write a hypothesis about chemical change when substances are mixed

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Chemical and Physical Changes of Matter

If you are confused about chemical and physical changes and how to tell them apart, you’ve come to the right place. Chemical and physical changes both are changes in the structure of matter . In a chemical change , a chemical reaction occurs and a new substance is produced. In a physical change , matter changes forms but does not change its chemical identity. So, the difference between chemical and physical changes is that a chemical change alters the chemical composition of matter, while a physical change does not

A chemical change produces a new substance, while a physical change alters the form of matter but not its chemical identity.

Chemical Changes

A chemical change involves a chemical reaction to produce a new product . It is a change at the molecular level of matter. Chemical bonds between atoms break and then form to connect different atoms.

Examples of Chemical Changes

In a chemical change, new product forms as atoms rearrange themselves. Chemical bonds are broken and reform to make new molecules. Examples of chemical changes include:

• Souring milk
• Digesting food
• Cooking an egg
• Baking a cake
• Rusting iron
• Mixing an acid and a base
• Burning a candle
• Mixing baking soda and vinegar

Physical Changes

A physical change is a change in matter that alters its form but not its chemical identity. The size or shape of matter often changes, but there is no chemical reaction. Phase changes are physical changes. These include melting, boiling, vaporization, freezing, sublimation and deposition. Breaking, crumpling, or molding matter also results in a physical change. Many physical changes are reversible.

• Examples of Physical Changes

Examples of physical changes include:

• Melting an ice cube
• Freezing an egg
• Boiling water
• Sublimation of dry ice into carbon dioxide gas
• Shredding paper
• Crushing a can
• Breaking a bottle
• Chopping vegetables
• Mixing sand and salt
• Making sugar crystals
• Dissolving sugar in water (the sugar mixes with the water, but can be recovered by evaporation or boiling)

How to Tell Chemical and Physical Changes Apart

The key to distinguishing between chemical and physical changes is determining whether there is a new substance that wasn’t there before. If you see signs of a chemical reaction, it’s probably a chemical change. Signs of a reaction include:

• Temperature change
• Color change
• Formation of a precipitate

If none of these signs are present, it’s a good bet a physical change occurred.

Are Physical Changes Reversible?

Some people use reversibility as a test for chemical and physical changes. The premise is that a physical change can be undone, while a chemical change can only be reversed by another chemical reaction. This is not a great test because there are too many exceptions. While you can melt and freeze an ice cube (a physical change), it’s much harder to reassemble shredded paper (another physical change).

Most physical changes can be reversed if energy is added. Some chemical changes are reversible, but only via another chemical reaction. For example, rusting of iron is a chemical change. Converting rust back into iron and oxygen is possible, but it requires a chemical reaction.

Practice Identifying Chemical and Physical Changes

Download and print this worksheet for practicing identifying chemical and physical changes. The worksheet and answer key are PDF files, or you can right-click, save, and print the PNG image.

[ PDF Worksheet ] [ Answer Key ]

Explore chemical and physical changes in greater detail and learn how they relate to chemical and physical properties of matter:

• Examples of Chemical Properties
• Is Dissolving Salt a Chemical or Physical Change?
• Examples of Physical Properties
• Atkins, P.W.; Overton, T.; Rourke, J.; Weller, M.; Armstrong, F. (2006).  Shriver and Atkins Inorganic Chemistry  (4th ed.). Oxford University Press. ISBN 0-19-926463-5.
• Chang, Raymond (1998).  Chemistry  (6th ed.). Boston: James M. Smith. ISBN 0-07-115221-0.
• Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001).  Organic Chemistry  (1st ed.). Oxford University Press. ISBN 978-0-19-850346-0.
• Kean, Sam (2010).  The Disappearing Spoon – And Other True Tales From the Periodic Table . Black Swan, London. ISBN 978-0-552-77750-6.
• Zumdahl, Steven S.; Zumdahl, Susan A. (2000).  Chemistry  (5th ed.). Houghton Mifflin. ISBN 0-395-98583-8.

Related Posts

Middle School Chemical Engineering For Girls

Great Activities for Middle School Outreach in Chemical Engineering

The Alka Seltzer Reaction

Introduction & motivation.

