How does temperature affect the rate of dissolving of a substance?
How does stirring affect the rate of dissolving of a substance?
How does the size of the solute grains affect the rate of dissolving of a substance?
What is dissolving?
In this section we are going to make more solutions. We will perform some experiments to see if we can make our solutes dissolve faster. Before we do that, we have to answer an important question: is melting and dissolving the same? What do you think?
The difference between melting and dissolving is very relevant, and mistaking dissolving for melting is a pervasive misconception at this stage of the learners' development.
Is melting and dissolving the same?
In this activity we are going to explore the differences between melting and dissolving. We have learnt that matter can exist as 3 different states: solid, liquid and gas.
INSTRUCTIONS (Part 1)
Watch the following short video clip to remind yourself how a solid, liquid and gas are different from each other. You should look out for the part where the states change. The solid changes to a liquid and then the liquid changes into a gas. Remember that a state change is when a substance changes from one state (i.e. solid) to another state (i.e. liquid).
The following picture shows how the different states of matter are related to each other. It also shows what the different state changes are called.
QUESTIONS:
What is needed to change a solid into a liquid?
Energy or heat
What is the process called when a solid changes into a liquid?
Melting
Fill in the missing words:
The particles in a _____ have fixed positions. When the solid melts, the particles are free to move from their positions. The state in which particles are close to each other but free to move around is called the _____ state.
Answers: solid, liquid
INSTRUCTIONS (Part 2)
Do you remember dissolving sugar in water in the previous chapter?
The following picture shows what we would see if we could shrink ourselves down to the size of water and sugar particles (molecules).
A solution of water and sugar molecules (particles)
Look carefully at the picture and then answer the following questions.
QUESTIONS:
Why is the sugar no longer visible? Give a reason.
The sugar does not disappear. It is not visible anymore as the sugar crystals have broken up into the individual sugar molecules, which we cannot see with the naked eye.
How can we be sure the sugar has not "disappeared" and that it is still there?
We can get the sugar back in crystal form by evaporating all the water away.
How is the picture of the sugar and water mixture different from the picture of the liquid which is shown below?
The picture of the sugar and water has two kinds of particles. The picture of the liquid only has one kind of particle.
The particles in liquid water.
What do we call it when we have two or more substances combined?
A mixture.
Can we say that the sugar melts? Give a reason.
No, the sugar does not melt. The sugar particles are free, but they are mixed with the water.
Rates of dissolving
Explanation of new words:
Temperature: tells us how hot or cold an object is. We measure temperature with a thermometer.
Grain size: the size of the particles of the substance. A big lump has a large grain size while powder has a small grain size.
Variable: something (an aspect of the investigation) that we change during an experiment. For instance, in an experiment to compare how fast a solute dissolves at different temperatures, we would change the temperature of the solvent. This means temperature is a variable in this experiment.
This section offers a great opportunity for learners to experience aspects of scientific experimentation that are a little more advanced than what they have done thus far. You could talk about controlling variables (changing just one variable in a set, and keeping all the others constant), and how to identify the independent and dependent variables. They will also be introduced to coming to a conclusion at the end of an investigation. Finally, they are introduced to the kinetic molecular model of matter when they learn that particles move faster when they are given energy (just like children!) and when they are moving faster they collide more, which speeds up dissolving. All of this provides a solid foundation for building a more complex understanding of these concepts in later years.
The rate of dissolving refers to how quickly a solute dissolves in a solvent. The word "rate" has many meanings. In science, when we use the word "rate" we usually mean how fast or how slow.
Tom likes his coffee sweet, with 3 teaspoons of sugar. For the coffee to taste sweet, the sugar must be dissolved. Here is a picture of a cup of coffee. Answer the questions that follow.
Yes, because it is a combination of different substances.
Is the cup of coffee a solution? Give a reason.
No, it is not a solution as a whole. The milk is suspension, as it is cloudy, and does not dissolve in the water. The coffee granules are an infusion in the water. However, the sugar does dissolve in the water and this forms a solution.
Teacher note: This might be hard for learners to answer, but it is important to note here that "cloudiness" implies that tiny aggregates or globules are suspended in the liquid or gas base. Transparency implies the particles of the substances are separated from each other and dissolved. So the milk is a suspension and not a solution in the water.
Make a list of the components in the coffee mixture.
Coffee, sugar and milk in water
Which component is the solvent?
Water
Is the sugar a solute or a solvent?
Solute
What could Tom do to make sure that the sugar dissolves quickly?
Tom could stir the coffee, Tom could use finer sugar, or Tom could stir in the sugar before the milk is added (assuming the milk is not heated).
