Nature of Matter

Look around a kitchen in Chennai at lunchtime.

Steel vessels and a block of jaggery are solid; water, coconut oil and buttermilk are liquids; steam rises from the rice cooker and the air carries the smell of tempering.

All of these, and you too, are made of matter: anything that has mass and takes up space.

Light, sound and heat are not matter, because they have no mass and occupy no space.

Chemistry looks at matter in two ways.

You can describe it in bulk, as you see and handle it (the macroscopic view), or you can ask what its tiny particles are doing (the particle view).

This section uses both views: first to explain the three states, then to sort matter into types.

Matter exists in three physical states: solid, liquid and gas.

The difference between them lies in how close the particles are and how freely they move, as the diagram shows.

Why the properties follow from the particles.

• A solid keeps its shape because each particle is held in place by its neighbours and can only vibrate. Push on one side of a steel tumbler and the whole tumbler moves.

• A liquid keeps its volume because its particles are still touching, so there is no room to squeeze them closer. It flows and takes the shape of its container because the particles can slide past one another.

• A gas fills any container because its particles are far apart and moving fast in straight lines until they hit something. Most of a gas is empty space, which is why a gas can be compressed so much: you are squeezing the space, not the particles.

The three states can be turned into one another by changing the temperature or the pressure.

• Heating gives the particles more energy. A solid usually melts to a liquid, and the liquid boils or evaporates to a gas (vapour). Ghee melting in a hot kadhai and water boiling in a pressure cooker are examples.

• Cooling takes energy away. A gas condenses to a liquid, and the liquid freezes to a solid, as when water vapour condenses on the outside of a cold glass of lassi.

• Pressure pushes gas particles closer together. Squeeze a gas hard enough, at a low enough temperature, and the particles come close enough to cling together as a liquid. The LPG cylinder in your kitchen holds its fuel as a liquid under pressure; when you open the regulator, the pressure falls and the liquid turns back into gas.

Changing state does not change what the substance is.

Ice, water and steam are all H₂O; only the arrangement and motion of the molecules differ.

At the bulk level, every sample of matter is either a mixture or a pure substance.

Pure substances are either elements or compounds, and mixtures are either homogeneous or heterogeneous.

The tree below is the one to learn.

Pure substances. In a pure substance, every particle is chemically the same.

Copper, silver, gold, water and glucose are pure substances.

Glucose always contains carbon, hydrogen and oxygen in the same fixed ratio, and every glucose particle is identical, so a pure substance has a fixed composition.

Its constituents cannot be separated by physical methods such as filtering or boiling.

Mixtures. A mixture contains particles of two or more pure substances, in any ratio.

Sugar dissolved in water, air, tea and nimbu pani are all mixtures: you can make nimbu pani sweeter or sourer, so its composition is variable.

The pure substances in a mixture are called its components, and each keeps its own properties: the sugar in nimbu pani is still sweet.

• In a homogeneous mixture the components mix completely. Their particles are spread evenly, so the composition is the same throughout. Sugar solution, salt water, air and brass are homogeneous.

• In a heterogeneous mixture the composition is not the same throughout, and often you can see the different components. Salt mixed with sugar, rice with small stones in it, poha with peanuts, and sand stirred into water are heterogeneous.

The photograph shows the difference.

Sugar stirred into water (left) disappears completely, and every drop of the solution is equally sweet.

Sand stirred into water (right) stays visible as separate grains, and the mixture at the bottom of the beaker is quite different from the mixture at the top.

The components of a mixture can be separated by physical methods: hand-picking stones from rice, filtering sand from water, crystallising salt from sea water at the salt pans of Gujarat, or distilling.

Elements. The particles of an element contain only one kind of atom.

Sodium, copper, silver, hydrogen and oxygen are elements.

In some elements the particles are single atoms (sodium, copper, the noble gases).

In others, two or more atoms of the same kind join to make a molecule: hydrogen (H₂), nitrogen (N₂) and oxygen (O₂) are diatomic, ozone (O₃) has three atoms, phosphorus (P₄) four and sulphur (S₈) eight.

An element made of molecules is still an element, because all its atoms are of one type.

Compounds. When atoms of two or more different elements join in a definite ratio, a compound forms.

Water (H₂O) always has two hydrogen atoms for every oxygen atom; carbon dioxide (CO₂) one carbon atom for every two oxygen atoms; ammonia, carbon dioxide and sugar are other examples.

Two facts make compounds different from mixtures:

• A compound has properties different from those of its elements. Hydrogen is a gas that burns with a pop, oxygen is a gas that helps things burn, yet water, made from the two, is a liquid used to put out fires.

• A compound can be split into its elements only by chemical methods (for example, passing electricity through water), never by physical ones.

• "Pure" in the market is not "pure" in chemistry. A packet of "pure ghee" or "pure milk" means nothing has been added. Chemically both are mixtures of many substances. A chemist calls a sample pure only when it contains one substance.

• A uniform look does not make something a compound. Brass looks the same throughout, but it is a homogeneous mixture (an alloy) of copper and zinc, and its composition can vary.

• An element can be made of molecules. Oxygen gas is made of O₂ molecules, yet it is an element because both atoms in each molecule are oxygen.

1. Which of the following is a heterogeneous mixture?

• (A) A solution of copper sulphate in water
• (B) Iron filings mixed with sulphur powder
• (C) Brass
• (D) Air filtered free of dust [1]

2. Which of the following elements exists as molecules of eight atoms?

• (A) Copper
• (B) Neon
• (C) Sulphur
• (D) Phosphorus [1]

3. Given below are two statements labelled Assertion (A) and Reason (R). Select the most appropriate answer from the options (A), (B), (C) and (D) given below.

Assertion (A): Air is a compound because the gases in it always occur in a fixed ratio.

Reason (R): The components of a mixture can be separated by physical methods.

• (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

• (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).

• (C) Assertion (A) is true, but Reason (R) is false.

• (D) Assertion (A) is false, but Reason (R) is true. [1]

4. Differentiate between a homogeneous and a heterogeneous mixture, giving one example of each. (1 + 1) [2]

5. Hydrogen burns with a pop and oxygen supports burning, yet water puts out fires. What does this tell you about compounds? State one other way in which a compound differs from a mixture. (1 + 1) [2]

6. Explain, in terms of particles, (a) why a solid has a definite shape, (b) why a gas has neither a definite shape nor a definite volume, and (c) what happens to the particles when steam condenses to water. (1 + 1 + 1) [3]

7. Riya says, "Oxygen gas is a compound because it is made of O₂ molecules." (a) Is she right? Give a reason. (b) Name one element whose particles are single atoms. (1 + 1) [2]