The effect of increasing temperature on the solubility of two solids

Aim

Question :
What is the effect of increasing temperature on the solubility of
(a) a Salt
(b) a Sugar
when they are placed in
(1) Ethanol
(2) Water.

This is basically asking if solubility is proportional to the temperature of the solvent. The idea is to do it in ethanol and water with a salt and a sugar. This is to test the question in two different liquids (solvents) with two different solids (solutes) to reach a more accurate answer.

Predictions

Solubility is the number of grams of the solute that will dissolve in 100 g of the solvent. Some things may dissolve in water but not in other liquids and some things may dissolve in other liquids and not in water.

Water is a polar solvent. Polar solvents are liquids whose molecules display a permanent dipole. A dipole has two oppositely charged poles (like a magnet). A molecule with a dipole is a molecule with a positive and a negative end. Ionic compounds are compounds that will split into two or more ions when placed in a liquid. Ions are particles that are positively charged (cations) or negatively charged (anions). Ionising liquids (polar liquids capable of dissolving ionic compounds) will dissolve ionic compounds well because they can pull both anions (with the positive ends of the molecules) and cations off (with the negative ends of its molecules). Salts are ionic compounds so I would expect them both to dissolve well in water.

I am going to use sodium chloride (common or table salt) in my experiment.
e.g.

NaCl + polar liquid		Na+ + Cl- (dissolved in polar liquid)
NaCl + H2O		Na+(aq) + Cl-(aq)
Table Salt + Water		Sodium + Chloride

So when you heat a polar liquid (water) it should dissolve a greater quantity than at room temperature (22-25 degrees C). The liquid molecules have more energy to move around and break the chemical bonds between the sodium and the chloride ions in the compound. They do that by attracting an ion by the oppositely charged end of the dipole in the molecule.

Because liquid molecules have more energy to break the chemical bonds between component particles in compounds, I would expect most compounds to dissolve better at high temperatures rather than low temperatures.

Ethanol is an organic compound and therefore is likely to be a covalent compound and unlikely to be a polar liquid. I do not expect ethanol to dissolve ionic compounds such as sodium chloride.

Most compounds should follow the general rule (a solute will dissolve better when the solvent is at a higher temperature rather than a low temperature). Sugars are not ionic compounds and therefore do not have ionic bonds. Instead they have covalent bonds. Covalent compounds may act differently to ionic compounds and therefore, the liquid molecules may need more or less energy to break the chemical bonds.

Equipment

Unfortunately, due to time and other restrictions, I was not able to use ethanol in my experiments. If ethanol had been used, a water trough would have been needed because ethanol must not be allowed to heat too rapidly and because of its low boiling point (78°C). This is for safety, because ethanol can catch fire or even explode.

Chemicals

Method

Because I couldn’t use Ethanol, the question is now :
What is the effect of increasing temperature on the solubility of
(a) a Salt
(b) a Sugar
when they are placed in
(1) Water.

1. 
   100 ml of water were placed in a beaker.
    
2. 
   Solid was added in 5g lots at each temperature.
    
3. 
   The solid was stirred in with a stirring rod. When all the 5g had dissolved, another 5g was added. This carried on until there was some solid in the bottom of the beaker that wouldn’t dissolve.
    
4. 
   The beaker was heated on a tripod over a Bunsen burner, until it reached the right temperature. When that happened, solid was added in 5g lots until there was some left that wouldn’t dissolve.
    
   If the temperature needed was lower than room temperature, the beaker had to be cooled. This was done by placing it in the freezer for a while (done at home).
    
5. 
   When there was solid left on the bottom, which wouldn’t dissolve, it meant the solution had become saturated (no more of the solute could dissolve). When the solution had become saturated at a particular temperature, the amount was recorded (correct to 5g) and another temperature was done. This made each result accurate to 5g.
    

This process was repeated for both solutes (sugar and salt) at 5, 25, 45, 65, 85 °C

This experiment was a fair test because

• There was a constant amount of water in each beaker.
• When the experiment was repeated, all the conditions were kept the same.

