Water | ICSE Class 8 Chemistry Notes

This note covers water as a compound, its three states, evaporation and condensation, the water cycle, water as a solvent, concentration and saturation, solubility, dissolved gases, density, separation methods, hard water and water conservation.

Why is water a compound rather than a mixture?

An element is a substance that cannot be broken down into simpler substances. An atom is a particle of an element. A molecule is a stable particle formed when atoms combine. Water contains the elements hydrogen and oxygen chemically combined.

A compound contains different elements chemically combined in a fixed ratio. Its properties differ from those of its constituent elements. In a water molecule, the ratio of hydrogen atoms to oxygen atoms is 2:1: two hydrogen atoms for one oxygen atom.

The chemical formula H₂O represents water. Here H is the symbol for hydrogen, O is the symbol for oxygen, and the small 2 indicates two hydrogen atoms. The absence of a number after O indicates one oxygen atom.

How does water differ from salt water?

A mixture contains two or more substances whose individual properties are retained. Its constituent substances are called components. Salt water is a mixture: evaporation can separate the salt from the water. This physical separation separates components without changing their chemical identity; it does not split water into hydrogen and oxygen.

A pure substance consists of the same type of particles. Pure water is therefore a compound and a pure substance, while water containing dissolved salt is a mixture. A clear appearance alone does not establish that a sample contains just one substance.

The drawing shows repeated groups, each with one red sphere labelled oxygen and two smaller pale spheres labelled hydrogen. Each group represents a water molecule.

What happens when water is broken down?

Passing electric current through water containing a few drops of dilute sulphuric acid produces hydrogen and oxygen. Hydrogen gives a pop sound when tested with a burning candle; oxygen makes the flame glow brighter. These observations identify different substances, rather than water vapour.

Water → Hydrogen + Oxygen is the word equation; the arrow means “produces”. This demonstration requires adult supervision and care with the acid and gas tests. In contrast, freezing and boiling leave the water particles unchanged: they change water's state.

How do the three states of water differ?

A state is a physical form of a substance. Water occurs as solid ice, liquid water and gaseous water vapour. Ice and liquid water behave differently, but they are forms of the same substance. Heating or cooling can change one state into another.

Volume means the space occupied by an object or substance. Pouring water between differently shaped containers changes its shape, not its volume. An ice cube placed in another container keeps its own shape unless it melts.

What are melting and freezing?

Melting is the conversion of a solid into a liquid. Ice melts when supplied with heat. Freezing is the conversion of a liquid into a solid. Placing water in a sufficiently cold environment, such as a freezer, changes it into ice.

Water turns into ice at 0 °C. The symbol °C means degrees Celsius, a unit used to express temperature. Ice taken out of a freezer can melt back into water. No extra substance has to be added to explain the change.

Water vapour is invisible and is present in the air even at room temperature. The visible mist associated with steam contains tiny water droplets. Therefore, visible droplets and invisible water vapour should not be treated as identical observations.

Note: A change in appearance does not necessarily mean a new substance has formed. Ice, liquid water and water vapour remain water; breaking water down into hydrogen and oxygen produces different substances.

What controls evaporation and its cooling effect?

Evaporation is the conversion of water into its vapour state. It takes place continuously, even at room temperature. Wet clothes, a mopped floor and water left on washed utensils dry because water passes into the surrounding air as vapour.

Water in a puddle may also seep into the ground. Its disappearance can therefore involve both seepage and evaporation. Water disappearing from a steel plate provides a useful comparison because it does not seep through the plate.

Which conditions change the rate?

Humidity is the amount of water vapour in the air. When this amount is already high, water evaporates slowly. A fair comparison changes one condition while keeping the other relevant conditions the same.

1. Take the same amount of water in a bottle cap and a plate.
2. Keep the two containers near each other so their surroundings are similar.
3. Record the time required for complete evaporation in each case.
4. Compare the results with the exposed areas of water: the larger area allows faster evaporation.

Why does an earthen pot cool water?

Some water seeps through the surface of an earthen pot and evaporates, producing a cooling effect. Sprinkling water on a floor or roof in summer also gives cooling. Moving air helps sweat evaporate and cools the body.