Stirring a solution is just one of the things we can do to make a solute dissolve faster.
We are going to perform 3 investigations to explore the factors that affect the rate of dissolving of sugar (a solute) in water (a solvent).
In the first investigation, we will explore whether salt dissolves faster in hot or cold water. What do you expect?
Does salt dissolve faster in hot or cold water?
AIM:
To investigate how temperature affects the rate of dissolving
MATERIALS AND APPARATUS:
3clear containers (glass beakers are ideal, but small yoghurt tubs will also be suitable)
table salt
very hot water (not boiling)
tap water (at room temperature)
ice water
teaspoon
stopwatch or clock with second hand
Try to use sea salt (or any brand without "free flowing agent" - This is usually cornflour and obscures observations by forming a cloudy suspension). Alternatively, one can just use sugar here.
METHOD:
Measure the same quantity (100 ml) of tap water, hot water and ice water respectively into the three containers. Look at the diagram of the setup.
Place one teaspoon (5 ml) of salt into the container with the hot water.
Stir the solution by moving the teaspoon once back and forth through the water.
Stirring must occur at the same, regular rate for comparisons to be drawn. Let each group agree on a way to ensure this (perhaps by counting each full "stir" of the spoon).
Measure the time it takes for the salt to dissolve completely. Record the time in the table below.
Repeat steps 3-5 with the tap water and record the time it takes for the salt to dissolve in the table.
Repeat steps 3-5 with the ice water, and record the time it takes for the salt to dissolve in the table.
This step should be done with the ice OUTSIDE the water used for mixing. Either very cold water from a fridge can be used, or the mixing container should be placed on a bed of crushed ice. The presence of the ice in the mixing water obviously has nothing to do with the rate of dissolution. Additionally, some of the salt may end up on top of the ice and freeze there, creating a situation where it may appear that the salt dissolved quicker in the cold water than the room temperature water.
RESULTS:
The effect of temperature on dissolving
Situation
Time to dissolve (in seconds)
Salt in hot water
Salt in tap water
Salt in ice water
QUESTIONS:
What did we compare in this investigation?
We compared how fast salt dissolved in hot water, water at room temperature, and ice water.
Name three things that were the same about the three situations.
The amount of water we used
The amount of salt we added to the water
The amount of stirring
What did we change in this investigation?
The temperature of the water.
Teacher note: This is an opportunity to introduce the term variable (see New Words above). The variable that we change (manipulate) is called the independent variable.
We measured the temperature of the water. What else did we measure?
We measured the time it took for the salt to dissolve.
Teacher note: The variable that we measure is called the dependent variable. In this case time is the dependent variable.
In which situation did the salt dissolve the fastest?
The salt dissolved fastest in the hot water
CONCLUSION:
The salt crystals dissolved _____ in the hot water than the cold water. Temperature affects the rate of dissolving. When we increase the temperature of the solvent, the rate at which the solute dissolves _____.
Answer: faster, increases.
Extension question: Why do you think the salt dissolves faster in the hot water?
Teacher note: This is an opportunity to bring the energy concept into the conversation. Children move faster when they have more energy! Particles also move faster when they have more energy. When a solute dissolves in a solvent, the solute particles spread out among the solvent particles. This process happens much faster when all the particles are moving faster.
In the second investigation we will explore whether coarse salt dissolves faster than fine salt. What do you expect?
Does fine salt dissolve faster than rock salt?
To investigate how grain size affects the rate of dissolving
MATERIALS AND APPARATUS:
2 clear containers (glass beakers are ideal, but small yoghurt tubs will also be suitable)
fine table salt
coarse rock salt
teaspoon
tap water
stopwatch
METHOD:
Measure the same quantity (100 ml) of tap water into each container.
Place one teaspoon (5 ml) of fine table salt into the first container.
Stir the solution and measure the time it takes for the salt to dissolve completely. Record the time in the table below.
Repeat steps 2-3 with the coarse rock salt.
RESULTS:
The effect of grain size on dissolving
Situation
Time to dissolve (in seconds)
Fine salt in water
Coarse salt in water
QUESTIONS:
What did we compare in this investigation?
We compared which dissolved faster in water, fine salt or coarse salt.
Name three things that were the same about the three situations.
The amount of water we used
The temperature of the water
The amount of stirring
What did we change in this investigation?
The grain size of the salt
Teacher note: Here, you should explain the term 'grain size' by pointing out the differences in the sizes of the salt crystals. You should also try to convey that the same quantity (mass) of table salt would contain more grains if the grains were smaller.
Another opportunity to introduce the term variable (see New Words above). The variable that we change (manipulate) is called the independent variable.
What did we measure?
