6 15 r assignment conservation of mass lab

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8.6: The Law of Conservation of Matter

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• Page ID 221379

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Learning Objectives

• Correctly define a law as it pertains to science.
• State the law of conservation of matter.

In science, a law is a general statement that explains a large number of observations. Before being accepted, a law must be verified many times under many conditions. Laws are therefore considered the highest form of scientific knowledge and are generally thought to be inviolable. Scientific laws form the core of scientific knowledge. One scientific law that provides the foundation for understanding in chemistry is the law of conservation of matter. It states that in any given system that is closed to the transfer of matter (in and out), the amount of matter in the system stays constant. A concise way of expressing this law is to say that the amount of matter in a system is conserved .

With the development of more precise ideas on elements, compounds and mixtures, scientists began to investigate how and why substances react. French chemist A. Lavoisier laid the foundation to the scientific investigation of matter by describing that substances react by following certain laws. These laws are called the laws of chemical combination. These eventually formed the basis of Dalton's Atomic Theory of Matter.

Law of Conservation of Mass

According to this law, during any physical or chemical change, the total mass of the products remains equal to the total mass of the reactants.

\[ \overbrace{\underbrace{\ce{HgO (s)}}_{\text{100 g}}}^{\text{Mercuric oxide}} \rightarrow \underbrace{\overbrace{\ce{Hg (l) }}^{\text{Mercury}}}_{\text{92.6 g}} + \underbrace{\overbrace{\ce{O2 (g)}}^{\text{Oxygen}}}_{\text{7.4 g}} \nonumber \]

Another way of stating this is, "In a chemical reaction, matter is neither created nor destroyed." The law of conservation of mass is also known as the "law of indestructibility of matter."

Example \(\PageIndex{1}\)

If heating 10 grams of \(\ce{CaCO3}\) produces 4.4 g of \(\ce{CO2}\) and 5.6 g of \(\ce{CaO}\), show that these observations are in agreement with the law of conservation of mass.

• Mass of the reactants, \(\ce{CaCO3}\) : \(10 \,g\)
• Mass of the products, \(\ce{CO2}\) and \(\ce{CaO}\): \(4.4 \,g+ 5.6\, g = 10\, g\).

Because the mass of the reactants = the mass of the products, the observations are in agreement with the law of conservation of mass.

What does this mean for chemistry? In any chemical change, one or more initial substances change into a different substance or substances. Both the initial and final substances are composed of atoms because all matter is composed of atoms. According to the law of conservation of matter, matter is neither created nor destroyed, so we must have the same number and kind of atoms after the chemical change as were present before the chemical change.

It may seem as though burning destroys matter, but the same amount, or mass, of matter still exists after a campfire as before. Figure 5.1.1 shows that when wood burns, it combines with oxygen and changes not only to ashes, but also to carbon dioxide and water vapor. The gases float off into the air, leaving behind just the ashes. Suppose we had measured the mass of the wood before it burned and the mass of the ashes after it burned. Also suppose we had been able to measure the oxygen used by the fire and the gases produced by the fire. What would we find? The total mass of matter after the fire would be the same as the total mass of matter before the fire.

Exercise \(\PageIndex{1}\)

• What is the law of conservation of matter?
• How does the law of conservation of matter apply to chemistry?

The law of conservation of matter states that in any given system that is closed to the transfer of matter, the amount of matter in the system stays constant

The law of conservation of matter says that in chemical reactions, the total mass of the products must equal the total mass of the reactants.

Key Takeaway

The amount of matter in a closed system is conserved.

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LAB in Observations , Chemical Change , Conservation of Mass , Chemical Change . Last updated October 14, 2019.

In this lab, students perform three simple experiments, recording the mass of a substance before and after a reaction.  They then explain how the law of conservation of mass was not violated, even if the mass changed.

Grade Level

Middle and high school

By the end of this lesson, students should be able to

• explain the law of conservation of mass.
• determine whether or not a chemical reaction has taken place.

Chemistry Topics

This lesson supports students’ understanding of

• Conservation of mass
• Chemical changes

Teacher Preparation : 15 minutes

Lesson : 40 minutes

• Two 100-mL beakers
• Popcorn kernels
• 0.5-M lead(II) nitrate
• 0.5-M potassium iodide
• 1-M hydrochloric acid
• Safety goggles
• Marble chips
• Paper lunch sack
• Microwave or popcorn popper
• Always wear safety goggles when working with chemicals in the laboratory setting.
• Food in the lab should be considered a chemical, not for consumption.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• Make sure students return all of the popcorn to the bag after popping.
• Students should find the mass of the chemicals by subtracting the mass of the beakers and cups from the total mass.