A pot-in-pot cooler uses two earthen pots with wet sand between them and a lid or wet jute sack over the smaller pot. The sand needs regular watering to remain moist. Its cooling time depends on surrounding conditions.

How do condensation and the water cycle work?

Condensation is the conversion of water vapour into liquid water. When water vapour in air meets a cold surface, droplets form. This explains the droplets seen on the outside of a tumbler containing cold water and ice.

How can the origin of these droplets be checked?

1. Half-fill a tumbler with water containing a few ice cubes, cover it with a small steel plate and place it on a digital balance.
2. Record the reading every five minutes for thirty minutes and observe droplets on the outside.
3. An increase in the reading supports condensation, but this observation alone does not conclusively rule out seepage.
4. Repeat with the water level marked. The level does not fall while extra water collects outside, providing further evidence for condensation.

This investigation combines a proposed explanation with observations that can test it. Merely seeing droplets does not reveal their origin. Checking both the balance reading and the marked water level makes the explanation more convincing.

How does water return as rain?

As air moves higher above the Earth's surface, it becomes cooler. At certain heights, water vapour turns into droplets, generally around dust particles. These small droplets form clouds. Droplets join into larger drops, some of which become heavy enough to fall as rain.

Under special conditions, water might also fall as hail or snow. The water cycle is the circulation in which water evaporates from oceans and the Earth's surface, returns as rain, hail or snow, and ultimately flows back to the oceans.

The drawing includes an ocean, land, mountains, a winding river, clouds and falling drops. Upward arrows over water indicate evaporation, and downward arrows beneath clouds indicate rain or snow.

Groundwater is water present underground. The cycle links water above, on and below the Earth's surface. Repeated circulation does not mean that all water is immediately accessible or suitable for human use.

How does water act as a solvent?

A solution is a uniform mixture: its components are evenly distributed throughout. In a solution made by dissolving a solid in a liquid, the dissolved solid is the solute, and the liquid doing the dissolving is the solvent.

When salt dissolves in water, salt is the solute and water is the solvent. Sugar and water provide another example. The resulting mixture is uniform, even though the separate solid is no longer visible.

Definition: A substance is soluble in a liquid if it dissolves in that liquid. A substance that does not dissolve is insoluble in it. Water dissolves many substances, but it does not dissolve every substance.

Is the solvent always the larger component?

In solutions made by mixing two liquids, the component present in the smaller amount is called the solute, and the larger component is called the solvent. This rule must not replace the solid-in-liquid definition.

For example, the sugar syrup, or chashni, around gulab jamun contains a large amount of sugar dissolved in a small amount of water. Sugar is still the solute, and water is still the solvent.

Water's solvent role also explains why dissolved substances can be present in a transparent sample. Uniform appearance means that the components are evenly mixed; it does not mean that the sample is a pure compound.

How do concentration, saturation and solubility differ?

Concentration is the amount of solute present in a fixed quantity of solution or solvent. A dilute solution contains relatively less solute, while a concentrated solution contains relatively more. Dilute and concentrated are relative terms used when comparing solutions.

With the same amount of water, a solution containing one spoon of dissolved salt is more dilute than one containing two spoons. Comparing concentration requires attention to the water or solution quantity as well as the amount of solute.

What happens when more salt is added?

1. Half-fill a clean glass tumbler with water.
2. Add one spoon of salt and stir until it dissolves.
3. Continue adding salt a spoonful at a time, stirring after each addition.
4. Eventually, some salt remains undissolved and settles at the bottom, showing that the dissolving limit has been reached at that temperature.

An unsaturated solution can dissolve more of the solute at a given temperature. A saturated solution has dissolved the maximum amount possible and cannot dissolve more of that solute at that temperature.

Solubility is the maximum amount of a solute that dissolves in a fixed quantity of solvent at a particular temperature. It describes a limit; concentration describes how much solute is actually present. Temperature is essential when stating that a solution is saturated.