We measured the time it took for each type of salt to dissolve.
Teacher note: The variable that we measure is called the dependent variable. In this case the dependent variable is time, and the independent variable is grain size.
Which type of salt dissolved faster?
The fine salt dissolved faster.
Complete the conclusion below by filling in the missing word or words.
CONCLUSION:
The fine salt dissolved _____ than the coarse salt. Grain size affects the rate of dissolving. When we increase the grain size of the solute, the rate at which the solute dissolves _____.
Answer: faster, decreases.
Extension question: Why do you think the fine salt dissolves faster than the coarse salt?
Teacher note: This is an opportunity to bring the concept of surface area into the conversation.
Ask the learners to say which would contain more grains, a teaspoon of fine salt or a teaspoon of coarse salt.
Sketch the following two scenarios:
In the first scenario there are 30 stones and 30 children. Each child should pick up one stone, and carry it away. Only once the child has moved out of the way can the next child in line pick up a stone. If all the stones are in one large pile it will take a long time for all the stones to be picked up and carried away. This scenario can be compared to the larger grain size particles dissolving in the water.
In the second scenario there are also 30 stones and 30 children, but this time the stones are in 10 small piles of 3 stones each. In this scenario it is possible to carry 10 stones away at a time, because there are many more small piles. This scenario can be compared to the smaller grains size particles dissolving in the water.
This is roughly similar to the effect of smaller grain size, the more grains we have, the faster the solute particles can be 'carried away' by the solvent particles.
In the third investigation we will explore whether salt dissolves faster in water when it is stirred. What do you expect?
Does stirring increase the rate of dissolving?
In this investigation, the learners must come up with their own method (experimental design) to test whether stirring has an effect. You must provide the equipment. They can then look at the diagram of the setup and note what materials and apparatus they will need. After completing the investigation, they must also write out a method themselves. Learners can work in groups to do this.
In this investigation, you are going to design your own experiment to answer the question. Use your knowledge of the previous two investigations to help you. Look at the materials and apparatus that have been provided, as well as the diagram, to design your investigation and then write it out in the spaces below.
To investigate how stirring affects the rate of dissolving
MATERIALS AND APPARATUS:
Make a list of what you will need to complete this investigation:
2 clear containers (glass beakers are ideal, but small yoghurt tubs will also be suitable)
fine table salt
tap water
teaspoon
METHOD:
Write out the steps that you will follow to complete the investigation. Remember to include measurements.
Place a teaspoon of salt in each container.
Add the same amount of water (half a cup) to each container.
Stir the mixture in the first beaker but not the mixture in the second beaker.
Record your observations in the table.
RESULTS:
The effect of stirring on dissolving
Situation
Time to dissolve (in seconds)
Salt in water (with stirring)
Salt in water (without stirring)
QUESTIONS:
What did we compare in this investigation?
We compared which dissolved faster in water, a salt solution that is stirred or a salt solution that is not stirred.
Name 3 things that were the same about the 3 situations.
The amount of water we used
The temperature of the water
The amount of salt we used
What did we change in this investigation?
We investigated whether or not the mixture was stirred.
What made the salt dissolve faster: stirring, or not stirring?
The salt in the stirred solution dissolved faster.
CONCLUSION:
The mixture that was stirred dissolved _____ than the mixture that was not stirred. Stirring affects the rate of dissolving.
Answer: faster
Extension question: Why do you think stirring makes the salt dissolve faster?
Particles move faster when they are stirred. When a solute dissolves in a solvent, the solute particles spread out among the solvent particles. This process happens much faster when all the particles are moving faster.
The time it takes for a substance to dissolve is called the dissolving rate or the rate of dissolution.
The rate at which a substance dissolves can be affected by 3 factors, namely:
the temperature of the solution;
whether or not the solution is stirred (or shaken); and
the grain size of the solute.
A solute will generally dissolve faster if the solvent in which it dissolves is warm.
A solute will dissolve faster when the solution is stirred or shaken.
A solute will dissolve faster if the size of its grains is small.
What are the three factors that affect solubility? Write a sentence describing how they affect solubility.
The three factors that affect solubility are temperature, grain size and stirring. Increasing the temperature increases solubility, decreasing the grain size increases solubility and stirring increases solubility.
Write 3 examples where we use the factors that affect dissolving in our daily lives.
Any appropriate examples will do, for instance:
stirring tea or coffee to make the sugar dissolve
heating water to make salt dissolve when cooking; and
using fine salt as opposed to coarse grains when dissolving in water to make a salt solution for cooking or rinsing, etc.
Complete the following crossword puzzle:
Use the words in the box below, and the clues that are given underneath.
