Lower Secondary Chemistry: Matter, Separation Techniques, Criteria of Purity, and Chemical Changes

Comprehensive flashcards covering elements, compounds, mixtures, separation techniques (filtration, crystallisation, simple and fractional distillation, chromatography), purity criteria, and physical vs. chemical changes.

How is matter broadly classified based on its chemical composition?

Matter is classified into pure substances (which include elements and compounds) and mixtures (homogeneous and heterogeneous).

Why is an aircraft made from a mixture of metals (aluminium, copper, magnesium, and zinc) rather than pure aluminium?

The mixture of metals results in a light aircraft body that can withstand extreme stress, unlike pure aluminium.

What is the definition of an element?

An element is a pure substance that cannot be broken down into two or more simpler substances by chemical processes.

How are elements arranged in the Periodic Table in terms of groups and periods?

Elements in the same vertical column belong to the same group and share similar chemical properties; elements in the same horizontal row belong to the same period.

What general change in properties occurs as you move from left to right across a period in the Periodic Table?

The properties of elements gradually change from metallic to non-metallic.

Which metal is an exception to the rule that metals are solids at room temperature and pressure (r.t.p.)?

Mercury, which exists as a liquid at r.t.p.

Which non-metal is an exception to the rule that non-metals are poor electrical conductors?

Carbon in the form of graphite.

What are the definitions of ductile, malleable, and sonorous?

Ductile means capable of being drawn into wires; malleable means capable of being hammered into different shapes without breaking; sonorous means making a ringing sound when struck.

What is the difference between an atom and a molecule?

An atom is the smallest particle of an element that retains the chemical properties of that element, whereas a molecule is a group of two or more atoms chemically combined together.

What occurs when sugar is heated strongly, and what is this process called?

Sugar undergoes thermal decomposition to break down into carbon and water vapour ($\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 6\text{C} + 6\text{H}_2\text{O}$).

What is the volume ratio of hydrogen gas to oxygen gas collected during the electrolysis of water?

The ratio of hydrogen to oxygen is $2:1$.

What is the definition of a compound?

A compound is a pure substance containing two or more elements chemically combined in a fixed ratio.

What are the observations when magnesium burns in oxygen?

Magnesium (a shiny grey solid) burns with a brilliant white flame in oxygen (a colourless gas) to produce magnesium oxide (a white solid).

How many total atoms and elements are present in one formula unit of lead(II) nitrate, $\text{Pb}(\text{NO}_3)_2$?

There are $3$ elements ($\text{Pb}$, $\text{N}$, $\text{O}$) and a total of $9$ atoms ($1$ lead atom, $2$ nitrogen atoms, and $6$ oxygen atoms).

Compare a mixture of iron filings and sulfur with the compound iron(II) sulfide in terms of magnetic properties and formation ratio.

In the mixture, iron retains its magnetic property and components can mix in any proportion; in iron(II) sulfide, the substance is non-magnetic and combines in a fixed iron-to-sulfur mass ratio of $7:4$.

What three tests distinguish a solution from a suspension?

1. Appearance: solution is homogeneous and clear, suspension is heterogeneous and cloudy. 2. Light: solution allows light to pass through, suspension does not. 3. Filtration: solution leaves no residue, suspension leaves a residue.

What three factors affect the solubility of a solute?

The nature of the solvent, the nature of the solute, and temperature.

What three factors affect the rate at which a solute dissolves?

Temperature of the solvent, size of solute particles (surface area), and the rate of stirring.

Which four common metals are magnetic and can be separated using magnetic attraction?

Iron, steel, cobalt, and nickel.

What is sublimation in separation techniques, and which common substances sublime?

Sublimation separates a solid that sublimes from one that does not; substances that sublime include iodine and ammonium chloride.

In the filtration setup shown, what do labels X and Y represent?

X represents the residue (insoluble solid trapped on the filter paper), and Y represents the filtrate (liquid that passes through).

How can a solid mixture of salt and sand be separated to obtain pure samples of both?