One tumbler shows an unsaturated solution without a bottom deposit. The other shows liquid labelled “Solution” above a layer labelled “Undissolved salt”.

Note: “Concentrated” and “saturated” answer different questions. The first compares the amount of dissolved solute; the second states whether more of that solute can dissolve at the stated temperature.

How does temperature affect dissolved solids and gases?

For most substances, solubility increases when temperature increases. Heating can therefore allow more of a solid to dissolve in water. This is a general pattern with a qualification, not a rule that should be stated for every substance.

What does the baking soda demonstration show?

Baking soda is sodium hydrogen carbonate. Begin with about 50 millilitres of water at 20 °C. The abbreviation mL means millilitre, a unit of volume. Stir in baking soda until some remains undissolved.

1. Heat the mixture to 50 °C while stirring. The undissolved baking soda dissolves.
2. Add more baking soda at this temperature until some again remains undissolved.
3. Heat further to 70 °C while stirring. The undissolved solid dissolves again.
4. Compare the observations: water at 70 °C dissolves more baking soda than at 50 °C, and less dissolves at 20 °C.

The demonstration shows how a saturated solution can become unsaturated on heating for this solute. Heating equipment must be handled carefully. The temperature readings describe the experimental conditions; they are not quantities of baking soda.

A beaker stands on wire gauze over a tripod and spirit lamp. A laboratory stand supports a thermometer; a glass rod enters the beaker labelled as containing water and baking soda.

Do gases behave in the same way?

Many gases, including oxygen, dissolve in water. Oxygen dissolves only to a small extent, but this dissolved oxygen sustains aquatic life. Here aquatic means living in water. Dissolved gases form a uniform mixture with water.

The solubility of gases generally decreases as temperature rises. More oxygen can dissolve in cold water; when water warms, oxygen's solubility decreases. This differs from the usual temperature trend for dissolved solids.

What do density measurements tell us about water?

Mass is the quantity of matter in an object. Density is mass per unit volume. Comparing masses without comparing volumes can be misleading: equal-sized objects made of different substances may have different masses.

$\text{Density} = \frac{\text{Mass}}{\text{Volume}}$. The equals sign (=) means “is equal to”; the fraction means mass divided by volume. Mass may be measured in grams, abbreviated g, and volume in cubic centimetres, abbreviated cm³. Density is then expressed in grams per cubic centimetre, written g/cm³.

One millilitre equals one cubic centimetre: $1\,\mathrm{mL} = 1\,\mathrm{cm}^{3}$. At room temperature, the mass of one millilitre of water is close to one gram. Thus, 10 mL of water has a mass of approximately 10 g, and 100 mL has a mass of approximately 100 g.

How are mass and volume measured?

A balance measures mass. To measure a liquid, place its empty container on the balance and reset the reading to zero before adding the liquid. A measuring cylinder measures liquid volume using marked divisions.

The curved surface of water in the cylinder is the meniscus. Keep the cylinder on a flat surface and read the bottom of the water meniscus at eye level. This is the reading rule for water and other colourless liquids.

Worked example 1. A measuring cylinder has ten equal divisions between the 10 mL and 20 mL marks. What volume does one division represent?

Answer: The volume per division is $\frac{20\,\mathrm{mL} - 10\,\mathrm{mL}}{10} = 1\,\mathrm{mL}$. The sign − means subtraction; this cylinder's smallest readable division is therefore 1 mL.

Worked example 2. An aluminium block has a mass of 27 g and a volume of 10 cm³. Calculate its density.

Answer: $\text{Density} = \frac{27\,\mathrm{g}}{10\,\mathrm{cm}^{3}} = 2.7\,\mathrm{g/cm}^{3}$, or 2.7 g/cm³.

Relative density compares a substance's density with the density of water at the same temperature: $\text{Relative density} = \frac{\text{Density of substance}}{\text{Density of water at the same temperature}}$. It has no unit. Taking water's density as approximately 1 g/cm³, aluminium's relative density is approximately 2.7.

How can water measure a solid's volume, and why does ice float?

For a regular solid, volume can be calculated from its dimensions. A cuboid has rectangular faces. Its volume is the product of its length, width and height, with all three dimensions expressed in compatible units.