Chemical reactions are one of the primary focuses for Chemical Engineers. From synthesizing polymers to treating water to creating fertilizers, chemical reactions are important in nearly every aspect of daily life. One job of Chemical Engineers is to classify, understand, and control these reactions to speed them up or slow them down.

Chemical reactions occur when bonds within molecules are broken or formed. There are several things that signify that a chemical reaction took place. These include a change in color, the production of a gas or solid, and of course a change in chemical composition. The starting chemicals before a reaction are called the reactants , and the chemicals that are produced are called the products . The reaction in this activity involves using sodium bicarbonate and citric acid to produce water and carbon dioxide.

Reaction : HCO 3 – (aq) + H + (aq) → H 2 O (l) + CO 2 (g)

The tablets contain sodium bicarbonate (NaHCO 3 ) and citric acid. When the tablet is dissolved in water, bicarbonate (HCO 3 – ) and hydrogen ions (H + ) are formed. Once in solution, the two chemicals can then react according to the reaction listed above. For the reaction to occur, the HCO 3 – and H + must collide at the right angle with the right amount of energy. The chances of this happening are better when the tablet is crushed into more pieces since the molecules have more opportunities to collide and when the temperature is higher, since the molecules are moving faster.

In this activity, students will experiment with the reaction between Alka Seltzer tablets and water in different conditions. By changing temperature and the surface area available for reaction, students will begin to see what factors chemical engineers can control to get the desired result.

This activity introduces the reaction used for the Alka Seltzer Rockets activity, so it is typically performed before building rockets to understand the nature of the reaction before using it.

Chemical Safety:

• Sodium Bicarbonate
• Alka Seltzer tablets
• Large beakers
• Food coloring
• Stopwatches
• Metal spoons
• Thermometers

Before the experiment, ask students to hypothesize what will make the reaction go the fastest and what makes them think that. This can be anything, but try to seek answers with specific regard to the variables being changed in this activity.

The Effect of Temperature on Rate of Reaction

• Partially fill a large beaker with ice cubes. Fill the beaker with water up to the 250 mL mark with cold water and stir the ice water until the temperature equilibrates.
• Measure the temperature of the water and record it in the table.
• Add a tablet and record the time it takes for the tablet to react.
• Repeat 1-2 with room temperature water, then with hot water heated to 70 degrees C using a hot plate.

The Effect of Surface Area on Rate of Reaction

• A whole tablet
• A tablet broken into quarters
• A tablet ground into powder: Place the tablet it a piece of weighing paper (wax or parchment paper work as well) and break it either with your hands or crush it using the back of a metal spoon.
• Add 250 mL of water to a large beaker.
• Measure and record the temperature of the water and make sure it is consistent between trials.
• One student should be ready with a stopwatch and another student should be ready with the whole tablet. The student with the stopwatch should count to three and on three start the stopwatch. At the same time, the other student should drop the tablet into the water.
• Gently stir the water at a consistent speed and pattern.
• As soon as the last of the tablet disappears, yell “Stop!,” stop the stopwatch, and record the time in the table.
• Repeat Steps 2-6 with the quartered tablet and the crushed tablet.

At the end, collect and present all class data on the board. Highlight discrepancies and the general trend.

• Which combination of factors made the reaction go the fastest? The slowest? (Higher surface area and temperature make the reaction go faster. Since the reaction occurs on the surface of the tablet pieces, more access to it will make the reaction go faster because there are more molecules to make bumping together more likely. Higher temperature gives more energy to the molecules, meaning they are more likely to have enough energy for the reaction to continue. The opposite is true for the slowest rate – low surface area and temperature.)
• Why would we want reactions to happen faster or slower? (e.g. we want rusting reactions to be slower to protect metal products, but we want redox reactions that recharge our phone batteries to be fast.)
• Is there a limit to how fast we can make the reaction? Would we want to place a limit if there is not a physical one? (Reactions have maximum rates for a few reasons, like the amount of surface area available to react, if the mixture makes it difficult for molecules to move, etc. If the rate were increased too high, it becomes a safety concern! Sometimes reactions get too fast, too hot, and can’t be slowed down. This is a dangerous runaway reaction , the last thing a chemical engineer wants!)
• Why did any discrepancies come up in the data? What ways could we make our process better to limit those from affecting the class data as a whole? (Discrepancies come up from human error with measuring time, not having precise sizes of tablets, imprecise temperature control across trials, and how hard it is to see a reaction is finished! Let students get creative with suggesting improvements, but a few could include using a grid and knives to chop up tablets or putting the ground tablets through a sieve, using a robot to stir and observe the reaction, and putting the beakers in water baths.)
• We know Alka Seltzer is a medicine to make us feel better. Why might it be designed to fizz? (Fizzing helps the aspirin in the tablet quickly absorb into the bloodstream, making the medicine fast-acting. It might also make it more appetizing to drink!)

