Chemical reactions and conservation of mass

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Lesson notes

What is a Chemical Reaction?

A **chemical reaction** involves the **rearrangement of atoms** to form new substances.
Reactants are the starting substances; products are the substances formed.
During a reaction, **chemical bonds** break and new bonds form.
The atoms themselves are **not created or destroyed**—they just get rearranged.

Law of Conservation of Mass

The **law of conservation of mass** states that mass is neither created nor destroyed in a chemical reaction.
The total mass of the **reactants** equals the total mass of the **products**.
This law was established by **Antoine Lavoisier** in the late 18th century.
It applies to **closed systems** where no matter enters or leaves.

Balancing Chemical Equations

A **balanced chemical equation** has the same number of each type of atom on both sides.
Coefficients are used to balance the number of atoms.
Example: 2H₂ + O₂ → 2H₂O (4 H atoms and 2 O atoms on each side).
Balancing ensures the **conservation of mass** is satisfied.

Evidence of a Chemical Reaction

Common signs include **color change**, **gas production**, **temperature change**, and **formation of a precipitate**.
These observations indicate that new substances have formed.
Even though mass is conserved, the **properties** of the products differ from the reactants.

Open vs. Closed Systems

In a **closed system**, no matter can enter or leave, so mass is conserved.
In an **open system**, matter can escape (e.g., gas), so mass may appear to change.
To observe conservation of mass, experiments must be done in a **closed container**.

Conservation of Atoms

The **number of atoms** of each element remains constant during a reaction.
Atoms are simply **rearranged** into new molecules.
This is why balancing equations is possible—the atoms are all accounted for.

Real-World Examples

Burning wood: the mass of wood + oxygen equals the mass of ash + carbon dioxide + water vapor.
Rusting iron: iron + oxygen + water → rust; the total mass increases because oxygen is added.
In a closed container, the mass before and after any reaction is **exactly the same**.

Hydrogen atom: 1 proton, 0 neutrons, 1 electron. Atoms are the basic units that are conserved in reactions.

Oxygen atom: 8 protons, 8 neutrons, 2 electrons in first shell and 6 in second. Atoms rearrange but are not lost.

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Practice questions

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1.What does the law of conservation of mass state?
Easy
- AMass can be created but not destroyed.
- BMass can be destroyed but not created.
- CMass cannot be created or destroyed in a chemical reaction.
- DMass is created when a chemical reaction occurs.
2.Who is credited with confirming the law of conservation of mass?
Easy
- AIsaac Newton
- BAntoine Lavoisier
- CDmitri Mendeleev
- DJohn Dalton
3.In a chemical reaction, the total mass of the reactants is ________ the total mass of the products.
Easy
- Agreater than
- Bless than
- Cequal to
- Dunrelated to
4.If 10 grams of hydrogen gas react with 80 grams of oxygen gas to form water, what is the total mass of water produced?
Medium
- A70 grams
- B80 grams
- C90 grams
- D100 grams
5.During a chemical reaction, atoms are rearranged but the total number of each type of atom remains the same. This is an example of:
Medium
- Aconservation of energy
- Bconservation of mass
- Cconservation of charge
- Dconservation of volume
6.In an open system, the mass of the products may appear less than the mass of the reactants because:
Hard
- Amass is destroyed
- Bsome products are gases that escape
- Cthe law of conservation of mass does not apply
- Datoms are lost
7.Which of the following equations demonstrates conservation of mass?
Medium
- AH₂ + O₂ → H₂O
- B2H₂ + O₂ → 2H₂O
- CH₂ + O₂ → 2H₂O
- D2H₂ + 2O₂ → 2H₂O
8.A student burns 12 g of magnesium in air to produce magnesium oxide. If the mass of the product is 20 g, what mass of oxygen from the air combined with the magnesium?
Hard
- A8 g
- B12 g
- C20 g
- D32 g

