Acids and bases

To decide what makes an aqueous solution neutral, we focus on the balance between two ions: the hydrogen ion (${\text{H}}^{+}$) and the hydroxide ion (${\text{OH}}^{-}$). This idea sits at the heart of the Acids and bases topic: a neutral solution has no overall acidic or basic excess because these ions are balanced.

Water is not completely “inactive”—even in pure water, some molecules split and recombine continuously. This self-ionization can be shown by:

$\begin{array}{rl}{\text{H}}_2\text{O (l)}& \rightleftharpoons {\text{H}}^{+}\text{ (aq)}+{\text{OH}}^{-}\text{ (aq)}\end{array}$

Each time a water molecule dissociates, it produces one ${\text{H}}^{+}$ and one ${\text{OH}}^{-}$. That means they are created in a 1:1 ratio, and in a neutral solution nothing is present to disturb that equal production.

Now compare each option to the definition of neutrality:

• Option A: Having extra ${\text{H}}^{+}$ compared with ${\text{OH}}^{-}$ would make the solution acidic, so it cannot describe neutrality.
• Option B: Because of the equilibrium above, there are always some ions in water; it is never completely free of ions.
• Option D: In water, ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ appear together due to the equilibrium—an aqueous solution cannot contain one and completely lack the other.

Neutrality means the acidic and basic ions are present in equal amounts, so the correct description is that hydrogen and hydroxide ion concentrations are the same. At room temperature ($25^{\circ }\text{C}$ or $298\text{ K}$), both are $1.0\times 10^{-7}{\text{ mol dm}}^{-3}$.

In any water-based solution, a small fraction of water molecules dissociate into hydrogen ions and hydroxide ions. This auto-ionization happens continuously and is reversible, so it is written as a dynamic equilibrium:

${\text{H}}_2\text{O}(l)\rightleftharpoons {\text{H}}^{+}(aq)+{\text{OH}}^{-}(aq)$

This is essential background for Topic 3: Types of chemical reaction (Neutral solutions).

At room temperature ($25^{\circ }\text{C}$), the concentrations of these ions are linked by a constant called the ionic product of water, $K_w$:

$K_w=[{\text{H}}^{+}][{\text{OH}}^{-}]=1.0\times 10^{-14}$

So if one of these ion concentrations rises, the other must fall so that their product stays at this fixed value.

A neutral solution is defined by equal concentrations of hydrogen ions and hydroxide ions.

Let $x=[{\text{H}}^{+}]=[{\text{OH}}^{-}]$. Then:

$\begin{array}{rl}x\cdot x& =1.0\times 10^{-14}\\ x^2& =1.0\times 10^{-14}\\ x& =\sqrt{1.0\times 10^{-14}}\\ x& =1.0\times 10^{-7}{\text{ mol dm}}^{-3}\end{array}$

This gives $[{\text{H}}^{+}]=[{\text{OH}}^{-}]$, which corresponds to pH 7.

Using the equilibrium relationship:

• Option A: If $[{\text{H}}^{+}]>[{\text{OH}}^{-}]$, the solution is acidic, so it is not neutral.
• Option B: Auto-ionization ensures there are always some ions present (around $10^{-7}$ M), so “no ions” is incorrect.
• Option D: The $K_w$ relationship requires both ions to exist together; one cannot be absent in water.

Because neutrality requires equal acidic and basic ion concentrations, the correct choice is the one stating that the hydrogen ion concentration equals the hydroxide ion concentration.

Start with the definition: in water, some molecules always form ions through auto-ionization:

${\text{H}}_2\text{O}(\text{l})\rightleftharpoons {\text{H}}^{+}(\text{aq})+{\text{OH}}^{-}(\text{aq})$

So any aqueous solution contains both ${\text{H}}^{+}$ and ${\text{OH}}^{-}$. A solution is called 'neutral' when these two ion concentrations are equal.

Option A claims there is an excess of ${\text{H}}^{+}$ over ${\text{OH}}^{-}$ ($[{\text{H}}^{+}]>[{\text{OH}}^{-}]$). That situation is acidic rather than neutral, so this option must be eliminated.

Options B and D suggest that ions are missing.

• Option B says there are no ions, but water’s ongoing auto-ionization means ions are always present.
• Option D says ${\text{OH}}^{-}$ is not present, which cannot happen in an aqueous equilibrium because ${\text{H}}^{+}$ and ${\text{OH}}^{-}$ must coexist according to the ionic product of water ($K_w$).

