Solubility normally has the units of grams solute per 100 mililiters of solvent (g/100 mL)

A solution that has an amount of solute equals to its solubility at a given temperature for a given amount of solvent is a saturated solution.

EXERCISES1) FILL IN THE BLANKS WITH THE WORDS BELLOW

dispersed shaving flour immiscible micrometer physical undisturbed mayonnaise confused solvent look creating settle consists jelly

Solutions and dissolutions must not be …............................ Solution is a mixture of solute and …........................... while dissolution is the process of …........................... a solution. The solution has the same …........................... state as the solvent. There are some liquid mixtures that …........................... like solutions but are not: colloids, suspensions and emulsions are not considered solutions.A colloid ........................... of a substance from 5 to 200 nanometers …........................... in another substance. Examples: blood, milk ,…........................... and …........................... cream.A suspension is a mixture in which the solute is larger than one suspended particles will …........................... over time if left …............................ Examples: sand in water, …........................... in water, dust in air and andalusian gazpacho.An emulsion is a mixture of two or more liquids that are normally …............................Examples: …..........................., vinaigrette and homogenized milk.

2) THE RIGHT OPTION

Homogeneous / Homogenious means that the components of the mixture form a single phase. Heterogeneous means which / that the components of the mixture form two or more phases / faces.A dissolution / solution has the same chemical properties than / as its components but different physical / phisical properties. Example: pure water's density is not the same as / than salt water's.When a substance is diluted / dissolved, there is a breakdown of the crystal lattice /network into ions, atoms or molecules.In the process of solving / dissolution, energy may be absorbed or emited / released.Externally, this means an increase or a decrease in temperature.Solutions are very useful in chemistry / chemical reactions for several reasons:a) Reactants are usually cheap / expensive. b) In a liquid phase the contact area between the reactants is higher and the reaction makes / takes place better. c) If the reaction is violent, solutions prevent spitting / splashing.

PROJECT 3DDT is soluble in organic solvents. This explains why it concentrates in the fat of fishes, birds and other animals, sometimes with mortal effects. On the contrary, it´s nearly unsoluble in water, lingering in environment. DDT doesn´t disappear from the soil, even after so many rains.

a) Discuss the above test from a chemical and environmental point of view.b) Suggest a good solvent for DDT.c) Do some research about what for was used DDT in the past.

d) Do some research about what substances replace DDT nowadays.