For the Student

Chemical changes are changes in chemical bonds. In a chemical reaction, the identity of the substance changes. You can usually recognize a chemical change by seeing evidence of this new substance. Evidence includes color changes, precipitates (solid formed from two liquids) forming, gas (bubbles) released, or energy changes in the form of heat, light, or sound (think fireworks).

The Law of Conservation of Mass states that in chemical reactions mass cannot be created or destroyed. The mass of the substances that you start with have to equal the mass of the substances produced.

Prelab Questions

• Wood burning is a chemical change. How do you know this?
• After burning, the mass of the wood has changed. Does this violate the Law of Conservation of Mass?

EXPERIMENT 1

• Obtain two small (100-mL) beakers. Add approximately 20 mL of lead(II) nitrate to one beaker.
• Add approximately 20 mL of potassium iodide to the second beaker.
• Place both beakers on the balance and find the total mass of beakers and chemicals. Record observations.
• Pour contents of beaker one into beaker two. Find the total mass of beakers and chemicals. Record mass and observations in data table. Clean and dry beakers when finished.

EXPERIMENT 2

• Using the same beakers from experiment one, fill beaker one with a small handful of marble chips (up to approximately the 25-mL mark).
• Add approximately 25 mL of hydrochloric acid to beaker two. Place both beakers with substances on balance and record total mass.
• Leaving both beakers on the balance, carefully pour the hydrochloric acid from beaker two into beaker one. Leave both beakers on the balance and record your observations.
• Wait two minutes and record total mass again. Rinse marble chips with lots of water and pour into strainer. Clean and dry both beakers.

EXPERIMENT 3

• Obtain a Dixie cup and record the mass.
• Fill the Dixie cup about half way with popcorn kernels and record the mass. Determine the mass of the popcorn kernels alone.
• Obtain a lunch bag and record the mass.
• Using a hot air popper, heat the popcorn and collect the popped kernels in the lunch bag. Add any unpopped kernels back into the bag as well.

Record the mass of popcorn and lunch bag. Determine the mass of the popcorn alone.

• Did a chemical reaction take place? How do you know?
• What change did you notice in the mass of the substances before and after your experiment? (Explain using data)
• Did the mass of the popcorn change before and after popping? (Explain using data)
• Did the volume of the popcorn change before and after popping?

Write a paragraph summarizing how these experiments demonstrate the Law of Conservation of Mass. Include evidence from each experiment in your summary.

Conservation of Mass Experiments

Start with reviewing the difference between physical and chemical changes. (Chemical changes include: gas, color change, precipitate, temperature change, or light). Get some play doh and roll it into a ball. Place it on the scale and ask students if they think the mass will change if you change the shape of the play doh. You could also use legos or anything else you have handy.

Once they’ve seen that physical changes don’t cause a mass change, move on to chemical changes. Here are some labs you can use for different grade levels to teach the law of conservation of mass.

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Conservation of mass

Help students navigate the law of conservation of mass on dissolving and heating

It’s sort of obvious isn’t it, you can’t create or destroy matter. A sandwich will have the same mass as its ingredients and a smashed wine glass will weigh no more or no less than before it was dropped. But beyond the obvious sit a number of interesting observations that make the seemingly intuitive law of conservation of mass not so intuitive after all:

Source: © Getty Images

Even sandwiches are the same mass as their components

• A birthday cake will weigh less than its ingredients.
• Iron wool increases in mass when it burns in air.
• A candle decreases in mass when it burns in air.
• When sugar dissolves in water it disappears but mass is conserved.
• Plants get the majority of their mass from an invisible gas.

What students need to know

To help students navigate these observations that seem to undermine the law of conservation of mass, let’s delve a little deeper into their thinking.

The demonstration in the image below is based on a thought experiment from Johnston and Scott (1991) . The task explores students’ thinking about mass changes on dissolving.

Only 40% of 12–13 year olds in this study successfully predicted that mass is conserved when a substance dissolves. For some, the mass decreased on dissolving because the sugar got broken down into smaller pieces. Those who predicted the mass would increase believed the sugar clings to the water particles and so gets heavier.

