In this experiment, you will investigate how adding salt to water affects its boiling point. You heats 200 cm³ of distilled water in a beaker until it boils...
Question 1 Report
In this experiment, you will investigate how adding salt to water affects its boiling point. You heats 200 cm³ of distilled water in a beaker until it boils and records the boiling temperature. You then adds 10 g of salt to 200 cm³ of fresh distilled water, stirs until dissolved, heats until boiling and records the boiling temperature. You repeats with 20 g, 30 g and 40 g of salt. The apparatus is shown in Fig. 17.1.
(a) You records these boiling temperatures: 0 g salt = 100.0 °C, 10 g = 100.8 °C, 20 g = 101.5 °C, 30 g = 102.4 °C, 40 g = 103.1 °C. Record the boiling point of the water containing 30 g of salt. [1]
(b) Describe the relationship between the mass of dissolved salt and the boiling point. [1]
(c) Measure the increase in boiling point when 40 g of salt is dissolved compared with pure water. [1]
(d) Draw a graph of boiling point (y-axis) against mass of salt (x-axis). Include the point for pure water. Draw the line of best fit. [4]
(e) Use your graph to predict the boiling point of a solution containing 25 g of salt in 200 cm³ of water. Show your method. [1]
(f) Explain why you should stir the solution before recording the boiling temperature. [1]
(g) State why distilled water is used rather than tap water. [1]
(h) State one safety precaution you should take. [1]
Answer Details
(a) Boiling point with 30 g of salt
From the given data: 102.4 °C. [1]
(b) Relationship between mass of salt and boiling point
As the mass of dissolved salt increases, the boiling point increases. The relationship is approximately proportional: each additional 10 g of salt raises the boiling point by roughly 0.7-0.8 °C. Dissolved particles interfere with the escape of water molecules from the surface, requiring a higher temperature for boiling to occur. [1]
(c) Increase in boiling point with 40 g of salt
\(103.1 - 100.0 = \mathbf{3.1}\) °C [1]
(d) Graph of boiling point against mass of salt
- X-axis labelled "Mass of salt / g" (0 to at least 40), y-axis labelled "Boiling point / °C" (range around 99.5 to 104). [1]
- Suitable, even scale on both axes. [1]
- All five data points plotted correctly: (0, 100.0), (10, 100.8), (20, 101.5), (30, 102.4), (40, 103.1). [1]
- A straight best-fit line drawn through or close to all points. The points lie almost perfectly on a straight line with gradient approximately 0.078 °C per gram. [1]
(e) Predicted boiling point at 25 g
On the graph, draw a vertical line from 25 g on the x-axis up to the best-fit line, then draw a horizontal line across to the y-axis. The reading should be approximately 102.0 °C (accept 101.9-102.1). Construction lines must be shown on the graph for full credit. [1]
(f) Why stir before recording the boiling temperature
Stirring ensures the temperature is uniform throughout the solution and that all the salt is fully dissolved. Without stirring, localised hot spots could form near the heat source, giving a thermometer reading that does not represent the true boiling point of the whole solution. [1]
(g) Why distilled water is used
Tap water already contains dissolved minerals and impurities that would raise its boiling point above 100 °C. Using distilled (pure) water ensures the starting boiling point is the true value, so the effect of the added salt alone is measured fairly. [1]
(h) Safety precaution
Any one valid precaution: [1]
- Wear safety goggles to protect eyes from splashes of boiling liquid.
- Do not lean over the boiling beaker to avoid scalding from steam.
- Handle hot glassware with tongs or heat-resistant gloves.
- Tie back long hair to keep it away from the flame.
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