Chemical Change and Law of Conservation of Mass

The Law of Conservation of Mass states that mass is neither created nor destroyed in an ordinary chemical reaction. When an iron nail rusts, it seems to get heavier in mass. Does the iron nail follow the Law of Conservation of Mass?

• No, rusting is an exception to the Law of Conservation of Mass.

• No, since rusting is a chemical change it does not follow the Law of Conservation of Mass.

• Yes, the iron rearranges its protons so that the masses are the same before and after the reaction and rusting follows the Law of Conservation of Mass.

• Yes, iron chemically combines with the oxygen in the air so if you add the oxygen into the mass of the chemicals before the reaction, the mass after the reaction is the same.

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

When wood burns, a small amount of ashes is made. Why is the mass of the wood before the fire not equal to the mass of the ashes after the reaction?

• The mass of the wood has been destroyed.

• The mass of the wood and the oxygen that allowed it to burn will equal the mass of the ashes and the gas given off during the burning.

• The mass of the wood and the ashes equals the mass of the oxygen and the smoke given off during the time that the wood burned.

• The wood has holes in it so it is actually lighter in mass than it appears. The mass of just the wood will equal the mass of just the ashes after the burning.

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

How does the Law of Conservation of Mass apply to a burning candle?

• The amount of wax before the reaction equals the amount of energy afterwards.

• The mass of the wick before the reaction equals the mass of the smoke afterwards.

• The mass of the wick, wax that burned and the oxygen that helped the flame before the reaction equals the mass of the smoke and the gases released after the reaction.

• The mass of the molecules of the candle before the reaction equals the mass of the candle and burned wick after the reaction.

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NGSS.MS-PS1-2

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Two solutions are mixed and react to produce the solid precipitate AgCl. The chemical reaction is shown below. According to the Law of Conservation of Mass, what happens to the TOTAL mass as the reaction takes place?

The mass will ​ (a)   .

• remain constant

• increase

• decrease

• first decrease and then increase

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

The law of conservation of mass says the bonds between atoms are​ (a)   to form new substances with new properties.

• fused

• created

• destroyed

• broken and rearranged

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NGSS.MS-PS1-5

Examine the experiment shown, then fill in the blanks. The experiment shows a ​ (a)   system. The mass at the end of the experiment will be ​ (b)   the initial mass.

• closed

• open

• equal to

• less than

• greater than

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

In the following reaction: 2NaN₃ decomposes to form 2Na + 3N₂. If 500 grams of NaN₃ decomposes to form 323 grams of N₂. How much Na is produced?

• 100 grams

• 177 grams

• 323 grams

• 500 gram

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

In a reaction A + B ----> C,  reactant A has 5g and product  C has 9g. How many grams does reactant B should have? 

• 4g

• 5g

• 9g

• 14g

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NGSS.MS-PS1-5

Matter can not be created nor destroyed: it can only be

• Destroyed a little bit

• Invisible

• Transformed, changed

• None of the above

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NGSS.MS-PS1-5

If a reaction starts with a total 75g of reactants it should produce 

• a total of 40g of products

• a total of 75g of products

• a total of 180 g of products

• None of the above

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NGSS.MS-PS1-5