Practical 6 — Physical Properties of Water

Aim

To investigate how the temperature of water changes as heat is added continuously, and to identify the temperatures at which changes of state occur.

Hypothesis

Write your own before you start. A good one predicts and gives a reason — for example: "The temperature will rise steadily except while the ice is melting and while the water is boiling, because during a change of state the energy is used to break hydrogen bonds instead of raising the temperature."

Apparatus

• 🟢 kitchen A small pot and a stove plate
• 🟢 kitchen About 2 cups of ice cubes (crushed is better)
• 🟡 pharmacy A thermometer reading at least −10 °C to 110 °C (a cooking/candy thermometer works)
• 🟢 kitchen A stopwatch — your phone
• 🟢 kitchen Paper and pen, or the table below

Variables

Independent (what you change): time, and therefore the heat added.
Dependent (what you measure): the temperature of the contents.
Controlled (kept the same): the stove setting, the pot, the mass of ice, and the position of the thermometer.

Method

1. Put the crushed ice in the pot. Push the thermometer into the middle of the ice, not touching the base of the pot.
2. Record the starting temperature at time 0.
3. Turn the plate to a medium setting and do not change it again — the whole experiment depends on heat going in at a steady rate.
4. Record the temperature every 30 seconds. Keep going.
5. Also record what you can see in the last column: all ice · ice and water together · all water · first bubbles · boiling steadily.
6. Stop about two minutes after the water has been boiling steadily. Turn the plate off.

Analysis

1. 7Draw a line graph of temperature (y-axis) against time (x-axis). Label both axes with quantity and unit, choose a scale that uses more than half the page, and give it a heading.
2. 2Identify the two horizontal sections of your graph and state the temperature at each.
3. 4Explain why the temperature does not rise during those two sections, even though heat is still being added.
4. 2Name the energy absorbed during each horizontal section.
5. 2State which state(s) of matter are present during the first plateau.
6. 3Your second plateau is much longer than your first. Explain why, referring to hydrogen bonds.
7. 2Give one everyday marine example of each change of state you observed.

Extension

Repeat with salt water (2 tablespoons of salt per litre, frozen the night before). Compare the melting plateau. Does salty ice melt at 0 °C? This is the sea-ice question in miniature.

Aim

To investigate the effect of temperature on the density of water, using colour to make the movement visible.

Apparatus

• 🟢 kitchen A large clear jar or glass vase of room-temperature water
• 🟢 kitchen Two small bottles or jars that fit inside it
• 🟢 kitchen Food colouring — red and blue
• 🟢 kitchen Very hot tap water and iced water

Method

1. Fill the large jar with room-temperature water and let it settle completely still.
2. Fill one small bottle with hot water and red colouring; the other with iced water and blue colouring.
3. Gently lower the red hot bottle to the bottom of the large jar, keeping it upright, then tilt it slowly to release the water. Record what you see.
4. Let the jar settle again, or start with fresh water. Now gently pour the blue cold water in at the surface. Record what you see.

Analysis

1. 2State which water was more dense and give the evidence from your observation.
2. 4Explain, in terms of molecular movement and hydrogen bonds, why warm water is less dense than cold water.
3. 2Why was the food colouring necessary? What kind of variable does it help you observe?
4. 3Deep ocean water is cold. A learner writes: "Deep water is dense because of the huge pressure on it." Say whether this is correct and justify your answer.
5. 3Use your result to explain how a convection current forms in the ocean.

Extension

Repeat with salty room-temperature water in the big jar and fresh coloured water in the bottle, both at the same temperature. You have now isolated salinity from temperature — which is exactly how a real oceanographer would design it.

Aim

To determine the density of fresh water and of salt water by measurement and calculation, using ρ = m/V.

Apparatus

• 🟡 pharmacy A kitchen scale reading to 1 g (digital is much better than spring)
• 🟢 kitchen A measuring jug or cylinder marked in mℓ — remember 1 mℓ = 1 cm³
• 🟢 kitchen Table salt, a teaspoon, and a stirrer
• 🟢 kitchen Tap water at room temperature

Method

1. Put the empty jug on the scale and zero it (tare). If your scale can't tare, weigh the empty jug and subtract later.
2. Add exactly 200 mℓ of tap water. Read the volume at the bottom of the meniscus, at eye level. Record the mass.
3. Empty and dry the jug. Make salt water: dissolve 7 g of salt (about 1¼ level teaspoons) in 200 mℓ of water — that is roughly real seawater at 35 g/kg. Stir until completely clear.
4. Measure exactly 200 mℓ of your salt water and record its mass.
5. Repeat each measurement three times and average. One reading is an anecdote; three is data.