Additional Resources

• How Does Alka Seltzer Work?
• VIDEO: Why Does Alka Seltzer Fizz?
• ← Alka Seltzer Rockets
• Separations Activity →

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The Conservation of Matter During Physical and Chemical Changes

Matter makes up all visible objects in the universe, and it can be neither created nor destroyed.

Chemistry, Conservation, Earth Science, Geology, Physics

Water in Three States

Water can exist in three different physical states—as a gas, liquid, and a solid—under natural conditions on Earth. Regardless of its physical state, they all have the same chemical composition. Water is 2 hydrogen atoms bonded to 1 oxygen atom.

Photograph by OJO Images Ltd.

Matter makes up everything visible in the known universe, from porta-potties to supernovas . And because matter is never created or destroyed, it cycles through our world. Atoms that were in a dinosaur millions of years ago—and in a star billions of years before that—may be inside you today. Matter is anything that has mass and takes up space. It includes molecules , atoms, fundamental particles , and any substance that these particles make up. Matter can change form through physical and chemical changes, but through any of these changes matter is conserved . The same amount of matter exists before and after the change—none is created or destroyed. This concept is called the Law of Conservation of Mass . In a physical change, a substance’s physical properties may change, but its chemical makeup does not. Water, for example, is made up of two hydrogen atoms and one oxygen atom. Water is the only known substance on Earth that exists naturally in three states: solid, liquid, and gas. To change between these states, water must undergo physical changes. When water freezes, it becomes hard and less dense, but it is still chemically the same. There are the same number of water molecules present before and after the change, and water’s chemical properties remain constant. To form water, however, hydrogen and oxygen atoms must undergo chemical changes. For a chemical change to occur, atoms must either break bonds and/or form bonds. The addition or subtraction of atomic bonds changes the chemical properties of the substances involved. Both hydrogen and oxygen are diatomic —they exist naturally as bonded pairs (H 2 and O 2 , respectively). In the right conditions, and with enough energy, these diatomic bonds will break and the atoms will join to form H 2 O (water). Chemists write out this chemical reaction as:

2H 2 + O 2 → 2H 2 O

This equation says that it takes two molecules of hydrogen and one molecule of oxygen to form two molecules of water. Notice that there are the same number of hydrogen atoms and oxygen atoms on either side of the equation. In chemical changes, just as in physical changes, matter is conserved. The difference in this case is that the substances before and after the change have different physical and chemical properties. Hydrogen and oxygen are gases at standard temperature and pressure, whereas water is a colorless, odorless liquid. Ecosystems have many chemical and physical changes happening all at once, and matter is conserved in each and every one—no exceptions. Consider a stream flowing through a canyon—how many chemical and physical changes are happening at any given moment? First, let’s consider the water. For many canyon streams, the water comes from higher elevations and originates as snow. Of course that’s not where the water began —it’s been cycled all over the world since Earth first had water. But in the context of the canyon stream, it began in the mountains as snow. The snow must undergo a physical change —melting—to join the stream. As the liquid water flows through the canyon, it may evaporate (another physical change) into water vapor. Water gives a very clear example of how matter cycles through our world, frequently changing form but never disappearing. Next, consider the plants and algae living in and along the stream. In a process called photosynthesis , these organisms convert light energy from the sun into chemical energy stored in sugars. However, the light energy doesn’t produce the atoms that make up those sugars—that would break the Law of Conservation of Mass —it simply provides energy for a chemical change to occur. The atoms come from carbon dioxide in the air and water in the soil. Light energy allows these bonds to break and reform to produce sugar and oxygen, as shown in the chemical equation for photosynthesis :

