Why do solutions go cloudy

The branch of chemistry that deals with the study of reaction rate and its mechanism is called Chemical Kinetics. In all the chemical reactions the reactants are consumed and new products are formed.

So the rate of a reaction is defined as the rate of decrease in concentration of any one of the reactants or the rate of increase in concentration of any one of the product.

The concentration of the reactant plays an important role in the rate of a reaction. As the concentration of the reactant increases, the number of reacting molecules increases. Because of the increase in the number of molecules, the number of collisions also increases as a result the rate of the reaction increases. The effect of concentration of the reactant on the rate of a reaction can be studied easily by the reaction between sodium thiosulphate and hydrochloric acid.

Sodium thiosulphate reacts with dilute acid to produce sulphur dioxide, sulphur and water. Sulphur dioxide is a soluble gas and dissolves completely in aqueous solution.

The sulphur formed however is insoluble and exist in the mixture as a white or pale yellow precipitate or a colloid that gives a milky appearance and makes the solution opaque. Therefore the rate of the reaction can be studied by monitoring the opaqueness of the reaction. You can see one below.

As a teacher, it was fun to watch their level of excitement over something so seemingly simple. Though I used this experiment to primarily investigate collision theory and different factors that affect the time it takes for a reaction to complete, it could easily be used to determine something more complex like reaction order see the entire Flinn video from which the above clip is taken.

I facilitated the design of the experiment by asking my students a series of questions that were meant to feel like it was a genuine conversation happening between scientists interested in answering a question. The PowerPoint that I used to help facilitate this discussion can be found as Supporting Information at the bottom of this post if you are logged in to ChemEd X, but the general theme followed these questions:.

The total volume of each solution should be the same in each beaker. You can give it an initial stir to uniformly distribute the HCl. The timer starts after this initial swirl. After everyone had finished the experiment and analyzed their results, I was thrilled to see that the data from each group produced a graph that displayed the relationship I was looking for. Not a single group had one weird outlier or a graph with seemingly random points all over the place! This provided a great opportunity to talk about the benefits of qualitative evidence as well.

I attribute these consistent results to two primary things:. Minimizing chances for experimental error was huge. Effect of Concentration on Reaction Time Graph. It is the arguments I am most interested in developing after students complete their data analysis. Though most boards had similar claims, they often differed in what evidence they chose to present. They all had access to the same evidence and yet different groups intentionally left out certain pieces of evidence—why? Where their boards differed the most was in their reasoning, which is meant to have them justify why their evidence makes sense based on known scientific principles.

I should mention that the students had not been presented anything about collision theory before this lab and yet many of them were able to come up with a valid particle-based explanation while others either circled around ambiguity, lacked detail, or simply displayed some form of misconception. The important part of this was that they tried their best, based on the models they had running around in their heads, to explain the phenomenon and knew that it was up to the scientific community our class to act as a filter for sorting out valid explanations from ones that either lacked detail or could not quite account for the evidence.

This is the process I love doing the most. The lab itself took about 30 mins to do but because I involved them in the experimental setup and dedicated time to construct arguments that were presented, debated, and refined, the entire process took 3 periods 1 hr each.

Accessed 17 Jan. Analyzing data in 9—12 builds on K—8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.

Asking questions and defining problems in grades 9—12 builds from grades K—8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations. Scientific questions arise in a variety of ways. They can be driven by curiosity about the world e.

Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli 17th-century inventor of the barometer to his discoveries about the atmosphere and the identification of a vacuum.

Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem?

What are the criteria specifications for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution? Constructing explanations and designing solutions in 9—12 builds on K—8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.

Modeling in 9—12 builds on K—8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds. Use a model to predict the relationships between systems or between components of a system. Engaging in argument from evidence in 9—12 builds on K—8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds.

Arguments may also come from current scientific or historical episodes in science. Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits of arguments. Planning and carrying out investigations in builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data e.

Students who demonstrate understanding can apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs. Assessment is limited to simple reactions in which there are only two reactants; evidence from temperature, concentration, and rate data; and qualitative relationships between rate and temperature.

Emphasis is on student reasoning that focuses on the number and energy of collisions between molecules. This is awesome. I found this lab to be very useful too, and appreciate how you've shared how it's run in your classroom. This table gives some example results. Describe the effect of increasing the temperature of the reaction mixture on the rate of reaction.

Use your graph to help you. The rate of reaction increases as the temperature increases. The rate increases by a greater amount at higher temperatures. Suggest a reason why the same person should look at the black cross each time. Different people may decide that they cannot see the cross at different amounts of cloudiness, leading to errors in deciding when to take the reaction time.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:. Fran Scott demonstrates how to measure the rate of reaction and how to increase it. Practical - effect of changing the temperature on the rate of reaction Investigate the effects of changing the conditions of a reaction on the rate of a reaction by observing a colour change. Aims To investigate the effect of changing the temperature on the rate of a reaction. Method Using a measuring cylinder, add 50 cm 3 of dilute sodium thiosulfate solution to a conical flask.