So both of these options are eliminated.

With the incorrect statements removed, the remaining description matches neutrality. At room temperature ($298\text{ K}$), a neutral solution has:

$[{\text{H}}^{+}]=[{\text{OH}}^{-}]=1.0\times 10^{-7}{\text{ mol dm}}^{-3}$

This equality means the solution is neither acidic nor alkaline.

We need to name a typical physical sensation associated with bases when they touch skin. This is a basic fact from Topic 3: Types of chemical reaction (Acid and base characteristics) about how acids and bases can be identified by simple observations.

Check each description. A base solution doesn’t normally feel like something rough or gritty. It also isn’t typically sticky like syrup, and it isn’t described as dry when it’s in solution.

A standard memory cue in chemistry is that bases (alkalis) feel soapy or slippery. This helps distinguish them from acids, which are more often linked with sour taste or a stinging sensation.

That slippery feeling happens because bases can trigger saponification. When a base contacts skin, it reacts with fatty acids (natural oils) on the surface and produces a tiny amount of soap, which makes the skin feel slick.

Since bases are known for a soap-like, slippery texture, the correct choice is soapy.

We’re identifying a common physical clue for bases. We already know bases have a $\text{pH}>7$, and we’re looking for the typical touch sensation they produce on skin. Now we can remove the options that don’t fit.

Rough suggests something abrasive, like sand or a coarse surface. A basic solution such as aqueous sodium hydroxide ($\text{NaOH}$) doesn’t feel gritty. Since bases are common in cleaning products and don’t create an abrasive feel, this option doesn’t match.

Sticky is typical of thick substances like honey or glue, often due to sugars or long-chain polymers. Even if some mixtures can be sticky, that isn’t a key identifying feature of bases in general.

Dry implies little moisture or a dusty/powdery feel. Most bases we test in class are aqueous solutions, so they feel wet. Also, bases tend to feel slippery rather than dry because they interact with skin oils, so this choice can be eliminated.

After removing the other textures, soapy is left—and it’s the scientifically correct description. Bases can cause saponification on contact with skin oils, producing small amounts of soap and giving a slippery/soapy sensation.

We need the precise term for a substance that fits both parts of the description:

1. It is a base.
2. It is base that is soluble in water and, when dissolved, produces hydroxide ions, ${\text{OH}}^{-}(\text{aq})$.

This is a core vocabulary definition in the MYP Acids and bases unit.

This choice does not match. Using the Brønsted–Lowry idea (and the earlier Arrhenius model used at this level), an acid in water increases $H^{+}$ (or $H_3{O}^{+}$) in solution. The question is about making ${\text{OH}}^{-}$ instead, so it cannot be an acid.

Indicators (such as litmus or phenolphthalein) are substances used to test whether a solution is acidic or basic by changing color. They help you notice the presence of bases/${\text{OH}}^{-}$, but they are not the base being described.

Some bases are metal oxides (for example magnesium oxide, $MgO$), but the word “oxide” only tells you the compound contains oxygen. Many oxides are not bases, and even basic oxides are not necessarily soluble in water. So “oxide” does not match the specific water-soluble base definition.

An alkali is defined as a base that dissolves in water to give an aqueous solution containing hydroxide ions, ${\text{OH}}^{-}$.

For instance, sodium hydroxide ($NaOH$) is a base, and because it dissolves in water it is classified as an alkali:

$NaOH(s)\stackrel{H_{2}O}{\to }N{a}^{+}(aq)+O{H}^{-}(aq)$

So the correct word is alkali.

We are looking for the name of something that matches both conditions:

1. It is a base.
2. It is soluble in water and produces hydroxide ions, OH^-(aq).

Now we can rule out options that do not fit.

An acid produces ${\text{H}}^{+}(\text{aq})$ (or ${\text{H}}_3{\text{O}}^{+}(\text{aq})$) when dissolved in water. Since the question specifically mentions hydroxide ions, ${\text{OH}}^{-}$, this option contradicts the requirement and must be removed.

An indicator is used to show whether a solution is acidic or basic through a color change. It can reveal the presence of ${\text{OH}}^{-}$, but it is not itself the water-soluble base described in the question.

An oxide simply means a compound made of oxygen plus another element. Some oxides (like ${\text{Na}}_2\text{O}$) behave as bases, but many oxides do not dissolve in water, and some (like ${\text{CO}}_2$) are acidic. Therefore, “oxide” does not specifically mean a water-soluble base.

