Describe methods of separations and purification for the components of the following types of mixtures:

(i) solid-solid(ii) solid-liquid(iii) liquid-liquid (miscible and immiscible)

Done by: Chew Yun Jin (3)3H/2011

1.2 Methods of

purification and analysisa)

Techniques to be covered for separations and purification include:(i) Use of suitable solvent, filtration and crystallization

or evaporation(ii) Sublimation(iii) Distillation and fractional distillation(iv) Use of a separating funnel(v) Paper chromatography

Separating a Solid from Solid. . .

H o w d o w e u s e f i l t r a t i o n t o

s e p a r a t e t w o s o l i d s ?

A m i x t u r e o f t w o s o l i d s c a n a l s o

b e s e p a r a t e d b y f i l t r a t i o n i f o n e

o f t h e m a r e s o l u b l e i n a s o l v e n t

b u t t h e o t h e r i s n o t . F o r

e x a m p l e , a m i x t u r e o f s a l t a n d

s a n d c a n b e s e p a r a t e d b y

f i l t r a t i o n u s i n g w a t e r a s t h e

s o l v e n t . S a l t i s s o l u b l e i n w a t e r

Filtration

I t i s u s e d t o

s e p a r a t e o n e

s o l u b l e s o l i d a n d

a n o t h e r s o l i d .

E x p e r i me n t 3T o s e p a r a t e a m i x t u r e o f c o m m o n t a b l e s a l t

a n d s a n d

P r o c e d u r e :

1) P o u r s o m e d i s t i l l e d w a t e r i n t o t h e

m i x t u r e o f c o m m o n t a b l e s a l t a n d s a n d .

S t i r a n d w a r m t h e m i x t u r e .

2) P o u r t h e w a r m m i x t u r e i n t o a f i l t e r

f u n n e l l i n e d w i t h f i l t e r p a p e r . C o l l e c t

t h e f i l t r a t e i n a c o n i c a l f l a s k (F i g . 3.14).

3) Wa s h t h e r e s i d u e w i t h a l i t t l e d i s t i l l e d

w a t e r t o r e m o v e a l l t h e s a l t s o l u t i o n

f r o m i t . T h e r e s i d u e i s s a n d .

4) P o u r t h e f i l t r a t e i n t o a n e v a p o r a t i n g

d i s h a n d e v a p o r a t e t h e f i l t r a t e t o

d r y n e s s (F i g . 3.15). T h e w h i t e s o l i d l e f t i n

t h e e v a p o r a t i n g d i s h i s s a l t .

A n o t h e r E x a m p l e :

T o s e p a r a t e s a l t a n d

c h a l k p o w d e r

-D i s s o l v e t h e m i x t u r e u s i n g

d i s t i l l e d w a t e r .

-T h e s a l t w i l l d i s s o l v e i n

w a t e r w h i l e

t h e s a n d w i l l r e m a i n i n t h e

b e a k e r .

-T h e f i l t e r p a p e r o n l y a l l o w s

t h e

s o l u t i o n t o p a s s t h r o u g h .

-T h e c h a l k p o w d e r w i l l b e

-C o l l e c t t h e

f i l t r a t e a n d p l a c e

i t

i n t o a n

e v a p o r a t i n g d i s h .

-E v a p o r a t e t h e

w a t e r / s o l v e n t

u n t i l

t h e s o l u t i o n i s

s a t u r a t e d . (D o n o t

h e a t t o d r y n e s s )

-C o o l t h e s o l u t i o n

a n d c r y s t a l s w i l l

a p p e a r .

C r y s t a

l l i z a t

i o n

Separating a Solid from a Liquid. . .DecantingThe simplest way to separate the water from the

pebbles is to just pour the water away. This

method is called decanting (Fig 3.10) We use

decanting to separate a dense, insoluble solid

from a liquid. Decanting is carried out everyday

during many activities. For example during

cooking, water is poured off various cooked foods

by decanting.

FiltrationTo separate sand from water, we use a method known as filtration. In general, we use filtration to separate small solid particles from liquid. Examples of small solid particles include sand, clay, dust particles and precipitates (small solid particles produced in a liquid by chemical reactions)

How can we remove a precipitate from a reaction mixture by filtration?

A white precipitate of lead (II) sulfate is produced when we add sodium sulfate solution to lead (II)nitrate solution.

A s o l i d c a n b e s e p a r a t e d f r o m a l i q u i d b y

f i l t r a t i o n b e c a u s e t h e f i l t e r p a p e r a c t s

a s a s i e v e . A l i q u i d c a n p a s s t h r o u g h t h e

p o r e s (s m a l l h o l e s ) o f t h e f i l t e r p a p e r b u t

a s o l i d c a n n o t d o s o .

U p o n f i l t r a t i o n , t h e s o l i d t h a t r e m a i n s o n

t h e f i l t e r p a p e r i s c a l l e d t h e r e s i d u e . T h e

l i q u i d o r s o l u t i o n t h a t p a s s e s t h r o u g h t h e

f i l t e r p a p e r i s c a l l e d t h e f i l t r a t e .

Evaporation to Dryness and Crystallization

When a solid is insoluble in water, the easiest way to separate it from a liquid is by filtration. However, many substances, like common salt, dissolve in water to form solutions. To separate such substances, we evaporate water from the solution. This separation technique is called evaporation to dryness. Fro example, when we evaporate salt solution to dryness, we recover solid salt. This method can be used to recover salt from seawater.

However, many substances decompose when they are heated strongly. For example, sugar will decompose to give water and carbon when it is heated strongly. Most crystals, such as copper (II) sulfate crystals, give off water to become powders when heated. For such substances, evaporation to dryness is nota good method of purification. Also, when all the water is removed during evaporation, any soluble impurities present will be left on the crystals.

The best method of obtaining a pure solid sample solution is called crystallization.

How do we purify by crystallization?In crystallization, water is removed by heating the solution. Heating is stopped at the stage when a hot saturated solution is formed. If the resulting solution is allowed to cool to room temperature, the dissolved solid will be formed as pure crystals.

How do we test for a saturated solution?A clean glass rod can be used to test whether a solution is saturated. It is dipped into the solution and removed. There will be a small amount of solution on the rod. If small crystals form on the rod as the solution cools, the solution is saturated. The solution is at its saturation point or crystallization point.

Example: Obtaining copper (II) sulfate crystals from copper (II) sulfate solutionFlowchart below shows the steps involved.

Copper (II) sulfate solution

Saturated copper (II) sulfate solution

Heat until the solution is saturated

Copper (II) sulfate crystals + solution (contains soluble

impurities)

Allow solution to cool

Copper (II) sulfate solution

Filter the mixture

Websites for extra information : http://www.youtube.com/watch?v=aFONmzLQJSY&

1)Chemistry Matters (Marshall Cavendish)

textbook

2) www.chemcompass.pbworks.com

3) www.google.com

Done by:

Chew Yun Jin (3)3H/2011