6CO 2 + 6H 2 O + light → C 6 H 12 O 6 (sugar)+ 6O 2

This equation says that six carbon dioxide molecules combine with six water molecules to form one sugar molecule and six molecules of oxygen. If you added up all the carbon, hydrogen, and oxygen atoms on either side of the equation, the sums would be equal; matter is conserved in this chemical change. When animals in and around the stream eat these plants, their bodies use the stored chemical energy to power their cells and move around. They use the nutrients in their food to grow and repair their bodies—the atoms for new cells must come from somewhere. Any food that enters an animal’s body must either leave its body or become part of it; no atoms are destroyed or created. Matter is also conserved during physical and chemical changes in the rock cycle. As a stream carves deeper into a canyon, the rocks of the canyon floor don’t disappear. They’re eroded by the stream and carried off in small bits called sediments. These sediments may settle at the bottom of a lake or pond at the end of the stream, building up in layers over time. The weight of each additional layer compacts the layers beneath it, eventually adding so much pressure that new sedimentary rock forms. This is a physical change for the rock, but with the right conditions the rock may chemically change too. In either case, the matter in the rock is conserved. The bottom line is: Matter cycles through the universe in many different forms. In any physical or chemical change, matter doesn’t appear or disappear. Atoms created in the stars (a very, very long time ago) make up every living and nonliving thing on Earth—even you. It’s impossible to know how far and through what forms your atoms traveled to make you. And it’s impossible to know where they will end up next. This isn’t the whole story of matter, however, it’s the story of visible matter. Scientists have learned that about 25 percent of the universe’s mass consists of dark matter—matter that cannot be seen but can be detected through its gravitational effects. The exact nature of dark matter has yet to be determined. Another 70 percent of the universe is an even more mysterious component called dark energy, which acts counter to gravity. So “normal” matter makes up, at most, five percent of the universe.

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Rate the lesson plan, field trips, changes in matter.

Arches National Park , Canyonlands National Park , Hovenweep National Monument , Natural Bridges National Monument

Program Outline

Essential Question:  What physical and chemical changes in matter can be observed in nature?

Utah Science with Engineering Education Standards:

Strand 5.2: PROPERTIES AND CHANGES OF MATTER All substances are composed of matter. Matter is made of particles that are too small to be seen but still exist and can be detected by other means. Substances have specific properties by which they can be identified. When two or more different substances are combined a new substance with different properties may be formed. Whether a change results in a new substance or not, the total amount of matter is always conserved.

Standard 5.2.1 Develop and use a model to describe that matter is made of particles on a scale that is too small to be seen. Emphasize making observations of changes supported by a particle model of matter.

Standard 5.2.3 Plan and carry out investigations to determine the effect of combining two or more substances. Emphasize whether a new substance is or is not created by the formation of a new substance with different properties.

Matter is the “stuff” of the universe. Everything that has mass and volume, no matter how small, is made of matter. Air, water, rocks, trees, stars, and animals all consist of matter. Matter can exist as a solid, liquid, or gas (or plasma) and can change in many ways. Physical changes are those in which the shape, size, or state of the matter changes, but the substance is still essentially the same. For example, chopping up a carrot or ice melting into water are both physical changes. Chemical changes are those where one or more substances are combined to produce a new substance. At the end of a chemical change, you have a new substance. Burning a piece of paper would be a chemical change, as would baking a cake.

Ozone is an invisible gas made of 3 oxygen atoms. High levels of ozone in the lower atmosphere can cause human health problems and can contribute to the greenhouse effect. Car exhaust, the result of a chemical change in fuel, is a major contributor of ozone to the lower atmosphere. Canyonlands National Park also monitors ozone at ground level. Scientists monitor ozone levels to provide real time data on air quality to the public.

