Students identify elements, compounds, and mixtures from a list of chemical formulas and common substances.

Problem Analysis:

The task requires identifying whether each given substance is an element, compound, or mixture. Additionally, there are questions about specific substances like sea water, ice tea, and bottle water. Below is the explanation and classification for each item.

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Definitions:

1. Element: A pure substance that consists of only one type of atom. Examples: Gold (Au), Oxygen (O₂).
2. Compound: A pure substance made up of two or more different types of atoms bonded together in a fixed ratio. Examples: Water (H₂O), Carbon Dioxide (CO₂).
3. Mixture: A combination of two or more substances that are not chemically combined. The components retain their individual properties. Examples: Sea water, Ice tea.

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Classification of Each Substance:

#### 1. Au
- Classification: Element
- Explanation: Au is the chemical symbol for gold, which is a pure element consisting of only gold atoms.

#### 2. CaH₂ (Calcium hydride)
- Classification: Compound
- Explanation: Calcium hydride is composed of calcium (Ca) and hydrogen (H) atoms bonded together in a fixed ratio (1:2).

#### 3. Ga
- Classification: Element
- Explanation: Ga is the chemical symbol for gallium, which is a pure element consisting of only gallium atoms.

#### 4. Cu
- Classification: Element
- Explanation: Cu is the chemical symbol for copper, which is a pure element consisting of only copper atoms.

#### 5. H₂O
- Classification: Compound
- Explanation: H₂O is water, which is composed of hydrogen (H) and oxygen (O) atoms bonded together in a fixed ratio (2:1).

#### 6. Rf
- Classification: Element
- Explanation: Rf is the chemical symbol for rutherfordium, which is a synthetic element consisting of only rutherfordium atoms.

#### 7. H₂SO₄ (Sulfuric acid)
- Classification: Compound
- Explanation: Sulfuric acid is composed of hydrogen (H), sulfur (S), and oxygen (O) atoms bonded together in a fixed ratio (2:1:4).

#### 8. HCl (Hydrogen chloride)
- Classification: Compound
- Explanation: Hydrogen chloride is composed of hydrogen (H) and chlorine (Cl) atoms bonded together in a fixed ratio (1:1).

#### 9. O₂
- Classification: Element
- Explanation: O₂ is molecular oxygen, which is a diatomic molecule consisting of two oxygen atoms. It is still considered an element because it contains only one type of atom.

#### 10. CO
- Classification: Compound
- Explanation: CO is carbon monoxide, which is composed of carbon (C) and oxygen (O) atoms bonded together in a fixed ratio (1:1).

#### 11. CO₂
- Classification: Compound
- Explanation: CO₂ is carbon dioxide, which is composed of carbon (C) and oxygen (O) atoms bonded together in a fixed ratio (1:2).

#### 12. Sea water
- Classification: Mixture
- Explanation: Sea water is a solution containing various dissolved substances such as sodium chloride (NaCl), magnesium chloride (MgCl₂), and other minerals. It is not a pure substance but a mixture of different compounds.

#### 13. Ice tea
- Classification: Mixture
- Explanation: Ice tea is a mixture of water, tea (which contains various compounds), sugar (if added), and ice. It is not a pure substance but a combination of different components.

#### 14. Bottle water
- Classification: Mixture
- Explanation: Bottle water can be either pure water (H₂O) or mineral water, which contains dissolved minerals like calcium, magnesium, and others. Even if it is "pure" water, it may contain trace amounts of dissolved gases or minerals, making it a mixture in most cases.

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Final Answers:

1. Au: Element
2. CaH₂: Compound
3. Ga: Element
4. Cu: Element
5. H₂O: Compound
6. Rf: Element
7. H₂SO₄: Compound
8. HCl: Compound
9. O₂: Element
10. CO: Compound
11. CO₂: Compound
12. Sea water: Mixture
13. Ice tea: Mixture
14. Bottle water: Mixture

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Final Answer Box:

\[
\boxed{
\begin{array}{ll}
\text{Au} & \text{Element} \\
\text{CaH₂} & \text{Compound} \\
\text{Ga} & \text{Element} \\
\text{Cu} & \text{Element} \\
\text{H₂O} & \text{Compound} \\
\text{Rf} & \text{Element} \\
\text{H₂SO₄} & \text{Compound} \\
\text{HCl} & \text{Compound} \\
\text{O₂} & \text{Element} \\
\text{CO} & \text{Compound} \\
\text{CO₂} & \text{Compound} \\
\text{Sea water} & \text{Mixture} \\
\text{Ice tea} & \text{Mixture} \\
\text{Bottle water} & \text{Mixture} \\
\end{array}
}
\]