An alkali is exactly a base that dissolves in water to produce hydroxide ions, ${\text{OH}}^{-}(\text{aq})$, so it fits every part of the description.

Example: $\text{NaOH}(\text{s})\stackrel{{\text{H}}_2\text{O}}{\to }{\text{Na}}^{+}(\text{aq})+{\text{OH}}^{-}(\text{aq})$

In IB Chemistry, especially in the section on pH, titrations and indicators, we learn that different tools can be used to judge how acidic or basic a solution is.

A universal indicator matches the statement given: it is created by blending multiple acid–base indicators (for example, phenolphthalein, methyl red, and bromothymol blue) so that the colour changes gradually across a broad span of $pH$ values, rather than switching at just one point.

A single indicator (such as litmus) gives only a simple message—acid or base. Universal indicator, however, produces many possible colours (red through to violet).

Because each colour corresponds to a range on a pH chart (usually covering $pH$ 0 to 14), you can estimate a particular $pH$ by matching the colour. The statement describes this use correctly, so it should be accepted as correct.

Since the statement correctly describes what universal indicator is and what it’s used for, choosing "False" would not make sense.

The sentence given is essentially the standard description of a universal indicator, so the correct selection is "True".

The statement begins by saying universal indicator is a mixture of several different indicators.

Many common indicators (like litmus or phenolphthalein) are single chemicals that change colour over a limited pH interval. Universal indicator is different because it is deliberately made by combining several indicators (such as methyl red, bromothymol blue, and phenolphthalein). So this first claim is correct.

Next, the statement says it can provide a range of colours.

Because the mixture contains several indicators, each component shifts colour at different acid/base strengths. Together, they form a wide colour spectrum—for instance, red in strong acids ($pH<3$), green at neutral ($pH=7$), and purple in strong bases ($pH>11$). This part is also correct.

Finally, it says the colours can be used to estimate the specific pH value.

In practice, you compare the solution’s colour to a universal indicator colour chart to get an approximate $pH$. It won’t be as exact as a digital $pH$ meter, but it is a standard quick visual method in school labs. This claim is correct as well.

Since each part of the statement checks out scientifically, the option "False" must be the incorrect choice.

Overall, the statement accurately explains what universal indicator is made of, what you observe (many colours), and how it’s used (to estimate $pH$), so the correct answer is "True".

This question is about what happens when an ammonium salt (for example, ammonium chloride, $N{H}_{4}Cl$) is treated with a base such as sodium hydroxide, $NaOH$. That combination is commonly used as a test for ammonium ions because it releases a characteristic gas with a noticeable smell.

From Types of chemical reaction, a typical rule is: ammonium salts react with strong bases to form a salt, water, and a particular gas. The word equation is:

$\text{ammonium salt}+\text{base}\to \text{salt}+\text{water}+\text{ammonia gas}$

A common example is: $N{H}_{4}Cl(aq)+NaOH(aq)\to NaCl(aq)+H_{2}O(l)+N{H}_3(g)$

The gas $N{H}_3$ is famous for an intense, irritating, pungent smell.

Hydrogen ($H_2$) is usually made when an acid reacts with a reactive metal (such as $Mg+2HCl\to MgC{l}_2+H_2$). Also, hydrogen has no smell (it is colorless and odorless), so it does not match a “sharp and pungent” gas.

We can also remove Nitrogen and Carbon dioxide:

• Nitrogen ($N_2$) is the main gas in air and is very unreactive; it is odorless.
• Carbon dioxide ($C{O}_2$) is typically produced when acids react with carbonates. Like hydrogen and nitrogen, it is odorless and colorless, so it doesn’t fit the smell clue.

Ammonia ($N{H}_3$) is the only option that is produced when an ammonium salt reacts with a base and is also known for a sharp, pungent odor. A further check is that it turns damp red litmus paper blue, showing it is basic.

We need to identify the gas released in the reaction, and the standout clue is that it has a “sharp and pungent smell.” So we compare the typical odors (or lack of odor) of the gases in the choices.

Hydrogen ($H_2$) is a colorless, odorless gas. Since it doesn’t have a smell, it cannot be the gas described.

Nitrogen ($N_2$) is about $78\%$ of the atmosphere. It is colorless and odorless, so it doesn’t match the pungent smell either.

Carbon dioxide ($C{O}_2$) is the gas we breathe out and the gas in fizzy drinks. It is odorless, so it cannot be responsible for a sharp, pungent odor.

