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Your guide: Miss SanaCounts atoms the way you count rupees: carefully.
A glass of tap water, the copper wire inside a charger cable and pure water from a lab bottle. One is an element, one is a compound and one is a mixture. Which is which? Two questions settle it: what kind of particles does it contain, and is the ratio fixed?
An element is the simplest form of matter: a pure substance containing the same kind of atoms. Ordinary chemical reactions cannot break it into anything simpler. Most elements are solids; only a few are liquids or gases (Fig 1.2: iron rods, copper wire, zinc screws and nuts).
A compound is also pure, but it is made of two or more different elements chemically combined in a fixed ratio. The elements react and form chemical bonds that are not easy to break (Fig 1.3: iron sulphide, copper chloride). Water, ammonia, methane and carbon dioxide are compounds.
A mixture forms when more than one type of element or compound is mixed together in any ratio. Air, soil, milk and tap water are everyday mixtures. A salt solution is homogeneous: its concentration is the same throughout. A rock is heterogeneous: different parts contain different amounts of its minerals (Fig 1.4: rock, chocolate).
Elements may be metals, non-metals, metalloids or noble gases.
Elements can exist as atoms, molecules, ions and isotopes.
Compounds may be molecular, ionic, intermetallic or coordination complexes, and inorganic or organic.
The composition and properties of an element or compound are uniform throughout a sample and from one sample to another.
Any ratio: you can add a little more salt to salt water and it is still salt water.
Homogeneous: the same concentration everywhere (salt solution).
Heterogeneous: the concentration differs from part to part (rock).
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A glass of tap water, the copper wire inside a charger cable and pure water from a lab bottle. One is an element, one is a compound and one is a mixture. Which is which? Two questions settle it: what kind of particles does it contain, and is the ratio fixed?
An element is the simplest form of matter: a pure substance containing the same kind of atoms. Ordinary chemical reactions cannot break it into anything simpler. Most elements are solids; only a few are liquids or gases (Fig 1.2: iron rods, copper wire, zinc screws and nuts).
A compound is also pure, but it is made of two or more different elements chemically combined in a fixed ratio. The elements react and form chemical bonds that are not easy to break (Fig 1.3: iron sulphide, copper chloride). Water, ammonia, methane and carbon dioxide are compounds.
A mixture forms when more than one type of element or compound is mixed together in any ratio. Air, soil, milk and tap water are everyday mixtures. A salt solution is homogeneous: its concentration is the same throughout. A rock is heterogeneous: different parts contain different amounts of its minerals (Fig 1.4: rock, chocolate).
Elements may be metals, non-metals, metalloids or noble gases.
Elements can exist as atoms, molecules, ions and isotopes.
Compounds may be molecular, ionic, intermetallic or coordination complexes, and inorganic or organic.
The composition and properties of an element or compound are uniform throughout a sample and from one sample to another.
Any ratio: you can add a little more salt to salt water and it is still salt water.
Homogeneous: the same concentration everywhere (salt solution).
Heterogeneous: the concentration differs from part to part (rock).
Pure substances have a fixed composition: an element has one kind of atom, and a compound has different elements in a fixed ratio. A mixture has no fixed ratio.
Q1. Classify a solution of common salt in water.
A homogeneous mixture.
Q2. Oxygen gas is made of O2 molecules. Is it an element or a compound?
An element that exists as molecules.
Q3. Why is a piece of rock a heterogeneous mixture?
The concentration of its constituents differs from one part to another.
✗ Calling O2 or N2 a compound because it is a molecule.
✓ One kind of atom means an element, even when the atoms are joined in molecules.
✗ Writing that a mixture has its components in a fixed ratio.
✓ Only a compound has a fixed ratio; a mixture can be made in any ratio.
✗ Calling tap water or air pure because it looks the same everywhere.
✓ Looking uniform means homogeneous, not pure. Both are mixtures in your book.
1. Which of these is a heterogeneous mixture?
(b) Concrete varies from part to part. Salt solution is homogeneous, copper is an element and ammonia is a compound.
2. Which of these is a compound?
(a) Iron sulphide is iron and sulphur bonded in a fixed ratio (Fig 1.3). Mercury and iodine are elements; tap water is a mixture.
3. A pure substance containing the same kind of atoms, which ordinary chemical reactions cannot break down, is:
(d) That is the book's definition of an element (p. 6).
4. In a compound, the elements are combined:
(b) A fixed ratio is what separates a compound from a mixture. Compounds can be molecular as well as ionic.
5. Nitrogen gas (N2) is best described as:
(c) Both atoms in N2 are nitrogen: one kind of atom, so it is an element that exists as molecules.
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