Chemical reactions and conservation of mass

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Chemical Reactions: Basics

A chemical reaction occurs when matter changes chemically into an entirely different substance with different properties.
During a chemical reaction, reactants (starting substances) are used to make products (new substances).
Chemical reactions involve a rearrangement of atoms: bonds in reactants break, and new bonds form to create products.
The only way to be positive a chemical reaction occurred is to detect a new substance formed.
Chemical reactions cannot be undone by simple physical means (e.g., you can't unbake a cake).

Signs of a Chemical Reaction

New substance forming – the most reliable sign; e.g., plants turn carbon dioxide into oxygen.
Change in energy – heat, light, sound, or coldness (e.g., fireworks, vinegar+baking soda gets cold).
Bubbles of a new gas released (e.g., baking soda and vinegar produce carbon dioxide bubbles).
Color change not due to mixing colors (e.g., leaves changing color, cooked egg white turning white).
New odor produced (e.g., burning logs produce a smoky smell).
Formation of a precipitate – a solid comes out of a solution (e.g., acid added to milk makes cottage cheese).
Extreme or unusual property changes – e.g., density, flammability, or tarnishing of metals.

A solid comes out of solution when acid is added to milk, forming cottage cheese.

Reactants and Products

Reactants are the substances present before a chemical reaction; they are used up to make products.
Products are the new substances formed during the reaction.
In the eggshell experiment, reactants are calcium carbonate (eggshell) and acetic acid (vinegar); products are calcium acetate, carbon dioxide, and water.
In fermentation, glucose is the reactant; alcohol and carbon dioxide are products.
In respiration, reactants are glucose and oxygen; products are carbon dioxide, water, and energy.
In photosynthesis, reactants are carbon dioxide and water; products are glucose and oxygen.

Representing Chemical Reactions

Chemical reactions can be represented in three ways: macroscopic (observable), submicroscopic (particle diagrams), and symbolic (chemical equations).
A word equation uses names: e.g., magnesium + oxygen → magnesium oxide.
A chemical equation uses symbols and formulae: e.g., 2 Mg + O₂ → 2 MgO.
A picture equation uses colored circles to represent atoms and shows the rearrangement.
The arrow (→) separates reactants (left) from products (right).

The three levels of representing chemical reactions: symbolic, submicroscopic, and macroscopic.

Balanced Chemical Equations

A balanced equation has equal numbers of each type of atom on both sides.
Coefficients (numbers in front of formulae) indicate the number of molecules or atoms involved.
Subscripts (numbers inside formulae) indicate the number of atoms of an element in one molecule.
When balancing, you may only add coefficients; you may not change subscripts or add/remove reactants/products.
Example: 2 H₂ + O₂ → 2 H₂O is balanced because there are 4 H and 2 O atoms on each side.

Methane and oxygen react to form carbon dioxide and water: CH4 + 2O2 -> CO2 + 2H2O.

Conservation of Matter

The law of conservation of matter states that matter cannot be created or destroyed.
In a chemical reaction, atoms are rearranged, but the total number of atoms remains the same.
The total mass of products equals the total mass of reactants.
Example: Vinegar and baking soda in a sealed bag had mass 34 g before and after the reaction, even though new substances formed.
Counting atoms in reactants and products shows the same number of each element, proving conservation.

A sealed bag of vinegar and baking soda has a mass of 34g before and after the reaction.

Examples of Chemical Reactions

Burning coal: carbon + oxygen → carbon dioxide.
Rusting: iron + oxygen → iron oxide (rust).
Tarnishing: copper + oxygen → copper oxide.
Fermentation: glucose → alcohol + carbon dioxide (used in brewing and making ginger beer).
Ammonium dichromate volcano: ammonium dichromate + oxygen → nitrogen gas + water + chromium oxide.
Limewater test: carbon dioxide + calcium hydroxide → calcium carbonate (white precipitate) + water.

A diagram showing the fermentation of glucose into alcohol and carbon dioxide.

Detecting Substances via Reactions

Some reactions produce unique and characteristic results, allowing detection of substances.
Limewater turns milky when carbon dioxide is bubbled through it, indicating the presence of CO₂.
Ammonium dichromate burning produces bright orange sparks and a dark-green product, used as a spectacular demonstration.
These unique reactions help chemists identify unknown substances.

Limewater that has turned milky in a test tube.