Key factors which could influence the results were

1. The amount of water in each beaker.
2. The length of time each amount of solid was given to dissolve.

(To save time in the next lesson, a beaker of sugar solution was left in a beaker inside a sealed bag. A week later there were organisms growing in it. They were white and filamentous. This obviously had to be thrown away.)

Results

SALT

The first results seemed odd, so they were repeated. The second experiment produced identical results. Because it produced the same results, it is likely that they are right and that the first results were not anomalous.

SUGAR

It was necessary to do the experiment twice, to make sure that the results were correct and not anomalous. When that had been done, the average of the two results could be found. In this case, because both results were the same, the average came out as the same figure. Ideally, if there had been more time, it would have been better to do the experiment three times. As these results came out exactly the same, it is quite a good indicator that they are accurate.

(Click on the graph to see an enlarged version of it.)

(Click on the graph to see an enlarged version of it.)

(Click on the graph to see an enlarged version of it.)

On these graphs. The best fit line was calculated by a regression equation (y = a + bx), i.e. Best fit line = Intercept + ( Slope * Temperature ). Because of this the line can only be used to predict solubility within the temperature range in the experiment. It should not be used for predictions outside this range.

SALT

SUGAR

Conclusion

The results show that the solubility of sugar (sucrose) did increase with temperature but the solubility of salt (sodium chloride) stayed the same at all temperatures.

This simplified extract comes from page 161 in the book ‘Principles of Chemistry’:

If heat is given off when a particular substance is dissolved in a solvent, then the solubility of that salt in that solvent will decrease with increasing temperature. On the other hand, if heat is absorbed when a particular substance is dissolved in a solvent, then the solubility of that salt in that solvent will increase with increasing temperature.

SALT

Sodium chloride doesn’t appear to fit into either category because its solubility doesn’t decrease or increase with increasing temperature.

I referred to two books (GCSE Chemistry Classbook and GCSE Chemistry) to check my results. They both confirmed that sodium chloride does keep a constant solubility with increasing temperature. But, they both showed the solubility of salt at 35g. These differed from my results of 30g. My results are probably slightly inaccurate because of two reasons :

1. Salt was added in 5g lots so the results were only accurate to 5g. This may mean that my results could have in fact been 32-4g.
2. I used tap water which would have sodium ions and chloride ions in. These may account for the last few grammes.

The effect of increasing temperature on the solubility of a salt (Sodium Chloride [NaCl] )when placed in water (H₂O) (g solute/100g solvent) is nothing. There is no change of solubility at any of the temperatures I have done. Because of this, it may mean that Sodium Chloride is very strongly ionic and can be broken down so easily by water. This would mean that the solution becomes completely saturated straight away.

SUGAR

On page 163 of ‘Principles of Chemistry’ it says :

An ionizing solvent such as water, will not dissolve not only ionic substances but also substances of high polarity. Because sugar dissolves in water, it means that a sugar compound is a substance of high polarity.

The effect of increasing temperature on the solubility of a sugar (Sucrose [C₁₂H₂₂O₁₁] ) when placed in water (H₂O) is a change in solubility. If the temperature is lowered the solubility gets less and if the temperature is raised the solubility rises. This is because the water molecules have more or less energy to move around and break the chemical bonds. When there is more energy, the molecules can break more of the bonds between the component particles in the compound.

Improvements

• A water bath could have been used to heat the beakers in. This would have allowed the contents to heat less rapidly.
• The solid could have been added in smaller amounts (e.g. 2g). Or, the solid could have been added (in 5g lots) until the solution was saturated and then the rest of the salt in the last lot could be weighed. You could then take that amount away from 5g and it would give you a more accurate saturation point.
• Using distilled water to cut out Na⁺ and Cl^- ions.

References

Dunstan, S. 1968. Principles of Chemistry. Van Nostrand Reinhold

• Chapter 13 Solution, Solubility and Solvents.
• Pages 159-164
• Page 165 Table 13.3

McDuell, B. 1997. GCSE Chemistry Classbook. Letts

McDuell, B. 1997. GCSE Chemistry Study Guide. Letts

Computer programs used

Microsoft Encarta 97

Microsoft Excel Version 5.0

Microsoft Word

Paint Shop Pro

Encyclopaedia Britannica 97