Unlike in the lower atmosphere, Ozone plays a positive role in the upper atmosphere. The stratospheric ozone layer blocks much of the sun’s UV light from reaching the earth’s surface. Normal quantities of UV light are good for such things as plant growth and suntans. But, increased UV light from a damaged ozone layer leads to increased incidences of skin and eye disease in humans as well as damage to some wildlife and plants.

The single largest factor in the destruction of the ozone layer is a family of chemicals called chlorofluorocarbons (CFCs). This reaction is a chemical change in the ozone molecule. Humans used CFCs to manufacture hundreds of different products, including Styrofoam packaging, aerosol spray cans, and as the coolants in refrigerators and air conditioners. Since 1987, The Montreal Protocol outlawed the use of CFCs in the United States and many countries. Even if all countries quit using CFCs, however, they will linger in the upper atmosphere for decades. UV light is also monitored, which indirectly reflects the condition of the upper atmosphere ozone layer.

Weathering is the physical breakup of rocks, and is often confused with erosion, which is the removal of rock by gravity, wind, and water once it has weathered. This confusion is understandable because they are often intertwined. There are two types of weathering: physical and chemical. Physical weathering breaks the rock into smaller pieces, but the components remain virtually the same. An example might be ice expanding a crack on the side of an arch. Each time the water freezes, the crack gets bigger, but both pieces are still sandstone. Eventually, the piece falls out of the arch, which is erosion. The piece is still sandstone, just smaller and no longer connected to the arch.

Chemical weathering involves breaking down of the minerals that hold the individual grains of sand together. On the Colorado Plateau, this breakdown is often caused by rainwater, picking up CO2 in the atmosphere and becoming slightly acidic. This weak carbonic acid reacts with the calcium carbonate in the stone, slowly eating away at the rock. Chemical weathering dissolves approximately one sand grain per year from the rock. Wind and rain then removes these tiny grains. In places on the stone where water stands, this process speeds up, causing depressions to slowly deepen. Thus, desert potholes grow deeper over time.

PH is the measurement of hydrogen (H+) and hydroxyl (OH-) in a solution that contains water. The pH scale is typically measured from 0 to 14. A substance with a pH of 7 is a neutral solution, having equal numbers of H+ and OH- ions. Solutions with more H+ ions are acidic and will have a pH between 0 and 7. Solutions with more OH- ions are basic and will have a pH between 7 and 14. pH will increase or decrease when the proportions of each ion change as solutions react with other substances. An acid mixed with a base will neutralize both solutions and bring the pH closer to 7.

  Pre-Trip Activity

  field trip station #1,   field trip station #2,   field trip station #3.

• Cryptobiotic Soil Crust filaments forming clumps – Sand sticks to the cyanobacteria, which holds it in clumpy shapes, but individual sand grains do not change
• Washes next to the path – water carries sand grains away, but those grains are still sand
• Sand and gravel in the path 
• Footprints or animal tracks in the sand – Sand is moved around, but the individual sand grains do not change
• Cairns on trail - Humans moved these rocks, but they are still rocks
• Animal holes - Sand is moved around by the animals but it remains sand.
• Arches forming - Rocks that fall away and increase the opening of an arch are smaller pieces of the same rock
• Branches falling off trees/ branches lining trail - The wood is no longer connected to the living tree, but it remains wood
• Rocks breaking away from other rocks – New rocks are smaller, but remain rocks
• Pine cones/needles dropping from the tree
• Rain falling from the sky – When water falls as rain, it changes from a gas to a liquid, which is a change in state. It is still water.
• Snow in the mountains melting – Snow melting from a solid to a liquid is a change of state. It is still water.
• Animals eating and digesting pine nuts
• Berries /leaves/ logs rotting or decomposing into smaller organic matter – decomposers like bacteria and insects digest once-living material creating waste. (a new substance)
• Wood beetle eating trails in the logs
• Animals eating berries/nuts/leaves/seeds
• Water poured on dormant moss, turning it green – Moss turns green as it begins to photosynthesize and produce chlorophyl
• Cryptobiotic soil growing and fixing nitrogen – Cyanobacteria uses photosynthesis to grow. Growth is chemical change because something new is created. In addition, one of crypto's most important jobs is to pull nitrogen molecules from the air and break them apart so plants can absorb the individual nitrogen atoms. 
• Pine trees growing pinecones
• Juniper berries growing berries
• Leaves turning green through photosynthesis, turning red/yellow when photosynthesis stops
• Yuccas growing flowers
• Cacti growing spines
• Potholes are holes in the sandstone bedrock
• Arches getting larger
• Natural holes in the sandstone wall
• Airplanes/Cars flying overhead or driving along the road – Engines burn fuel, creating heat, which we have harnessed to power engines
• Burned tree limbs – evidence combustion occurred
• The red color of the surrounding sandstone– the iron in the rocks is rusting.