Analysis

1. 2Write down the formula for density and state the unit you are working in.
2. 4Calculate the average density of your fresh water and of your salt water. Show all working, and round to three decimal places.
3. 2The accepted value for pure water at 4 °C is 1.000 g/cm³. Calculate your percentage error for fresh water.
4. 3Give two reasons your fresh-water value may differ from 1.000 g/cm³, and say for each whether it would push your answer up or down.
5. 3Rearrange the formula and calculate the mass of 5 ℓ of seawater at your measured density.
6. 3A rock has a mass of 78 g. Dropped into a measuring cylinder holding 50 cm³ of water, the level rises to 80 cm³. Calculate the rock's density and state whether it will float in seawater.

Extension

Make up three jars at different salinities (0, 17 and 35 g/kg), calculate each density, and plot density against salinity. You should get a straight line — and you have just measured the relationship that drives global ocean circulation.

Aim

To compare the viscosity of several household liquids, and to determine the effect of temperature on viscosity.

Apparatus

• 🟢 kitchen A baking tray or a smooth board, propped at a fixed angle
• 🟢 kitchen Equal volumes (5 mℓ / 1 teaspoon) of: water, cooking oil, dish-washing liquid, syrup or honey
• 🟢 kitchen A stopwatch and a ruler
• 🟢 kitchen A cup of hot water to warm one sample

Method

1. Prop the tray at a fixed angle. Mark a start line near the top and a finish line 20 cm below it.
2. Release 5 mℓ of the first liquid at the start line and time how long its leading edge takes to reach the finish line.
3. Wash and dry the tray completely between liquids. Repeat for all four.
4. The temperature test: stand a sealed container of the syrup in hot water for 5 minutes, then race it again at the same angle over the same distance.

Analysis

1. 2Define viscosity and name the molecular property that causes it.
2. 2Rank your liquids from lowest to highest viscosity, using your timings as evidence.
3. 3Draw a bar graph of flow time for the four room-temperature liquids.
4. 4State the effect of temperature on the viscosity of syrup, quote your two timings, and explain the effect in terms of molecular movement.
5. 3Name three variables you had to control for this to be a fair test, and say what would have happened if you hadn't.
6. 3Cold seawater is more viscous than warm seawater. Explain how this helps plankton stay in the sunlit surface layer.

Extension — the Algoa Bay version

Fill two identical bottles with seawater, one chilled in the fridge and one left in the sun. Drop an identical small bead into each and time the sink. You have just measured the viscosity difference that keeps plankton afloat in cold water.

Aim

To demonstrate the surface tension of water, and to investigate the effect of a detergent on it.

Apparatus

• 🟢 kitchen A coin (a R5 is ideal), a bowl, water
• 🟡 pharmacy A dropper or a 5 mℓ syringe — a clean teaspoon will do at a push
• 🟢 kitchen Dish-washing liquid
• 🟢 kitchen Ground pepper or dried herbs, and a paper clip

Method

Part A — how many drops fit on a coin?

1. Dry the coin and lay it flat. Add water one drop at a time, counting as you go, until the water bursts over the edge.
2. Look at the water from the side before it bursts — note the shape of the dome.
3. Repeat three times and average.
4. Now add one drop of dish-washing liquid to your water and repeat the whole count.

Part B — the floating paper clip and the pepper escape

5. Lower a paper clip flat onto still water so it rests on the surface without sinking.
6. Scatter pepper across the surface of a bowl of still water.
7. Touch the surface with a cocktail stick dipped in dish-washing liquid. Record what happens — and how fast.

Analysis

1. 3Define surface tension and explain, using hydrogen bonds, why the water forms a dome instead of running off.
2. 2Compare your plain and soapy drop counts. Calculate the percentage decrease.
3. 3Explain what the detergent does to the water molecules at the surface.
4. 2The paper clip is denser than water, yet it floats. Explain — and note this is not the same reason a boat floats.
5. 3Explain why the pepper flees when soap touches the surface.
6. 3Detergent and oil pollution both reduce the surface tension of seawater. Suggest one consequence for a marine organism that relies on the surface film, and name such an organism.

Extension

Bluebottles (Physalia) hang from the surface film of Algoa Bay. Research one other organism that lives on or under the sea surface, and write a short paragraph on how a detergent spill would affect it.