Source: Adapted from Johnston and Scott (1991)

Johnston and Scott’s thought experiment for exploring students’ ideas about conservation of mass on dissolving

What’s interesting about these explanations is that they are based on correct science. Sugar crystals do get smaller when they dissolve in water and sugar molecules do cling to water molecules. However, these correct ideas were being used to draw incorrect conclusions about the overall change in mass. This was because students were thinking about specific parts of a system, eg just the sugar crystals or just the water particles. Instead, they needed to think about the system as a whole.

Ideas for your classroom

Thinking about the whole system like this can help students understand what is happening when mass seems not to be conserved. The thermal decomposition of copper(II) carbonate is a wonderful example of this. Students often get confused by the decrease in mass when the green CuCO 3 powder is heated to form black copper(II) oxide.

Equation 1: CuCO 3 (s) → CuO(s) + CO 2 (g)

There are two ways to help students here. First, discuss how the equation links with their practical observations. Second, identify where all the components of the system are. In particular, students need to look beyond the test tube that is being heated to ‘find’ the carbon dioxide gas.

Discuss the equation

Chemical equations can be a pretty misleading representation of what happens in a chemical reaction. So, rather than starting with the overall equation that shows all substances together, break the reaction up into at least three steps and introduce them separately like this:

It is now easier for students to see that there are no reactants when the reaction has finished. You can show how atoms get rearranged using colour, like this:

Examine the whole system

Help students to identify all the components of the system, both within and outside the test tube. Provide a diagram of the apparatus and ask them to identify the locations and names of all the substances. Do this for the start and the end of the reaction. You can see what this looks like in the accompanying classroom activity.

Download this

• A classroom activity to help students visualise the components of a reaction using the examples of dissolving sugar and heating copper carbonate as a starter slide (as  MS Powerpoint or pdf ), or worksheet (as MS Word or pdf ) with an answer sheet (as MS Word or pdf ).
• Some thinking questions (as MS Powerpoint or pdf ) to challenge students to consider what happens to matter during and after a reaction.

Download all

Classroom activities to help students visualise the components of a reaction using the examples of dissolving sugar and heating copper carbonate from the Education in Chemistry website:  rsc.li/2BlIRxb

Common misconceptions

Source: © Science Photo Library

Thermal decomposition of CuCO 3 : Students don’t always understand why the mass decreases after heating

Understanding conservation of mass requires an understanding of particle theory. Research by Vanessa Kind found that some students think that gases have no mass and that substances in their solid state weigh more than their liquid form, confusing density with mass. Use carefully structured demonstrations to address these misunderstandings before introducing chemical changes that appear to violate the law of conservation of mass.

Demo 1: Gases have a mass

Blow up two balloons with air and attach each balloon to either end of a 1-metre ruler. Balance the ruler on a fulcrum. Ask students to predict what will happen if you pop one balloon. Pop one balloon and ask the students to observe and explain. Alternatively, replace balloons with brown paper bags, then pour carbon dioxide from a mixture of acid and marble chips into one of the bags and watch the ruler tilt.

Demo 2: Matter is lost in open systems

Place some vinegar and baking powder on a digital balance. Record their mass. Now add these two chemicals together inside a Ziploc bag and students will see the bag expand but the mass remain the same. Alternatively, use the reaction of calcium carbonate with hydrochloric acid for this demonstration.

Demo 3: Solid particles have the same mass as liquid particles

Record the mass of two beakers, one containing sodium hydroxide solution and the other containing copper(II) sulfate solution. Now pour them together to make the blue precipitate (solid), copper(II) hydroxide. Place both beakers back onto the balance. Without showing students the reading, ask them to predict what the new mass will be.

Formative assessment

Diagnostic multiple-choice questions are a great way to explore students’ reasoning behind their answers. Best Evidence Science Teaching resources provide a great starting point to explore their ideas about conservation of mass. Students are given a question and multiple plausible explanations for an observation. They then choose and justify which explanation they agree with. You can also provide students with thought experiments and ask them to provide their own explanations (see the thinking questions resource).

If students sketch graphs of changes in mass during a reaction and talk about their reasoning in pairs, you can listen to their thinking. Use the burning iron wool in air demonstration as a stimulus for this.