  Post-Trip Activity

References and resources,   references, lesson plans.

Last updated: April 8, 2022

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• Fifth Grade

Lesson 4.1 - Conservation of Mass

Lesson overview for teachers.

View the video below to see what you and your students will do in this lesson. 

Youtube ID: AqrzrUVcA50

Lesson Plan (PDF)   |   Student Activity Sheet (PDF)   |  Student Activity Sheet Answers (PDF)   |   Student Reading (PDF)   |   Teacher Background (PDF)   |   Connections to NGSS (PDF)

Students will be able to make measurements showing that whether the process is a change of state, dissolving, or a chemical reaction, the total mass of the substances does not change. 

Note: In the demonstrations and activities in this lesson, substances will be weighed before and after various processes have occurred – either melting, dissolving, or a chemical reaction. The basic principle students should observe and conclude is that mass is conserved in these processes, so the mass should not change. Students may observe slight variations of plus or minus 0.1 grams, depending on the sensitivity of the balance or whether the mass is actually somewhere between two values. If there is a minor change in mass, explain to students that small differences may be caused by a slight lack of precision in the scale readout, or by errors in the weighing methods, but that the overall results suggest that mass is conserved in all of these processes. 

Key Concepts

• When a substance changes state, the mass of the substance does not change.
• When a substance dissolves in a liquid, the total mass of the substance and the liquid it dissolves in does not change.
• When substances react to form new substances as products, the mass of the products is the same as the mass of the reactants.

NGSS Alignment

• NGSS 5-PS1-2:  Measure and graph quantities to provide evidence that regardless of the type of change that occurs when heating, cooling, or mixing substances, the total weight of matter is conserved.

Note: In this lesson, students measure and observe that mass is conserved during the processes of melting, dissolving, and chemical change. The students will not make a graph. 

• Students check to see whether the mass of ice and water in a cup changes as the ice melts.
• Students also test whether the combined mass of sugar and water changes after sugar is dissolved in the water.
• As a demonstration, students will observe that a precipitate forms in a reaction between solutions of magnesium sulfate and sodium carbonate, and that the mass of the products is the same as the mass of the reactants.

Download the student activity sheet (PDF)  and distribute one per student when specified in the activity. The activity sheet will serve as the Evaluate component of the 5-E lesson plan.  

Make sure you and your students wear properly fitting safety goggles.  Sodium carbonate may cause skin and serious eye irritation. Follow all safety precautions regarding the use, storage, and disposal of sodium carbonate. 

Clean-up and Disposal 

Remind students to wash their hands after completing the activity. All common household or classroom materials can be saved or disposed of in the usual manner. 

Materials needed for each group

• 1 Clear plastic cup
• 1 Teaspoon of sugar

Materials for the ENGAGE demonstration 

Materials for the extend demonstration .

• 2 Clear plastic cups
• Sodium carbonate
• Magnesium sulfate (Epsom salt)
• Graduated cylinder

1.  Do a demonstration to show that melting ice in water does not cause the mass of the combined water and ice to change.  

Question to investigate: will the combined mass of water and ice stay the same as the ice in the cup melts  .

• Pour water into a clear plastic cup so that it is about 1/3-full.
• Add 1 piece of ice.

Ask students: 

• If we weigh this cup with the water and ice, do you think the combined mass will change as the ice melts? No.
• Why or why not? Because the ice is just melting. It is still the same amount of water, but it’s just changing from a solid to a liquid. It should have the same mass.