More questions

9.In a chemical reaction, the number of atoms of each element is:
Easy
- Aalways increased
- Balways decreased
- Cconserved (same before and after)
- Dchanged depending on the reaction
10.When iron rusts, the mass of the rusted iron is greater than the original iron. This is because:
Medium
- Airon gains mass from the air
- Bmass is created
- Cthe law of conservation of mass is violated
- Diron expands
11.Which statement is true regarding the conservation of mass in nuclear reactions?
Hard
- AMass is always conserved exactly.
- BMass can be converted to energy; total mass-energy is conserved.
- CMass is not conserved at all.
- DNuclear reactions violate the law of conservation of mass.
12.Which of the following is NOT a reason why mass appears not to be conserved in some experiments?
Easy
- AGases escaping from an open container
- BHeat loss to the surroundings
- CThe reaction is not complete
- DMass is actually created
13.A student places 25.0 g of sodium hydrogen carbonate (NaHCO₃) in a sealed plastic bag and adds 15.0 g of acetic acid (CH₃COOH). The bag is sealed immediately. After the reaction finishes, the bag and its contents are weighed. Which mass is most likely recorded?
Medium
- A40.0 g
- Bless than 40.0 g because gas escaped
- Cmore than 40.0 g because a solid formed
- D10.0 g because the reactants disappeared
14.The diagram shows an experimental setup for investigating conservation of mass. Aqueous solutions of lead(II) nitrate and potassium iodide are mixed in a conical flask that is stoppered. The total mass before mixing is 150.0 g. After the reaction, the mass is 150.0 g. Which conclusion is correct?
Medium
- AMass is conserved because the flask was sealed.
- BMass is not conserved because a precipitate formed.
- CMass increased because the precipitate is heavier.
- DMass decreased because gas was produced.
15.In an experiment, 2.43 g of magnesium ribbon is burned in a crucible open to the air. The mass of the white ash (magnesium oxide) produced is 4.03 g. What mass of oxygen from the air combined with the magnesium?
Hard
- A1.60 g
- B2.43 g
- C4.03 g
- D6.46 g
16.The table shows the masses of reactants and products in four different reactions. Which reaction does NOT obey the law of conservation of mass? | Reaction | Mass of Reactants (g) | Mass of Products (g) | |----------|-----------------------|----------------------| | A | 10.0 | 10.0 | | B | 25.5 | 25.5 | | C | 12.0 | 11.9 | | D | 8.0 | 8.0 |
Medium
- AReaction A
- BReaction B
- CReaction C
- DReaction D
17.Which of the following is the best example of the law of conservation of mass?
Easy
- AA 10 g ice cube melts to form 10 g of liquid water.
- BA 10 g log burns to produce 10 g of ash.
- CA 10 g iron nail rusts to form 12 g of rust.
- DA 10 g sugar cube dissolves in water to form 10 g of solution.
18.A student investigates the reaction between hydrochloric acid and sodium carbonate. The apparatus is an open beaker on a balance. The initial mass is 50.0 g. After adding the acid, fizzing occurs and the final mass is 47.5 g. What is the most likely reason for the decrease?
Hard
- ACarbon dioxide gas escaped from the beaker.
- BThe acid evaporated.
- CThe reaction absorbed mass from the air.
- DThe balance malfunctioned.
19.A chemical equation is balanced when:
Medium
- Athe number of atoms of each element is the same on both sides.
- Bthe total mass of reactants equals the total mass of products.
- Cthe number of molecules is the same on both sides.
- Dboth A and B are true.
20.In a chemical reaction, atoms are:
Easy
- Acreated.
- Bdestroyed.
- Crearranged.
- Dconverted into energy.
21.The graph shows the mass of a reaction mixture in an open flask over time. At 2 minutes, the mass drops sharply. Which event most likely occurred at 2 minutes?
Hard
- AA gas was produced and escaped.
- BA solid precipitate formed.
- CThe temperature increased.
- DThe flask was sealed.
22.A student claims that the law of conservation of mass does not apply when a candle burns because the candle gets smaller. How should the student's claim be evaluated?
Medium
- AThe claim is correct; mass is not conserved in burning.
- BThe claim is incorrect; the mass of the candle and oxygen consumed equals the mass of products (including gases).
- CThe claim is partially correct; mass is conserved only if the candle is in a sealed container.
- DThe claim is incorrect; mass is conserved because the candle's mass decreases but the air's mass increases equally.
23.In a closed system, 10.0 g of calcium carbonate (CaCO₃) is heated and decomposes into calcium oxide (CaO) and carbon dioxide (CO₂). After heating, the mass of the solid residue (CaO) is 5.6 g. What mass of CO₂ was produced?
Hard
- A4.4 g
- B5.6 g
- C10.0 g
- D15.6 g
24.Which scientist is credited with confirming the law of conservation of mass through careful experiments?
Easy
- AAntoine Lavoisier
- BIsaac Newton
- CJohn Dalton
- DDmitri Mendeleev
25.A student places 5.0 g of sodium hydrogen carbonate in a sealed plastic bag and adds 3.0 g of acetic acid. The bag is sealed immediately. After the reaction stops, the mass of the bag and its contents is measured. What is the expected mass?