After eliminating the odorless options, the remaining choice is Ammonia ($N{H}_3$). Ammonia is well known for a strong, sharp, pungent smell (often associated with some cleaning products), which matches the description.

The prompt is describing the usual lab check for carbonates: you add an acid and look for bubbling. To answer, we need to remember what products form when an acid reacts with a metal carbonate.

There is a key word equation for reactions between acids and metal carbonates that you should know:

$\text{acid}+\text{metal carbonate}\to \text{salt}+\text{water}+\text{carbon dioxide}$

This is one of the core reaction patterns in the Types of chemical reaction content.

Effervescence means gas escaping quickly from a liquid, seen as fizzing/bubbles. From the general equation, the only product that is a gas under room conditions is carbon dioxide ($C{O}_2$).

We can dismiss the other gases by thinking about where they usually come from in school chemistry:

• Oxygen is commonly made when hydrogen peroxide breaks down, not when acids react with carbonates.
• Hydrogen is produced when an acid reacts with a metal, rather than a metal carbonate.
• Ammonia is released when ammonium salts are warmed with a base.

Using the acid + metal carbonate reaction pattern, the bubbles are caused by carbon dioxide being produced.

The situation is the classic lab test where a carbonate is mixed with an acid. This reaction is part of Topic: Acid and base characteristics. To solve it by elimination, compare each gas option with how it is typically produced.

In many school experiments, oxygen ($O_2$) is generated by breaking down hydrogen peroxide ($H_2{O}_2$) with a catalyst or by heating potassium chlorate. It is not expected from an acid reacting with a carbonate, so this option is wrong.

Hydrogen ($H_2$) is the typical gas when an acid reacts with a reactive metal (for example, magnesium with hydrochloric acid). Because the substance here is a carbonate, not a metal, hydrogen is not the gas produced.

Ammonia ($N{H}_3$) is usually prepared by heating an ammonium salt (such as ammonium chloride) with a strong base (such as calcium hydroxide). That does not match an acid–carbonate reaction, so this option is incorrect.

After eliminating the others, carbon dioxide remains. This also fits the chemistry because carbonates contain the $C{O}_3^{2-}$ ion. When acid is added, the overall reaction is:

$\text{acid}+\text{metal carbonate}\to \text{salt}+\text{water}+\text{carbon dioxide}$

The fizzing is the $C{O}_2$ leaving the solution.

Start by deciding whether the sample is acidic or alkaline. The prompt tells us the sample is lemon juice and that it contains citric acid.

Since citric acid is an acid, lemon juice is an acidic solution with a $pH$ well below 7. Fresh lemon juice is commonly around $pH$ $2.0$ to $3.0$.

Now use the known colour changes for methyl orange.

From the IB Data Booklet (Section 22: Acid-base indicators), methyl orange changes from red to yellow as the $pH$ goes up, over the range $3.1–4.4$:

$\begin{array}{cc}pH<3.1& \text{Red}\\ 3.1-4.4& \text{Orange (transition)}\\ pH>4.4& \text{Yellow}\end{array}$

Lemon juice is roughly $pH$ $2.2$ to $2.5$, which is lower than $3.1$. That places it in the “red” region for methyl orange.

• Option A cannot be correct because yellow is only seen once the $pH$ is above $4.4$.
• Option C cannot be correct because methyl orange does not show blue at any $pH$ (blue is linked to other indicators such as litmus or bromothymol blue).
• Option D cannot be correct because orange is mainly seen within the transition interval, closer to about $pH$ $3.5–4.0$.

Because the lemon juice is acidic enough to be below $pH$ $3.1$, methyl orange will display its acidic colour: red.

First, interpret what the sample is. The question refers to lemon juice, and it specifically notes citric acid is present.

That means the solution has a low pH, often in the $2.0$ to $3.0$ range, so it is clearly acidic on the pH scale.

Next, think about what an indicator does. methyl orange changes colour depending on how acidic the solution is (how much $H^{+}$ is present).

Its colour pattern is:

• Red: pH below $3.1$ (acidic)
• Yellow: pH above $4.4$ (basic/alkaline)
• Orange: Transition range between pH $3.1$ and $4.4$

Since lemon juice is strongly acidic, we can remove answers that match neutral/alkaline conditions or impossible colours for this indicator:

• Option A and C: Yellow corresponds to higher pH values (not a strong acid), and blue is not a methyl orange colour at all (that’s more like litmus or bromothymol blue).
• Option D: Orange would suggest the pH is inside the transition window, but lemon juice is usually more acidic than that.