Worked example 3. A notebook is 25 centimetres long, 18 centimetres wide and 2 centimetres high. Calculate its volume. The abbreviation cm means centimetre.

Answer: $V = l \times w \times h$, where V is volume, l is length, w is width, h is height and × means multiplication. $V = 25\,\mathrm{cm} \times 18\,\mathrm{cm} \times 2\,\mathrm{cm} = 900\,\mathrm{cm}^{3}$. The volume is 900 cm³.

An irregular solid, such as a stone, does not have a convenient regular shape for calculating its volume. The displacement method measures the volume of water displaced when the object is lowered into a measuring cylinder.

1. Record the initial volume of water in the cylinder.
2. Tie the stone with thread and lower it carefully into the water.
3. Record the final reading after the water level rises.
4. Subtract the initial reading from the final reading to obtain the stone's volume.

Worked example 4. A stone has a mass of 16.400 g. When it is immersed, the cylinder reading rises from 50 mL to 55 mL. Find its volume and density, using 1 mL = 1 cm³.

Answer: $\text{Volume displaced} = 55\,\mathrm{mL} - 50\,\mathrm{mL} = 5\,\mathrm{mL}$. The stone's volume is 5 cm³. $\text{Density} = \frac{16.400\,\mathrm{g}}{5\,\mathrm{cm}^{3}} = 3.28\,\mathrm{g/cm}^{3}$.

Two measuring cylinders appear side by side. The first contains water without an object; the second contains a stone suspended by a thread, with the water level higher.

What is unusual about freezing water?

Water has its highest density at 4 °C. When water freezes into ice at 0 °C, its particles arrange themselves in a way that occupies more space. Expansion means an increase in volume.

The same mass now occupies a larger volume, so its density decreases. Ice is therefore less dense than liquid water and floats on its surface. Floating ice forms a layer that keeps the water underneath warm enough for aquatic animals to survive extremely cold weather.

Density is useful when explaining why objects float or sink, but it is not the only factor involved. Avoid deciding solely from whether an object feels heavy. A comparison of mass per unit volume is more informative than mass alone.

How can substances be separated from water?

The choice of a separation method depends on what is mixed with water and which component is required. Insoluble particles, such as mud, behave differently from dissolved salt. A method that removes visible particles need not remove a dissolved substance.

How are sedimentation, decantation and filtration different?

Sedimentation is the settling of a heavier insoluble component at the bottom of a liquid. Decantation removes the liquid by carefully tilting the vessel. Some particles may remain in the poured liquid, so decantation does not necessarily give complete separation.

Filtration separates particles using a filter with pores, or small openings. Filter paper has very fine pores. The solid left on it is the residue; the liquid that passes through and is collected is the filtrate.

Photograph: Filtration. The first photograph shows a hand holding a funnel lined with filter paper. The second shows muddy water in the filter-lined funnel, supported by a tripod above a conical flask that collects the liquid.

How can the water itself be recovered?

Evaporation leaves dissolved salt behind, but the water is lost to the surroundings. Distillation combines vaporisation with cooling the vapour back into liquid. A condenser is the part of the apparatus in which the vapour is cooled and condensed.

The collected liquid is the distillate; water purified by distillation is called distilled water. Distillation can recover the solvent from a solution containing dissolved solids. Crystallisation, by comparison, separates a pure solid as crystals from a saturated solution. The required product determines which process is useful.

What is hard water, and how does it behave with soap?

Hard water contains dissolved calcium and magnesium salts that make soap form foam with difficulty. Soap reacts with these salts to produce an insoluble substance called scum. Insoluble means that the substance does not dissolve in the water.

The scum appears as a white, curdy solid. More soap is needed when washing with hard water. This behaviour concerns the dissolved salts; visible mud and hardness are different properties of a water sample.

How can soap be used to compare water samples?

1. Place about 10 mL of distilled water and 10 mL of hard water in separate test tubes.
2. Add a couple of drops of soap solution to each tube.
3. Shake both tubes vigorously for the same length of time.
4. Compare the foam and look for white curdy scum: the hard-water sample forms scum and gives foam with greater difficulty.