Medium
- A8.0 g
- B5.0 g
- C2.0 g
- Dmore than 8.0 g
26.A student investigates the reaction between hydrochloric acid and sodium carbonate in an open beaker on a balance. The initial mass is 50.0 g. After adding acid, fizzing occurs and the mass decreases to 47.5 g. What is the most likely explanation for the decrease?
Medium
- AGas (carbon dioxide) escaped from the beaker.
- BMass was destroyed during the reaction.
- CThe balance was not calibrated correctly.
- DWater evaporated from the solution.
27.The table shows the masses of reactants and products in four different reactions. Which reaction does NOT obey the law of conservation of mass?
Medium
- AReaction A: reactants 10 g, products 10 g
- BReaction B: reactants 15 g, products 15 g
- CReaction C: reactants 20 g, products 20 g
- DReaction D: reactants 25 g, products 24 g
28.In a closed system, 10.0 g of calcium carbonate (CaCO₃) is heated and decomposes into calcium oxide (CaO) and carbon dioxide (CO₂). After heating, the mass of the solid residue is 5.6 g. What mass of carbon dioxide gas was produced?
Medium
- A4.4 g
- B5.6 g
- C10.0 g
- D15.6 g
29.A student designs an experiment to verify conservation of mass for the reaction between iron and sulfur. They heat a mixture of 7.0 g of iron powder and 4.0 g of sulfur powder in a test tube. After reaction, the mass of the product (iron sulfide) is 11.0 g. Which statement best explains this result?
Hard
- AMass is not conserved because some iron was lost.
- BMass is conserved because 7.0 + 4.0 = 11.0.
- CThe result is incorrect because sulfur should have a different mass.
- DThe product mass should be less than 11.0 g due to gas loss.
30.The diagram shows an experimental setup for investigating conservation of mass. Aqueous solutions of lead(II) nitrate and potassium iodide are mixed in a conical flask. The flask is stoppered. The total mass before mixing is 150 g. After mixing, a yellow precipitate forms. What is the total mass after reaction?
Medium
- Aless than 150 g
- B150 g
- Cmore than 150 g
- Dimpossible to determine
31.A student places 25.0 g of sodium hydrogen carbonate (NaHCO₃) in a sealed plastic bag and adds 15.0 g of acetic acid (CH₃COOH). The bag is sealed immediately. After the reaction, the bag is opened and the mass of the contents is found to be 39.0 g. What is the mass of gas that escaped when the bag was opened?
Hard
- A1.0 g
- B10.0 g
- C40.0 g
- D0 g
32.The law of conservation of mass states that in a chemical reaction, mass is neither created nor destroyed. This means that the total mass of the reactants is:
Easy
- Aequal to the total mass of the products
- Bgreater than the total mass of the products
- Cless than the total mass of the products
- Dunrelated to the total mass of the products
33.Who is credited with confirming the law of conservation of mass through careful experiments in the late 18th century?
Easy
- AAntoine Lavoisier
- BJohn Dalton
- CDmitri Mendeleev
- DIsaac Newton
34.The diagram shows a Bohr model of an atom. Which particle is located in the nucleus and has a positive charge?
Medium
- Aproton
- Bneutron
- Celectron
- Dion
35.In the particle model of matter, which state of matter has particles that are closely packed in a regular pattern and only vibrate in fixed positions?
Medium
- Asolid
- Bliquid
- Cgas
- Dplasma
36.Which of the following statements about atoms in a chemical reaction is true?
Easy
- AAtoms are rearranged to form new substances.
- BAtoms are created during the reaction.
- CAtoms are destroyed during the reaction.
- DAtoms change into different elements.
37.The table shows the masses of reactants and products in four different reactions. Which reaction does NOT obey the law of conservation of mass? | Reaction | Mass of reactants (g) | Mass of products (g) | |----------|----------------------|----------------------| | A | 10.0 | 10.0 | | B | 15.5 | 15.5 | | C | 20.0 | 19.8 | | D | 25.0 | 25.0 |
Hard
- AA
- BB
- CC
- DD
38.A student places 25.0 g of sodium hydrogen carbonate (NaHCO₃) in a sealed plastic bag and adds 15.0 g of acetic acid (CH₃COOH). The bag is sealed immediately. After the reaction stops, the mass of the bag and its contents is measured. What should the mass be?
Hard
- A40.0 g
- Bless than 40.0 g
- Cmore than 40.0 g
- D10.0 g
39.A student mixes 25.0 g of sodium sulfate solution with 15.0 g of barium chloride solution in a sealed flask. After the reaction, the flask and contents are weighed. The total mass is 40.0 g. Which statement best explains this result?
Medium
- AMass is conserved because the system is closed.
- BThe reaction did not occur because mass remained the same.
- CMass was lost because a gas escaped.
- DThe law of conservation of mass only applies to open systems.
40.The table shows the mass of reactants and products for four reactions. Which reaction does NOT obey the law of conservation of mass? | Reaction | Mass of Reactants (g) | Mass of Products (g) | |----------|----------------------|----------------------| | A | 10.0 | 10.0 | | B | 15.5 | 15.5 | | C | 20.0 | 19.5 | | D | 12.0 | 12.0 |
Hard
- AA
- BB
- CC
- DD

