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Dissolving.

posted Dec 22, 2016, 10:54 AM by Upali Salpadoru   [ updated Apr 11, 2017, 8:37 PM ]


                1. Dissolving a solid in a liquid.



George:- “Why is the tea sweet?”

Linda:-    “'Cause of the sugar”.

Gearge:-   “How do you know there’s sugar?”

Linda:- "Surely, 'cause of the taste"


Fig.1 What a sweet cuppa?

               Question:-How can Linda prove there is sugar in the tea?


A Demonstration.

Sample

Salt+ Water

Canvas.jpg

Soil + Water

soil.jpg

Step 1. Sedimentatiuon

Observations

On  stirring

saly.jpg

Clear

No sarolid particles remain.

soil.jpg

Brownish

Cloudy (Opaque)


After sometime.

saly.jpg


No sediment


soil.jpg


Brownish sediment

Liquid almost clear.


Step 2. Filtering

Salt + Water

saly.jpg

Clear and colourless filtrate

No deposit.

Soil + Water

soil.jpg

Clear and colourless filtrate

Brown deposit

Filtrate




Deposit on filter paper

Step 3. Evaporating the filtrate.

Observation

Salt.jpg

white residue

Salt.jpg

Off white residue

Conclusion

All the salt added has dissolved.

Sand and clay has not dissolved.

Some substance in the soil has dissolved.

 

2. Is milk a solution?


Milk is white, which shows that the light gets scattered. But on keeping sedimentation does not take place. (Particles do not separate. ) When milk powder or floor is mixed with water particles do not disappear. Therefore they cannot be true solutions. Such homogenous mixtures are called colloids.


Characteristic Properties of a Solution.

1.       The solid dissolved is invisible.

2.       If the solute is completely dissolved the solution has to be clear. If not dissolved, a light ray passing through can be seen. (Tyndall effect)

Td.jpg

Fig. Light beam passes invisibly through a true solution but gets scattered by a colloid or a suspension.

3.       The solute or the solvent can impart a colour but a true solution cannot be white.

4.       A solution can pass through a filter paper without leaving a residue.

5.       On evaporating the filtrate the solute would be left behind. (Unless a chemical change takes place as in sugar)

6.       The solvent may be recovered by distillation.



3. Is dissolving a Physical or a Chemical Reaction ?

Dissolving usually is a physical reaction. Here are a few instances of chemical dissolving:

1.    Egg shells in vinegar. ……….CaCO3 +2 CH3COOH → Ca (CH3COO)2  + H2O + CO2

2.    Quick lime in water. ………. CaO  +  H2O →  Ca(OH)2

3.    Zinc in Hydrochloric acid. Zn + 2HCl →  ZnCl2 + H2

4. Solubility


This is the mass of the solute that can dissolve in 100g of water  at a particular temperature.

Most substances have a high solubility at a high temperature.

The solubility of gases decrease with the temperature.








5. Distillation.

This is a process by which the solute and the solvent can be separated in a solution. When a solution is heated the solvent escape as vapour leaving behind the solute. The vapour can be condensed by cooling.

dist.jpg

6. Saturated solution

The condition reached by dissolving the maximum amount of solute at a particular temperature is termed ‘saturation’.  By heating a saturated solution it is usually possible to dissolve a little more. On cooling the excess has to precipitate. In some substances this may not happen. Then the solution is a ‘supersaturated solution’.  One such a substance is ‘sodium thiosulphate’ (hypo).


7. Gases dissolving in liquids

If you keep some fresh cool water in a glass inside a warm room tiny bubbles will appear. This is not evaporation but the dissolved gases getting out due to heat.  There is an easy test to find out whether a gas is soluble or not..

Canvas.jpg

Courtesy:- engineeringtoolbox.com

Method

Fill the gas you wish to test into a test tube and invert it in a cup of water. Care should be taken not to allow the gas to leak out.


Solubility of some gases at 20 C and atmospheric pressure.

Oxygen …. 0.045g/kg ,   Carbon dioxide……..1.7g/kg,   Nitrogen…….0.02  Chlorine….7g/kg, Hydrogen…0.0016   (Courtesy..Engineeringtool box.com)

Solubility of gases decrease with temperature and increases with pressure.


8. Liquids dissolving in liquids …(Miscible liquids)

If you mix 20ml of oil with 20ml of water you will get 40ml a mixture. Within a few minutes the two liquids will separate. That means they are not soluble in each other. But if you mix 20ml of ethyl alcohol with 20ml of water you will not get 40 ml. There will be reduction of volume  in dissolving a liquid in another liquid

Che.jpg


1.0    What kind of mixing would occur when the following are mixed?

1.1    Salt and water.

1.2    Garden soil and water.

1.3    Coconut oil and water.

1.4    Egg shells and lime juice

1.5    Carbon dioxide and water                                           6x5 =30 Marks



2.0    Three solid substances A,B and C were tested as follows:

TEST

A      lime

B

C

Mixing..

with water gave a milky liquid.

with water gave a clear transparent liquid.

with alcohol gave a dark liquid.

Filtering

Clear filtrate.

White Residue present

Clear filtrate.

Residue absent

Dark liquid

Residue absent

Evaporating filtrate.

White

White powder.

Dark deposit.

Conclusion

……………

   …………….

     …….


                                                                                          5x3=15 Marks


3.0    Multiple choice

3.1    When a solid completely dissolves in water which is not likely to happen?

       A-    Sedimentation   B – Increase of density  C-  Get coloured  D-  Colourless

3.2    Water taken at 20°C was warmed to 30°C degrees some bubbles appeared. This can be cited as evidence for which of these?      

A-    Water boils at 30°C.   B- Less gas will dissolve at a high temperature.   C- More gas will dissolve at a higher temperature?   D- Water molecules squeeze out the dissolved gases as the contract..

3.3    When a crystal of the solute is added to a saturated solution what may happen?

      A-    Dissolve  B – Crumble   C  - Grow   D – Get super saturated.  

3.4    Adding a sugar crystal to a sugar  solution resulted in a large cluster of crystals

forming.  Which is the correct description of the solution.

      A- Dilute solution.   B- Concentrated solution  C- Saturated solution D- Super

saturated solution.

3.5 If an aqueous solution containing the maximum amounts of  sodium chloride and potassium nitrate  at 100°C was cooled to room temperature, what can you say about the residue formed?

  A- A mixture of NaCl and KNO3, B-  Mostly Nacl   C- Mostly KNO3.  D- Mixed crystals.

                                                                                                      5x5=25arks

4.0  Use solubility curves of two solids A (red) and B (blue) for 100g.of water are given here. sc.jpg

4.1  Find the solubility at 10°C  for A -------- and B -------- .  

        Find the solubility at 50°C  for A --------and B---------.

2x4=8marks.

4.2   What mass of solute A  is necessary to saturate 100g of water at 30°C?

        What mass of B is necessary to saturate  50g of water at 10°C ?

2x5=10 marks.


marks

                                                   






                                    


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