Equal sample volumes and equal shaking times make the comparison fair. The difference is linked to the water samples rather than to deliberately different treatment. Distilled water provides the comparison with water containing hardness-producing salts.

Why are detergents useful in hard water?

Detergents are cleansing agents that remain effective in hard water because they do not form the same insoluble precipitates with its calcium and magnesium components. A precipitate is an insoluble solid formed in a liquid during a reaction.

Consequently, detergent foam is not a reliable replacement for the soap comparison when checking hardness. The choice of cleansing agent changes the observation. Using a detergent successfully does not itself demonstrate that the dissolved hardness-producing salts have been removed.

Note: Removing visible particles by filtration and investigating dissolved salts with soap answer different questions. A sample can look clear and still contain dissolved substances that affect its use.

What is water of crystallisation?

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. A formula unit represents the composition of the salt in the ratio shown by its chemical formula. Crystals can therefore contain water even when they appear dry.

What happens when copper sulphate crystals are heated?

Blue copper sulphate crystals contain water of crystallisation. Heating removes this water, and the salt turns white. Water droplets can be observed inside the boiling tube. Adding water again restores the blue colour.

1. Take a few crystals of copper sulphate in a dry boiling tube.
2. Heat the crystals and observe the change from blue to white.
3. Look for water droplets in the tube, showing that water has been released.
4. Add two or three drops of water to the sample after heating and observe the return of the blue colour.

Hydrated means containing water of crystallisation.

This fixed water content differs from water acting as the liquid solvent in a salt solution. Dry-looking crystals and a wet solution should therefore not be confused simply because both contain water.

Why must water sources be protected and water conserved?

Water is needed for drinking, cooking, bathing, washing and cleaning. It is also used to grow crops and in industry. Freshwater, rather than saline or salty ocean water, supplies these needs. Most of the Earth's water is in oceans and seas.

Freshwater occurs in ice sheets and snow, rivers and lakes, and underground. Much of it is difficult to access. Only a very small fraction is easily accessible in sources such as ponds, rivers, lakes and wells.

How do human activities affect the supply?

Water pollution occurs when unwanted waste contaminates water sources. Waste dumped from homes and industries, along with plastic bags and wrappers, pollutes freshwater. Polluted water is not fit for consumption by living beings.

Rising population increases demand for water. Limited freshwater sources and unequal access mean that people in some places must travel long distances for drinking water. The existence of a water cycle does not remove these difficulties.

What is rainwater harvesting?

Rainwater harvesting is collecting and storing rainwater for later use. Buildings can collect substantial quantities. Turning off unused taps, fixing leaks, recycling water and harvesting water all help reduce waste and conserve supplies.

Traditional stepwells include Bawadi in Rajasthan and Vav in Gujarat. They collect rainwater and water seeping from nearby lakes, ponds and rivers. Stone-lined trenches allow water to seep into these storage structures.

Conservation also requires keeping water sources free from waste. Saving water and preventing pollution support each other: conserving a quantity of water is less useful if that water becomes unfit for use.

World Water Day is observed on 22 March each year. Its connection with daily life is practical: water must be used carefully, its sources protected, and avoidable losses reduced.

Glossary

• Compound — A substance formed from different elements chemically combined in a fixed ratio, with properties different from those elements.
• Evaporation — The conversion of liquid water into water vapour, occurring even at room temperature.
• Condensation — The conversion of water vapour into liquid water, as on a cold surface.
• Solution — A uniform mixture in which the components are evenly distributed throughout the mixture.
• Solute — The dissolved component, such as salt in a solution made from salt and water.
• Solvent — The component that dissolves the solute, such as water when salt dissolves in it.
• Concentration — The amount of solute present in a fixed quantity of solution or solvent.
• Saturated solution — A solution that cannot dissolve any more of a particular solute at the given temperature.
• Solubility — The maximum amount of solute that dissolves in a fixed quantity of solvent at a particular temperature.
• Density — The mass present in a unit volume of a substance, calculated by dividing mass by volume.
• Meniscus — The curved surface of a liquid in a container such as a measuring cylinder.
• Filtration — Separation using a filter that retains solid particles while allowing the liquid to pass through.
• Distillation — A separation process involving vaporisation followed by condensation so that the liquid can be collected.
• Hard water — Water containing dissolved calcium and magnesium salts that react with soap to form insoluble scum.
• Rainwater harvesting — The collection and storage of rainwater so that it can be used later.