Revision notes

## Chemical Reactions and Conservation of Mass ### Key Idea In a chemical reaction, atoms rearrange to form new substances. The **total number of atoms** and **total mass** stay the same — mass is conserved. ### Law of Conservation of Mass - Mass cannot be created or destroyed in a chemical reaction. - Mass of reactants = mass of products. - Discovered by Antoine Lavoisier in the late 1700s. ### Balancing Equations - To show conservation, chemical equations must be balanced: same number of each type of atom on both sides. - Example: \(2H_2 + O_2 \rightarrow 2H_2O\) (4 H atoms, 2 O atoms on each side). ### Important Notes - The law holds for **closed systems** (no matter enters or leaves). - In nuclear reactions, mass can convert to energy (not covered here). - For middle school, assume mass is always conserved in chemical reactions. ### How to Study 1. Identify reactants and products. 2. Count atoms of each element on both sides. 3. Adjust coefficients (numbers in front) to balance. 4. Check that total mass is equal.

Chemical reaction: A process where atoms rearrange to form new substances.
Conservation of mass: The principle that mass is neither created nor destroyed in a chemical reaction; total mass of reactants equals total mass of products.
Reactants: Starting substances in a chemical reaction.
Products: Substances formed as a result of a chemical reaction.
Balanced equation: A chemical equation with the same number of each type of atom on both sides.
Coefficient: A number placed before a chemical formula in an equation to indicate the number of molecules or atoms.

Flashcards

What does the law of conservation of mass state?

In a closed system, mass cannot be created or destroyed; it remains constant over time.

Who is credited with confirming the law of conservation of mass?

Antoine Lavoisier in the late 18th century.

In a chemical reaction, how does the total mass of reactants compare to the total mass of products?

They are equal; mass is conserved.

What happens to atoms during a chemical reaction?

Atoms rearrange to form new substances, but no atoms are created or destroyed.

Why must chemical equations be balanced?

To show that the number of atoms of each element is the same on both sides, reflecting conservation of mass.

What is a closed system in terms of mass conservation?

A system that does not allow matter to enter or leave.

Does the law of conservation of mass apply to nuclear reactions?

No, because mass can be converted to energy in nuclear reactions (e.g., annihilation).

What is stoichiometry?

The calculation of reactants and products in chemical reactions, based on conservation of mass.

If 10 grams of hydrogen react with 80 grams of oxygen, what is the mass of water produced?

90 grams, assuming complete reaction and no loss.

What does it mean to say mass is conserved in a chemical reaction?

The total mass of the reactants equals the total mass of the products.

Can mass be created or destroyed in ordinary chemical reactions?

No, mass is only rearranged; atoms are not created or destroyed.

What is the difference between an open system and a closed system?

An open system allows matter to enter or leave; a closed system does not.

Why is the law of conservation of mass important for balancing equations?

It ensures that the same number of each type of atom appears on both sides of the equation.

What happens to the total number of atoms during a chemical reaction?

It remains constant; atoms are rearranged but not created or destroyed.

Give an example of a balanced chemical equation that shows conservation of mass.

2H₂ + O₂ → 2H₂O (4 H atoms and 2 O atoms on each side).

What is the continuity equation in fluid dynamics related to?

It expresses conservation of mass for a fluid: mass cannot be created or destroyed.

Does the law of conservation of mass hold exactly in all situations?

No, it holds in classical mechanics but not in nuclear reactions or relativistic contexts.

What is the mass of the products if 5 g of sodium reacts with 8 g of chlorine?

13 g, assuming no loss and complete reaction.

What does 'conservation' mean in the context of mass?

The total amount of mass stays the same; it is not gained or lost.

How does the law of conservation of mass relate to the idea that atoms are indestructible?

Atoms are not destroyed in chemical reactions; they simply rearrange, so mass is conserved.

What is a reactant?

A substance that starts a chemical reaction and is consumed.

What is a product?

A substance formed as a result of a chemical reaction.

Why can't mass be conserved in an open system?

Because matter can enter or leave the system, changing the total mass inside.

What is the significance of Lavoisier's work on conservation of mass?

It helped transform chemistry from alchemy to a modern science based on measurement.

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