Careers in Chemistry

Chemistry is used in many careers: agricultural chemistry, biochemistry, biotechnology, chemical education, forensic science, food science, medicine, pharmaceuticals, and more.
Chemists find ways to rearrange atoms to create new compounds with desired properties.
Marie Curie was a famous chemist and physicist, first woman to win a Nobel Prize, and only person to win in two sciences.
Studying chemistry can lead to careers in research, industry, environmental science, and space exploration.

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Câu hỏi luyện tập

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1.What are the substances that are present before a chemical reaction takes place called?
Easy
- AReactants
- BProducts
- CCoefficients
- DSubscripts
2.What are the substances that are produced during a chemical reaction called?
Easy
- AReactants
- BProducts
- CCatalysts
- DEnzymes
3.During a chemical reaction, atoms are rearranged to form new substances.
Easy
True or false?
4.In a balanced chemical equation, the total number of atoms of each element is the same on both sides of the equation.
Easy
True or false?
5.Which of the following is a sign that a chemical reaction has taken place?
Easy
- AFormation of a new substance
- BChange in state (e.g., melting)
- CDissolving sugar in water
- DCutting a piece of paper
6.Propane gas burns in oxygen to produce carbon dioxide and water. Which statement explains why this reaction shows conservation of mass?
Medium
- AEnergy is produced during the reaction
- BWater is produced during the reaction
- CThe mass of the water and carbon dioxide equals the mass of the propane and oxygen
- DThe volume of the water and carbon dioxide equals the volume of the propane and oxygen
7.In the eggshell experiment, what are the reactants?
Easy
- ACalcium carbonate and acetic acid
- BCalcium acetate, carbon dioxide, and water
- CEggshell and vinegar
- DCalcium carbonate and vinegar
8.What is the product in the reaction between hydrogen and oxygen to form water?
Easy
- AHydrogen
- BOxygen
- CWater
- DHydrogen peroxide