Common errors and misconceptions

• Misconception: Water is a mixture of hydrogen and oxygen. Correct: Water is a compound containing these elements chemically combined in a fixed atom ratio of 2:1.
• Misconception: Evaporation occurs only when water boils. Correct: Water evaporates continuously, including at room temperature, as shown by drying clothes and floors.
• Misconception: Drops outside a cold glass must have leaked through it. Correct: Water vapour in the air condenses on the cold surface; marking the water level helps test the explanation.
• Misconception: Water dissolves every substance. Correct: Salt and sugar dissolve, whereas sand and sawdust form non-uniform mixtures with water.
• Misconception: A concentrated solution is necessarily saturated. Correct: Concentration compares solute quantities; saturation means no more of that solute dissolves at the given temperature.
• Misconception: Heating increases the solubility of all substances. Correct: Most solids follow this pattern, but the solubility of gases generally decreases on heating.
• Misconception: Ice floats because it contains less water than an equal mass of liquid water. Correct: The same mass occupies more space as ice, making its density lower.
• Misconception: A clear water sample cannot be hard. Correct: Dissolved calcium and magnesium salts cause hardness; visible suspended dirt is a different issue.

Exam-style questions with model answers

Q1. Define a solute and a solvent using a solution made by dissolving salt in water. [2 marks]

1. The solute is the component that dissolves. In this solution, salt is the solute.
2. The solvent is the liquid in which the solute dissolves. Here, water is the solvent.

Q2. Explain three observations: an ice cube keeps its shape, poured water changes shape without changing volume, and water vapour spreads throughout available space. [3 marks]

1. Ice is water in the solid state. It retains its own shape rather than taking the shape of each container into which it is placed.
2. Liquid water flows and takes the shape of its container. Its shape can change while the amount of space it occupies remains constant.
3. Water vapour is the gaseous state. A gas has no fixed shape and spreads throughout the available space instead of keeping a definite shape.

Q3. Salt is added gradually to a fixed amount of water at a fixed temperature. Initially it dissolves, but eventually some remains undissolved after stirring. Explain the change and distinguish saturation from concentration. [4 marks]

1. Initially, the solution is unsaturated because it can dissolve more salt at the stated temperature.
2. Eventually, the water reaches its dissolving limit. Additional salt remains undissolved rather than becoming part of the solution.
3. The solution is then saturated: no more of that solute can dissolve at that temperature.
4. Concentration is the amount of solute present in a fixed quantity of solution or solvent. It describes the amount present, whereas saturation describes whether the dissolving limit has been reached.

Q4. A stone has mass 16.400 g. Immersing it changes a measuring-cylinder reading from 50 mL to 55 mL. Using 1 mL = 1 cm³, calculate its volume and density, showing both steps. [2 marks]

1. The stone displaces 55 − 50 = 5 mL of water, so its volume is 5 cm³.
2. Density is mass divided by volume: 16.400 g / 5 cm³ = 3.28 g/cm³.

Q5. Describe how evaporation and condensation contribute to the water cycle, from water leaving the surface to its return to the oceans. Include cloud formation and rain. [5 marks]

1. Water evaporates from oceans and the Earth's surface. Liquid water changes into water vapour and enters the surrounding air, beginning this part of its circulation.
2. As air moves higher above the Earth's surface, it becomes cooler. At certain heights it becomes cool enough for water vapour to condense.
3. The vapour changes into small liquid droplets, generally around dust particles. These small droplets remain in the air and form clouds.
4. Droplets join together into larger drops. Some drops become heavy enough to fall as rain; under special conditions, water might also fall as hail or snow.
5. Water returning to the Earth's surface ultimately flows back to the oceans. This repeated movement between the surface and atmosphere forms the water cycle.