Note: It is possible that some water may evaporate from the cup as the ice melts, causing the contents of the cup to weigh a little less at the end of the process. On the other hand, any water condensing on the outside of the cup could make it weigh a little more. Neither of these factors is likely to contribute much to the combined mass measurements, since very little water will evaporate or condense in the time it takes for the ice to melt.  

• Weigh the cup, water, and ice. Record the combined mass on the activity sheet. 

While the ice melts, have students conduct the experiment below. When they are done with that experiment and the ice has melted, show students the mass of the water and melted ice.  

Expected results 

The mass should be the same. 

Give each student an Activity Sheet (PDF). Students will record their observations and answer questions about the activity on the activity sheet. 

2. Have students weigh water and sugar before and after the sugar dissolves.

Question to investigate:  will the combined mass of sugar and water be the same after the sugar dissolves in the water  , ask students:.

• If you weigh a cup of water and a teaspoon of sugar and then dissolve the sugar in the water, do you think the mass will change? No.
• Why or why not? Because the same amount of sugar is still there. The solid sugar crystals break apart in water as the sugar dissolves, but the individual sugar particles or molecules are still present and do not change as a result of dissolving in the water. The combined mass of the sugar and water shouldn’t change.

Materials for each group: 

• Add water to the cup until it is about 1/4-full.
• Add 1 teaspoon of sugar to the water. 
• Weigh the cup with the water and sugar and record the mass.

Note: Evaporating water could make the water and sugar weigh a little less. This will probably not be a factor since very little water will evaporate in the time it takes for the sugar to dissolve. 

• Carefully swirl the cup to help the sugar dissolve.
• When the sugar is dissolved, place the cup back on the scale to measure the mass.

Expected results

The combined mass does not change.

• Did the mass change? No.

3. Do a demonstration to see if the mass changes during a chemical reaction.

Question to investigate: will the mass change when reactants combine to form products in a chemical reaction  , materials for the demonstration.

• In a clear plastic cup add 50 mL of water and 1 teaspoon of Epsom salt (magnesium sulfate). Gently swirl the cup so that the Epsom salt dissolves.
• Measure the combined mass of the cup with the Epsom salt solution in it. Tell students the mass and have them record it.
• To another cup, add 50 mL of water and add 1 teaspoon of sodium carbonate. Gently swirl the cup until the sodium carbonate dissolves.
• Measure the combined mass of the cup with the sodium carbonate solution in it. Tell students the mass and have them record it.
• Have students add the two masses together and record and announce the sum. 
• Hold the cups up so students can see them and then slowly and carefully add the sodium carbonate solution to the Epsom salt solution.

A white solid will form. At first the solid may appear or look like clouds of white particles floating in the liquid, but the particles should eventually settle out to form a solid precipitate at the bottom of the cup.  

Tell students that a chemical reaction took place and that a new substance, a solid, was formed.  

• Do you think the total mass of the two cups, the combined solutions, with the white solid will be more, less or the same as it was before the reaction took place? Same.

• Place both cups on the scale to measure the total mass. 

The total mass should be the same as the sum of the individual masses recorded before the contents of the cups were combined and the reaction took place.

Explain that the reactants have been transformed into a new substance, but that all the individual atoms making up the reactants are still present in the products. That’s why the mass stays the same.

4. Show an animation to help explain why mass is conserved in melting, dissolving, and in a chemical change.

Show the animation  Conservation of Mass in Physical and Chemical Changes.

Explain that whether a process involves melting, dissolving, or a chemical reaction, all the atoms that were there before the process takes place are still there after any changes have occurred, so the overall mass stays the same. 

5. Show an animation to help explain why mass is conserved when water is frozen.

Remind students that they observed that the overall mass of water and ice stayed the same as the ice melted.

• When water freezes to form ice, it takes up more room in the container, but does its mass change?  Even though the volume of water changes as it becomes ice, the mass of the water should remain the same before and after it turns into ice.

Show the animation  Mass is Conserved in Freezing . 

Explain that even though water takes up more room when frozen, the same number of water molecules are still there so the mass stays the same. 

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