Chapter Notes

Is Matter Around Us Pure?

In our daily lives, we often use the word 'pure' to describe food items like milk, ghee, butter, salt, spices, mineral water, or juice, assuming they are free from adulteration. However, in science, the term 'pure' has a more specific meaning.

For a common person, 'pure' means unadulterated.

For a scientist, a substance is considered pure when all its constituent particles are identical in their chemical nature. This means a pure substance consists of only one type of particle.

For example, milk, which we commonly consider pure, is actually a mixture of water, fat, proteins, and other substances. Therefore, from a scientific perspective, it is not pure.

As we observe our surroundings, we find that most matter exists as mixtures of two or more pure components. Examples include seawater, minerals, and soil.

What is a Mixture?

Mixtures are constituted by more than one kind of pure form of matter. For instance, sodium chloride (common salt) dissolved in water forms a mixture. We can separate the sodium chloride from water through evaporation, a physical process. However, sodium chloride itself is a pure substance that cannot be further separated into its chemical constituents by physical means. Similarly, sugar is a pure substance with a uniform composition throughout.

Soft drinks and soil are examples of substances that are not single pure substances. A pure substance, regardless of its source, will always exhibit the same characteristic properties.

Therefore, a mixture contains more than one pure substance.

Types of Mixtures

Mixtures can be classified based on the nature of their components.

• Divide the class into groups A, B, C, and D.
• Group A: 50 mL water + 1 spatula of copper sulphate powder.
• Group B: 50 mL water + 2 spatulas of copper sulphate powder.
• Groups C and D: Different amounts of copper sulphate and potassium permanganate, or common salt and mix.
• Report observations on uniformity in color and texture.

Homogeneous Mixtures (or Solutions) These mixtures have a uniform composition throughout. Examples include salt dissolved in water and sugar dissolved in water. Even though groups A and B both created copper sulphate solutions, the intensity of the color differs, indicating that homogeneous mixtures can have variable compositions.

Heterogeneous Mixtures These mixtures contain physically distinct parts and have non-uniform compositions. Examples include mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water.

• Divide the class into four groups- A, B, C and D.
• Distribute the following samples to each group:
  • Group A: Few crystals of copper sulphate.
  • Group B: One spatula full of copper sulphate.
  • Group C: Chalk powder or wheat flour.
  • Group D: Few drops of milk or ink.
• Each group should add the given sample in water and stir properly using a glass rod. Are the particles in the mixture visible?
• Direct a beam of light from a torch through the beaker containing the mixture and observe from the front. Was the path of the beam of light visible?
• Leave the mixtures undisturbed for a few minutes (and set up the filtration apparatus in the meantime). Is the mixture stable or do the particles begin to settle after some time?
• Filter the mixture. Is there any residue on the filter paper?
• Discuss the results and form an opinion.

Groups A and B have got a solution. Group C has got a suspension. Group D has got a colloidal solution.

What is a Solution?

A solution is a homogeneous mixture of two or more substances. Examples include lemonade and soda water. Solutions can be solid (alloys), liquid, or gaseous (air). The particles in a solution are evenly distributed at the particle level, as demonstrated by the uniform taste of lemonade.

Alloys Alloys are mixtures of two or more metals or a metal and a non-metal. They cannot be separated into their components by physical methods. Alloys are considered mixtures because they exhibit the properties of their constituents and can have variable compositions. For example, brass is an alloy of approximately 30% zinc and 70% copper.

A solution consists of two components:

Solvent The component that dissolves the other component (usually present in a larger amount).

Solute The component that is dissolved in the solvent (usually present in a smaller amount).

Examples

(i) Sugar in water: Sugar is the solute, and water is the solvent. (ii) Tincture of iodine (iodine in alcohol): Iodine (solid) is the solute, and alcohol (liquid) is the solvent. (iii) Aerated drinks (soda water): Carbon dioxide (gas) is the solute, and water (liquid) is the solvent. (iv) Air: A mixture of gases (gas in gas). Oxygen (21%) and nitrogen (78%) are the main constituents.

Properties of a Solution

• A solution is a homogeneous mixture.
• The particles of a solution are smaller than 1 nm ($10^{-9}$ metre) in diameter and cannot be seen with the naked eye.
• Solutions do not scatter a beam of light passing through them, making the path of light invisible.
• The solute particles cannot be separated by filtration.
• The solute particles do not settle down when left undisturbed; hence, a solution is stable.

Concentration of a Solution

The concentration of a solution refers to the relative proportion of the solute and solvent. Solutions can be described as dilute, concentrated, or saturated, depending on the amount of solute present. These terms are comparative.

• Take approximately 50 mL of water each in two separate beakers.
• Add salt in one beaker and sugar or barium chloride in the second beaker with continuous stirring.
• When no more solute can be dissolved, heat the contents of the beaker to raise the temperature by about $5^{\circ} \mathrm{C}$. Start adding the solute again.

A saturated solution is one in which no more solute can be dissolved at a given temperature. The amount of solute present in a saturated solution is called its solubility.

An unsaturated solution contains less solute than the saturation level.

Different substances have different solubilities in a given solvent at the same temperature.

The concentration of a solution is the amount (mass or volume) of solute present in a given amount (mass or volume) of solution.