Add water to dissolve the salt, filter to obtain sand as the residue, wash the sand residue with distilled water, and evaporate the filtrate to dryness to obtain salt.

What are the two major membrane purification stages in producing NEWater from treated sewage?

Microfiltration (traps suspended solids and most disease-causing micro-organisms) followed by Reverse Osmosis (removes micro-organisms, dissolved salts, and chemical molecules).

How does reverse osmosis purify water?

Water is forced under high pressure through a partially permeable membrane whose pores allow only tiny water molecules to pass through.

Why is crystallisation preferred over evaporation to dryness for substances like copper(II) sulfate or sugar?

Evaporation to dryness decomposes heat-sensitive substances (like sugar) and leaves behind any soluble impurities; crystallisation yields pure crystals without thermal decomposition.

What is the effect of cooling rate on crystal size during crystallisation?

Rapid cooling produces small crystals, while slow cooling produces large crystals.

Where should the thermometer bulb be positioned in a simple distillation setup, and why?

The thermometer bulb must be positioned just beside the side arm leading to the condenser to accurately measure the boiling point of the vapour entering the condenser.

Why must cooling water enter the bottom of a Liebig condenser and leave from the top?

To ensure the condenser jacket is completely filled with cold water, providing efficient cooling and condensation of vapour.

What apparatus is used to separate two immiscible liquids such as oil and water?

A separating funnel.

What is the function of the glass beads or spiral in a fractionating column during fractional distillation?

They provide a large surface area for repeated vapour condensation and vaporization, allowing efficient separation of miscible liquids with different boiling points.

What do the two horizontal plateaus at $78\,^\circ\text{C}$ and $100\,^\circ\text{C}$ represent in this distillation heating curve?

The plateau at $78\,^\circ\text{C}$ represents ethanol boiling and distilling over, and the plateau at $100\,^\circ\text{C}$ represents water boiling and distilling over.

How can paper chromatography determine if a substance is pure?

A pure substance will produce only a single spot on the chromatogram, whereas a mixture will separate into more than one spot.

What is the definition and formula for the retention factor ($R_f$ value) in paper chromatography?

$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}$$

Does the $R_f$ value of a substance change if the chromatography experiment runs for a longer time?

No, the $R_f$ value remains constant provided the solvent, temperature, and chromatography conditions remain the same.

Based on the chromatogram, which banned dye (A, B, C, or D) is contained within food colouring sample X?

Banned dye C, because sample X contains spots that match the exact vertical height and alignment of dye C.

What is a locating agent, and which locating agent is used to reveal invisible amino acid spots and fingerprints?

A locating agent is a chemical applied to a chromatogram that reacts with colourless substances to form coloured spots; ninhydrin is commonly used.

What are the two effects of impurities on the melting and boiling points of a substance?

Impurities lower the melting point of a solid (causing melting over a range of temperatures) and raise the boiling point of a liquid.

What are two key differences between physical changes and chemical changes?

Physical changes are reversible and do not form new substances; chemical changes are irreversible, form new substances, and involve energy changes (heat or light taken in or given out).

What are the products of the thermal decomposition of limestone?

Calcium oxide (a white solid) and carbon dioxide (a colourless gas) via $\text{CaCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{CO}_2$.

What is the chemical reaction that takes place when an X-ray film is exposed to light or X-rays?

Silver bromide decomposes photochemically into grey silver solid (forming dark areas) and bromine gas via $\text{silver bromide} \xrightarrow{\text{X-ray or light}} \text{silver} + \text{bromine}$.

What is the word equation representing photosynthesis?

$$\text{Carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{sunlight}} \text{glucose} + \text{oxygen}$$

How do sulfur dioxide and nitrogen dioxide pollutants cause acid rain?

Sulfur dioxide (from burning sulfur-containing fossil fuels) and nitrogen dioxide dissolve in cloud water droplets, lowering rainwater pH.

How does the burning of fossil fuels lead to global warming?

Combustion releases excess carbon dioxide into the atmosphere, enhancing the greenhouse effect by trapping longer wavelength infrared radiation.