Because lemon juice typically has a pH below $3.1$, methyl orange will show its acidic colour, red.

So, after adding methyl orange to the fresh lemon juice, the mixture should turn red.

The solution is lemon juice, and it contains citric acid. That tells us immediately the sample is acidic, with a low pH (commonly around 2 to 3).

Blue is typically linked with alkaline conditions (for example, litmus turns blue in a base). Also, methyl orange does not produce a blue colour at any point, so this option can be ruled out straight away.

Methyl orange can be red, orange, or yellow. Yellow is seen when the pH is above 4.4 (only slightly acidic to basic conditions). Since lemon juice is a fairly strong natural acid, it will not give yellow with methyl orange.

That leaves orange versus red. Even though the indicator’s name includes “orange,” methyl orange becomes clearly red when the pH is below 3.1. Lemon juice is acidic enough to be in that range, so the correct colour is red.

When a strong acid such as $\text{HCl}$ is put into water, it separates into hydrogen ions (${\text{H}}^{+}$) and chloride ions (${\text{Cl}}^{-}$). However, in an aqueous solution the ${\text{H}}^{+}$ ion is essentially just a proton and it does not remain “free” for long. It immediately attaches to a water molecule by bonding to a lone pair on oxygen:

${\text{H}}^{+}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}$

We can discard options by checking what each name normally refers to:

• Hydride ion: This corresponds to ${\text{H}}^{-}$ (hydrogen with an extra electron). It appears in compounds like metal hydrides (for example, $\text{NaH}$), not in acidic water.
• Hydroxide ion: This is ${\text{OH}}^{-}$. It forms when water gives up a proton and is typical in alkaline solutions.
• Hydrogenate ion: This is not a recognized standard name for a simple inorganic ion; it resembles the word “hydrogenation,” which describes a reaction type rather than a specific ion.

In the IB Chemistry Topic: Acids and Bases, the species ${\text{H}}_3{\text{O}}^{+}$ is specifically named the hydronium ion. It is the protonated water molecule found in acidic aqueous solutions.

A strong acid like $\text{HCl}$ dissolves in water and ionizes fully to produce hydrogen ions (${\text{H}}^{+}$) and chloride ions (${\text{Cl}}^{-}$). As described, those ${\text{H}}^{+}$ ions are so reactive that they attach to water molecules right away:

${\text{H}}^{+}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}$

The question is asking for the name of that ion, which is a key idea in Topic: Acids and bases.

The hydride ion is written as ${\text{H}}^{-}$. It has a negative charge because hydrogen has gained an electron, and it commonly occurs in substances such as $\text{NaH}$. Since the ion here is ${\text{H}}_3{\text{O}}^{+}$ (positive), this cannot be the correct name.

The hydroxide ion is ${\text{OH}}^{-}$. It is associated with basic solutions rather than the species formed when an acid is added to water. It also has a negative charge, which does not fit the positive charge on ${\text{H}}_3{\text{O}}^{+}$.

Hydrogenate is a word most often connected to hydrogenation (adding hydrogen in a reaction). It is not a standard IUPAC name for an ion like ${\text{H}}_3{\text{O}}^{+}$. This choice is included to be misleading because it sounds chemistry-related.

With the other choices eliminated, the correct term is hydronium ion. This is the accepted name for ${\text{H}}_3{\text{O}}^{+}$, formed when a proton becomes hydrated in water.

Start by identifying what is reacting. One reactant is an acid (hydrochloric acid), and the other is a metal carbonate (calcium carbonate).

In your chemistry learning, especially in Topic 3: Types of chemical reaction, you meet patterns like this where acids react in consistent, repeatable ways depending on what they are mixed with.

For an acid reacting with a metal carbonate, the general word equation is:

$\text{acid}+\text{metal carbonate}\to \text{salt}+\text{water}+\text{carbon dioxide}$

For these specific substances, that becomes:

$\text{HCl}+{\text{CaCO}}_3\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$

The "effervescence" described is simply the bubbling you see when a gas is produced.

Now remove the options that do not match an acid–carbonate reaction:

• Hydrogen (${\text{H}}_2$): Usually formed when acids react with metals (for example, $Mg+HCl$), not when they react with carbonates.
• Oxygen (${\text{O}}_2$): More commonly produced in reactions like hydrogen peroxide breaking down or some thermal decompositions, not in a typical acid + carbonate mix.
• Ammonia (${\text{NH}}_3$): Typically released when ammonium compounds react with strong bases, which is a different situation from this one.