Methods for Expressing Concentration

(i) Mass by mass percentage $= \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$

(ii) Mass by volume percentage $= \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100$

(iii) Volume by volume percentage $= \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100$

Given

• Mass of solute (salt) = $40 \text{ g}$
• Mass of solvent (water) = $320 \text{ g}$

To Find

Concentration in terms of mass by mass percentage.

Formula

$\text{Mass percentage of solution} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$

Solution

First, calculate the mass of the solution:

$\text{Mass of solution} = \text{Mass of solute} + \text{Mass of solvent}$

$\text{Mass of solution} = 40 \text{ g} + 320 \text{ g} = 360 \text{ g}$

Now, calculate the mass percentage of the solution:

$\text{Mass percentage of solution} = \frac{40 \text{ g}}{360 \text{ g}} \times 100$

$\text{Mass percentage of solution} = 11.1 \%$

Final Answer The concentration of the solution in terms of mass by mass percentage is $11.1 \%$.

What is a Suspension?

Suspensions are non-homogeneous systems in which solids are dispersed in liquids. In a suspension, solute particles do not dissolve but remain suspended throughout the medium. These particles are visible to the naked eye.

Properties of a Suspension

• A suspension is a heterogeneous mixture.
• The particles of a suspension can be seen with the naked eye.
• Suspensions scatter a beam of light passing through them, making the path of light visible.
• The solute particles settle down when the suspension is left undisturbed, making it unstable.
• Suspensions can be separated by filtration.

What is a Colloidal Solution?

A colloidal solution (or colloid) is a mixture where the particles are uniformly spread throughout the solution. Although they appear homogeneous, colloidal solutions are actually heterogeneous mixtures. Milk is an example of a colloid.

The particles in a colloid are small enough that they cannot be seen with the naked eye. However, they can scatter a beam of visible light. This phenomenon is known as the Tyndall effect.

The Tyndall effect can be observed when a beam of light enters a room through a small hole, scattering off dust and smoke particles. It can also be seen when sunlight passes through the canopy of a dense forest, where mist droplets act as colloidal particles dispersed in air.

Properties of a Colloid

• A colloid is a heterogeneous mixture.
• The particles of a colloid are too small to be individually seen with the naked eye.
• Colloids scatter a beam of light passing through them, making the path of light visible (Tyndall effect).
• Colloids do not settle down when left undisturbed and are quite stable.
• Colloidal particles cannot be separated by filtration, but a special technique called centrifugation can be used.

A colloidal solution has two components:

Dispersed Phase The solute-like component or the dispersed particles.

Dispersion Medium The component in which the dispersed phase is suspended.

Colloids are classified based on the state (solid, liquid, or gas) of the dispersed phase and the dispersion medium.

Common Examples of Colloids

Physical and Chemical Changes

Physical Properties Properties that can be observed and specified, such as color, hardness, rigidity, fluidity, density, melting point, and boiling point.

A physical change involves a change in the form or appearance of a substance, but not its chemical composition. For example, the interconversion of states (ice, water, and water vapor) is a physical change.

Chemical Properties Characteristics that describe how a substance reacts with other substances.

A chemical change (or chemical reaction) involves a change in the chemical composition of a substance, resulting in the formation of new substances with different properties. Burning is an example of a chemical change.

During the burning of a candle, both physical (melting of wax) and chemical (burning of wax) changes occur.

Types of Pure Substances

Based on their chemical composition, substances can be classified as elements or compounds.

Elements

An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements are normally divided into metals, non-metals, and metalloids.

Metals typically exhibit the following properties:

• Lustre (shine)
• Silvery-grey or golden-yellow color
• Conduct heat and electricity
• Ductile (can be drawn into wires)
• Malleable (can be hammered into thin sheets)
• Sonorous (make a ringing sound when hit)

Examples of metals include gold, silver, copper, iron, sodium, and potassium. Mercury is the only metal that is liquid at room temperature.

Non-metals typically exhibit the following properties:

• Variety of colors
• Poor conductors of heat and electricity
• Not lustrous, sonorous, or malleable

Examples of non-metals include hydrogen, oxygen, iodine, carbon (coal, coke), bromine, and chlorine.

Metalloids Some elements have intermediate properties between metals and non-metals and are called metalloids. Examples include boron, silicon, and germanium.

Compounds

A compound is a substance composed of two or more elements chemically combined in a fixed proportion.

• Divide the class into two groups. Give 5 g of iron filings and 3 g of sulphur powder in a china dish to both the groups.

Group I

• Mix and crush iron filings and sulphur powder.

Group II

• Mix and crush iron filings and sulphur powder. Heat this mixture strongly till red hot. Remove from flame and let the mixture cool.

Groups I and II

• Check for magnetism in the material obtained. Bring a magnet near the material and check if the material is attracted towards the magnet.
• Compare the texture and colour of the material obtained by the groups.
• Add carbon disulphide to one part of the material obtained. Stir well and filter.
• Add dilute sulphuric acid or dilute hydrochloric acid to the other part of the material obtained.

Observations

Group I: A mixture of iron and sulphur is formed. The properties of the mixture are the same as its constituents. The gas obtained by Group I is hydrogen.

Group II: A compound is formed. The properties of the compound are different from the combining elements. The gas obtained by Group II is hydrogen sulphide.

Mixtures and Compounds