Q6. A muddy-water mixture contains insoluble mud. Describe filtration with filter paper, naming the residue and filtrate. Then explain how distillation recovers water from a separate salt solution. [5 marks]

1. Fold filter paper into a cone and place it in a funnel positioned over a conical flask, so the liquid passing through can be collected.
2. Pour the muddy water into the filter. Its fine pores allow water through while retaining the mud particles on the paper.
3. The retained mud is the residue. The liquid collected in the flask is the filtrate; these names distinguish the two separated portions.
4. For the salt solution, heat the liquid so that water forms vapour. Unlike simple evaporation, distillation collects this vapour for further treatment.
5. Cool the vapour in a condenser so it returns to liquid water. Collect this distillate separately while the dissolved solid remains behind.

Q7. Compare equal 10 mL samples of distilled water and hard water using equal additions of soap solution and equal shaking times. State the expected difference, its cause and why detergent would be unsuitable as a replacement in this comparison. [3 marks]

1. The hard-water sample forms foam with greater difficulty and produces white, curdy scum. The distilled-water sample provides the comparison without this hardness-related soap reaction.
2. Calcium and magnesium salts in hard water react with soap to form insoluble material. This consumes soap, so a larger quantity of soap is needed.
3. Detergents remain effective in hard water because they do not form the same insoluble precipitates. Their foam therefore does not provide the same test for hardness.

Q8. State how increasing temperature generally affects the solubility of solids and gases in water, and explain the importance of dissolved oxygen. [3 marks]

1. For most substances in the solid-solute comparison, solubility increases with temperature. Heating the baking soda mixture allows additional solid to dissolve, illustrating this general pattern.
2. The solubility of gases generally decreases as temperature rises. More oxygen can dissolve in cold water than in warm water under the comparison described.
3. Oxygen dissolves only to a small extent, but this dissolved oxygen sustains aquatic life. Its small dissolved quantity does not mean that its role is unimportant.

Key takeaways

• Water is a compound with hydrogen and oxygen atoms combined chemically in the fixed ratio of two to one.
• Ice, liquid water and water vapour are states of the same substance, with different shapes and abilities to flow.
• Evaporation occurs at room temperature, becomes faster with greater exposed area and air movement, and produces cooling.
• Condensation forms liquid droplets from water vapour and helps return evaporated water to the Earth's surface.
• Saturation states a dissolving limit at a particular temperature; concentration describes the amount of solute present.
• Most solids become more soluble on heating, whereas the solubility of gases generally decreases as temperature rises.
• Density equals mass divided by volume; ice floats because the same mass occupies more space than liquid water.
• Filtration removes suitable insoluble particles, evaporation recovers dissolved solids, and distillation allows the liquid solvent to be collected.
• Calcium and magnesium salts cause hardness by reacting with soap to form scum; protecting and conserving freshwater remains essential.

Test yourself

What does the 2 in H₂O represent?

It represents two hydrogen atoms in each water molecule, combined with one oxygen atom.

Why can a wet floor dry without being heated to boiling?

Water evaporates continuously, even at room temperature, changing into water vapour in the air.

Why does sugar remain the solute in concentrated gulab jamun syrup?

In a solid-in-liquid solution, the dissolved solid is the solute. Water remains the solvent even when much sugar is dissolved.

What condition must accompany the statement that a solution is saturated?

The temperature must be stated because the amount of solute that can dissolve depends on temperature.

How does warming water generally affect the amount of oxygen it can dissolve?

Warming decreases oxygen's solubility, so less oxygen can dissolve than in colder water.

Where should the eye be when reading water in a measuring cylinder?

The eye should be level with the bottom of the curved water surface, called the meniscus.

What is the difference between residue and filtrate?

Residue is the material retained on the filter; filtrate is the liquid that passes through and is collected.

Why does using detergent successfully not establish that hardness-producing salts are absent?

Detergents remain effective in hard water, so successful cleaning does not establish removal of hardness-producing salts.