Progression to 14–16

Students’ understanding of conservation of mass will become more sophisticated. They will consider more contexts, eg neutralisation and electrolysis, and look at how the components of the system interact. They will apply ideas of conservation of mass to reversible reactions. They will monitor the rate of reactions by considering how much product is made (or reactants used up) in a specific amount of time. This will involve using more complex apparatus such as gas syringes or data loggers.

Students will represent mass changes in more sophisticated ways too, using balanced symbol equations. Find opportunites to practice with the conservation of mass video . These equations will help students to develop a quantitative understanding of matter, calculating relative formula masses and predicting masses of products formed.

Take-home points

• The law of conservation of mass states that atoms are neither created or destroyed during a reaction, they are simply rearranged.
• Many chemical reactions appear to involve a change in mass because not all matter has been accounted for.
• Use a range of reactions, in open and closed systems, so students identify all component parts of a system.
• Understanding conservation of mass allows understanding of balancing equations and calculating reaction masses.

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Education Standards

Wyoming science content and performance standards.

Learning Domain: Matter and Its Interactions

Standard: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Next Generation Science Standards

Science Domain: Physical Sciences

Topic: Chemical Reactions

Standard: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction. [Clarification Statement: Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem-solving techniques.] [Assessment Boundary: Assessment does not include complex chemical reactions.]

Balancing Chemical Equations

Changing matter not weight, chapter 7 notes, law of conservation of mass lab, mathematically balancing equations, law of conservation of mass.

This unit dicusses the law of conservation of mass, as well as teaches the fundamentals of balancing chemical equations. 

Students determine the portion of original mass of gum that is swallowed for sugar and sugar-free gum.

Matter is not created nor destroyed; it simply changes from one form to another.  This law of conservation of mass challenges elementary students’ ideas about matter, because many children may think that matter is created or destroyed in a chemical reaction.  In this lesson, students will challenge their preconceptions about matter by experimenting with physical and chemical changes to determine that the total weight of the matter does not change. Students will use math to show that the total weight of matter is equal to the sum of the weight of its component parts, and they will graph this information to show that the weight of matter is conserved during physical and chemical changes.

Notes PowerPoint

Follow the Powerpoint, and take notes. Make sure to include all vocabulary. 

Balancing Chemical Equations Article

This lesson will reinforce your knowledge about chemical formulas and introduce the concept of balanced chemical equations. 

Balancing Chemical Equations Activity

How do you know if a chemical equation is balanced? What can you change to balance an equation? Play a game to test your ideas!

This provides a mathematical method of balancing chemical equations as an alternative to the conventional method (by inspection)

Version History

Lab - Law of Conservation of Mass (Day 2)

Learning intentions.

• To understand the Law of Conservation of Mass
• To understand sources of error in labs
• Every group takes out their lab procedures and reads them over.
• The teacher randomly creates pairs and trios of groups.
• Each group reads their procedures to the other group(s).
• The groups discuss the similarities and differences between their procedures.
• The groups create a single set of procedures, combining the best elements of their procedures.
• The groups discuss possible sources of error in their procedures.
• The groups choose a spokesperson who will report the procedures and sources of error to the class.

As a class, we will discuss the pros and cons of the procedures that were generated. Every group will then be given the chance to change their procedures to incorporate the best ideas that they have heard.

Run the lab using the sodium bicarbonate and vinegar provided by the teacher.

At the end of the lab, be sure to clean all your equipment by rinsing with water. It is safe to dispose of down the drain. Leave your equipment to drain on the drying racks at the back of the class.

You will need to use the Internet to find the answers to some of these questions.

• What is the chemical formula for the sodium bicarbonate (baking soda)?
• What is the chemical formula for the vinegar (weak acetic acid)?
• The chemical reaction between baking soda and vinegar proceeds in 2 steps. What are the balanced chemical reactions for these two steps?
• For each of the 2 steps, what is the type of reaction ?
• Watch the following video of burning steel wool. What do you think will happen to the mass as it burns? What actually happens? Why? Burning iron wool and change in mass (1:28) watch
• Required Parts
• Materials (as a list)
• Procedure (as a numbered list)
• Data and Results
• Future Work
• Properly controlled experiment
• Analysis, Questions, Conclusion, and Future Work
• Do you have a clear explanation for your results? Can you link the lab to the theory?
• Are your sources of error well identified?
• Do you answer the questions correctly?
• Does your conclusion answer the hypothesis?

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