Thêm câu hỏi

9.In a chemical equation, the numbers in front of chemical formulae are called coefficients and they indicate the number of molecules or atoms involved.
Easy
True or false?
10.When balancing a chemical equation, you may change the subscripts in the chemical formulae.
Easy
True or false?
11.In the reaction between magnesium and oxygen to form magnesium oxide, what is the balanced chemical equation?
Medium
- AMg + O2 → MgO
- B2Mg + O2 → 2MgO
- CMg + O → MgO
- D2Mg + O2 → MgO2
12.In the vinegar and baking soda reaction, the total mass before and after the reaction in a sealed bag was 34g. This demonstrates:
Medium
- AThe law of conservation of matter
- BThat mass can be created
- CThat mass is destroyed
- DThat the bag leaked
13.The total mass of the products in a chemical reaction is always equal to the total mass of the reactants.
Medium
True or false?
14.What is the product in the fermentation reaction of glucose?
Medium
- AGlucose
- BAlcohol
- CCarbon dioxide
- DBoth alcohol and carbon dioxide
15.In the reaction between iron and oxygen to form iron oxide, the balanced equation is 4Fe + 3O2 → 2Fe2O3.
Medium
True or false?
16.Which of the following is NOT a sign of a chemical reaction?
Medium
- ABubbles of gas
- BColor change
- CChange in state from solid to liquid
- DFormation of a precipitate
17.In the reaction between carbon and oxygen to form carbon dioxide, what are the reactants?
Medium
- ACarbon and oxygen
- BCarbon dioxide
- CCarbon and carbon dioxide
- DOxygen and carbon dioxide
18.What does the subscript in a chemical formula, such as the 2 in H2O, indicate?
Medium
- AThe number of molecules
- BThe number of atoms of that element in one molecule
- CThe coefficient
- DThe mass of the molecule
19.In a chemical reaction, the total number of atoms is conserved.
Medium
True or false?
20.Nitrogen gas and hydrogen gas react to form ammonia (N2 + 3H2 → 2NH3). Which statement correctly describes how matter is conserved during this reaction?
Medium
- AThe kinetic energies of the nitrogen and hydrogen molecules do not change when ammonia forms
- BThe number of hydrogen and nitrogen atoms before the reaction is the same as after the reaction
- CA new substance with properties similar to nitrogen and hydrogen is formed
- DMatter is cycled to the atmosphere as ammonia gas is released
21.In the eggshell reaction, what are the products?
Medium
- ACalcium acetate, carbon dioxide, and water
- BCalcium carbonate and acetic acid
- CEggshell and vinegar
- DCalcium acetate and vinegar
22.When balancing a chemical equation, you may add new reactants or products.
Medium
True or false?
23.In the reaction between methane (CH4) and oxygen (O2) to produce carbon dioxide (CO2) and water (H2O), what is the balanced equation?
Medium
- ACH4 + O2 → CO2 + H2O
- BCH4 + 2O2 → CO2 + 2H2O
- CCH4 + O2 → CO2 + 2H2O
- DCH4 + 2O2 → CO2 + H2O
24.Four setups each undergo a chemical reaction. Their masses are measured before and after: | Setup | Mass before (g) | Mass after (g) | |---|---|---| | W | 128.8 | 126.3 | | X | 138.2 | 137.0 | | Y | 127.0 | 127.7 | | Z | 130.0 | 130.0 | Only one of the four setups is a closed system. Based on the results, which setup is most likely the closed system?
Medium
- ASetup W
- BSetup X
- CSetup Y
- DSetup Z
25.In the reaction between iron and oxygen to form iron oxide, what is the balanced equation?
Medium
- AFe + O2 → Fe2O3
- B2Fe + O2 → Fe2O3
- C4Fe + 3O2 → 2Fe2O3
- D3Fe + 2O2 → Fe3O4
26.The formation of a precipitate is a sign that a chemical reaction may have occurred.
Medium
True or false?
27.In the reaction between sodium bicarbonate (baking soda) and vinegar (acetic acid), the reactants are:
Medium
- ASodium bicarbonate and acetic acid
- BCarbon dioxide, water, and sodium acetate
- CBaking soda and vinegar
- DAcetic acid and water
28.In a chemical reaction, the total mass of the products is always less than the total mass of the reactants.
Medium
True or false?
29.In the reaction between nitrogen monoxide (NO) and bromine (Br2) to form NOBr, what is the balanced equation?
Hard
- ANO + Br2 → NOBr
- B2NO + Br2 → 2NOBr
- CNO + 2Br2 → 2NOBr
- D2NO + 2Br2 → 2NOBr
30.In the reaction between copper and oxygen to form copper oxide, the word equation is: copper + oxygen → copper oxide.
Hard
True or false?
31.Which of the following is a balanced chemical equation for the reaction between carbon monoxide and water to form carbon dioxide and hydrogen?
Hard
- ACO + H2O → CO2 + H2
- B2CO + H2O → 2CO2 + H2
- CCO + 2H2O → CO2 + 2H2
- DCO + H2O → CO2 + 2H2
32.When balancing a chemical equation, you are allowed to change the coefficients.
Hard
True or false?
33.In the reaction between sulfur and oxygen to form sulfur dioxide, what is the balanced equation?
Hard
- AS + O2 → SO2
- B2S + O2 → 2SO2
- CS + 2O2 → SO2
- DS + O2 → 2SO2
34.The diagram shows every atom in the vinegar and baking soda reaction, colour-coded and labelled. Using the diagram, count the total number of oxygen (O) atoms on the reactant side (left of the arrow).
Medium
35.What is the balanced equation for the reaction between nitrogen gas and hydrogen gas to form ammonia?
Hard
- AN2 + H2 → NH3
- BN2 + 3H2 → 2NH3
- CN2 + 2H2 → 2NH3
- D2N2 + 3H2 → 2NH3
36.In a chemical reaction, the total number of atoms of each element is conserved.
Hard
True or false?
37.In the reaction between carbon disulfide and oxygen to form carbon dioxide and sulfur dioxide, what is the balanced equation?
Challenge
- ACS2 + O2 → CO2 + SO2
- BCS2 + 3O2 → CO2 + 2SO2
- CCS2 + 2O2 → CO2 + 2SO2
- DCS2 + 3O2 → CO2 + SO2
38.The only way to be certain that a chemical reaction has occurred is to observe a change in color.
Challenge
True or false?
39.In the reaction between iron and oxygen to form iron oxide, how many iron atoms are needed on the reactant side to balance the equation?
Challenge
- A1
- B2
- C3
- D4
40.In a chemical reaction, the total mass of the reactants equals the total mass of the products.
Challenge
True or false?

Ghi chú ôn tập

## 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.

Thẻ ghi nhớ

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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