Water is considered a pure substance because it is composed of only one type of molecule, which is H2O. It has a constant chemical composition and cannot be separated into simpler substances by physical means, making it a pure substance. 1
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Key Takeaways: Why is Water a Pure Substance?
- Water is a pure substance because it is composed of only one type of molecule, H2O, with a constant chemical composition.
- Pure substances can be classified as elements or compounds, and water is classified into the category of compound
- Water is not a mixture because it consists of only one type of molecule, and mixtures involve the combination of different substances that can be separated physically.
Explanation: Why is water a pure substance?
Water is considered a pure substance because it is composed of only one type of molecule, which is H2O. A pure substance is a material that has a consistent and definite chemical composition throughout.
In the case of water, whether it is in the form of a solid (ice), liquid (water), or gas (water vapor), the fundamental composition remains the same: two hydrogen atoms bonded to one oxygen atom. 2
Pure substances can be classified into two categories:
- Elements and
- Compounds.
Elements consist of only one type of atom, such as oxygen (O) or hydrogen (H). Compounds, on the other hand, are substances formed by the chemical combination of two or more elements in fixed proportions. 3
Water falls into the compound category because it is formed by the bonding of hydrogen and oxygen atoms.
It’s worth noting that while water is a pure substance, it can still contain impurities or dissolved substances.
For example, tap water may contain minerals, ions, or small amounts of other substances. However, the presence of these impurities does not change the fact that water itself is a pure substance with a consistent chemical composition.
Why is water not a mixture?
Water is not a mixture because it is composed of only one type of molecule, which is H2O.
A mixture, on the other hand, is a combination of two or more substances that are not chemically bonded together and can be separated by physical means. A mixture does not have a definite or fixed chemical composition, unlike a pure substance. 4
Water can be mixed with other substances to form a mixture, but by itself, it is a pure substance.
For example, when salt is dissolved in water, the resulting solution is a mixture of salt and water. The salt molecules are not chemically bonded to the water molecules, and they can be separated from the water by physical means, such as evaporation. 5
In summary, water is not a mixture because it is a pure substance consisting of only one type of molecule with a fixed chemical composition.
Water is a pure substance, but is it an element or a compound?
Water is a pure substance and it is classified as a compound, not an element. 6
Elements are substances that cannot be chemically broken down into simpler substances. They consist of only one type of atom. Examples of elements include oxygen (O), hydrogen (H), and carbon (C).
On the other hand, compounds are substances formed by the chemical combination of two or more elements in fixed proportions. Water, with its chemical formula H2O, is a compound composed of hydrogen (H) and oxygen (O) atoms.
In a water molecule, two hydrogen atoms are bonded to one oxygen atom through covalent bonds. The arrangement and bonding of these atoms give water its unique properties and characteristics. It is important to note that the properties of a compound are distinct from those of its constituent elements.
So, to summarize, water is a compound because it is composed of two different elements, hydrogen and oxygen, chemically bonded together in a fixed proportion.
Further reading
Why is Iron a Pure Substance?
Why is Copper a Pure Substance?
Is Brass a Pure Substance?
Why is Ice a Pure Substance?
Is Steel a Pure Substance?
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References
- P. (n.d.). Water. Water | H2O | CID 962 – PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/962
- Phases of Matter. (n.d.). Phases of Matter. https://www.grc.nasa.gov/www/k-12/rocket/state.html
- Chemical compound – Wikipedia. (2020, December 8). Chemical Compound – Wikipedia. https://en.wikipedia.org/wiki/Chemical_compound
- 2.7: Mixture. (2022, March 18). Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.07%3A_Mixture
- How to Separate Salt and Water. (2019, December 3). ThoughtCo. https://www.thoughtco.com/separate-salt-from-water-in-saltwater-607900
- CLASS NOTES. (n.d.). CLASS NOTES. https://www.vanderbilt.edu/AnS/physics/panvini/p110b/s03/read2.html