The reaction rule shows that the gas released is carbon dioxide ($C{O}_2$). That $C{O}_2$ escaping from the liquid is what creates the visible fizzing (effervescence).

Check the formulas to see which atoms are available at the start:

1. Hydrochloric acid: $\text{HCl}$ (Hydrogen and Chlorine)
2. Calcium carbonate: ${\text{CaCO}}_3$ (Calcium, Carbon, and Oxygen)

So the elements that can appear in products are: $\text{Ca}$, $\text{Cl}$, $\text{H}$, $\text{C}$, and $\text{O}$.

Consider Option D: ammonia, ${\text{NH}}_3$.

Ammonia contains Nitrogen ($\text{N}$). But neither reactant ($\text{HCl}$ or ${\text{CaCO}}_3$) includes nitrogen. By conservation of mass, reactions rearrange existing atoms rather than creating new elements, so ammonia cannot be produced here.

Compounds with the carbonate group (${\text{CO}}_3^{2-}$) are known for giving off a carbon-based gas when treated with acids.

From the remaining choices:

• Hydrogen (${\text{H}}_2$)
• Oxygen (${\text{O}}_2$)
• Carbon dioxide (${\text{CO}}_2$)

Only carbon dioxide contains Carbon ($\text{C}$), which is present in calcium carbonate. In this type of reaction, hydrogen and oxygen atoms commonly end up as water ($H_{2}O$), while the carbon and oxygen from the carbonate group form the gas.

Even though ${\text{H}}_2$ and ${\text{O}}_2$ use elements found in the reactants, they are not what you normally get from an acid–carbonate reaction.

• Hydrogen gas is the typical gas when acids react with a metal, not with a carbonate.
• Oxygen gas is not usually produced directly in simple acid reactions at room temperature.

Putting the element check together with carbonate behavior, the gas released must be carbon dioxide.

The full balanced equation is: $\text{2HCl(aq)}+{\text{CaCO}}_3\text{(s)}\to {\text{CaCl}}_2\text{(aq)}+{\text{H}}_2\text{O(l)}+{\text{CO}}_2\text{(g)}$

The ${\text{CO}}_2$ produced is what you observe as bubbles (effervescence).

Begin by picking out the chemicals mentioned. Vinegar contains ethanoic acid ($C{H}_{3}COOH$), and the limescale deposit is calcium carbonate ($CaC{O}_3$). That combination is a typical case of an acid reacting with a metal carbonate (which often produces bubbles).

Chemists rely on common reaction patterns to predict products. Whenever an acid reacts with a metal carbonate, the products follow this pattern:

$\text{acid}+\text{metal carbonate}\to \text{salt}+\text{water}+\text{carbon dioxide}$

So we should expect a salt to form, plus water, and $C{O}_2$ gas (the fizzing).

Here, calcium carbonate reacting with ethanoic acid forms the salt calcium ethanoate. The balanced symbol equation is:

$2C{H}_{3}COOH(aq)+CaC{O}_3(s)\to (C{H}_{3}COO{)}_{2}Ca(aq)+H_{2}O(l)+C{O}_2(g)$

Cleaning works because the solid deposit ($CaC{O}_3$) is changed into calcium ethanoate, which is soluble in water, so it can be rinsed away.

Now check the statements that describe ethanoic acid incorrectly. Ethanoic acid is a weak acid, so it does not fully split into ions in water. Also, it is not acting as a strong oxidizing agent in this situation—its key role is donating $H^{+}$ in an acid-carbonate reaction.

One option suggests an insoluble precipitate is produced. If that happened, you would still be left with a solid coating (just a different one) stuck to the kettle. Since vinegar is known to remove limescale, the product salt must dissolve rather than form an insoluble solid.

The correct choice is the one that fits the acid + carbonate pattern and explains the fizzing and removal of the deposit: the reaction makes a soluble salt, water, and carbon dioxide.

We are dealing with ethanoic acid ($C{H}_{3}COOH$) and calcium carbonate ($CaC{O}_3$). An acid reacting with a metal carbonate follows a reliable rule:

$\text{acid}+\text{metal carbonate}\to \text{salt}+\text{water}+\text{carbon dioxide}$

This is the expected acid-base reaction type for carbonates.

Ethanoic acid is a carboxylic acid and is classified as a weak acid, meaning it only partially ionizes in water. So the statement claiming it completely dissociates (as a strong acid would) is not true for ethanoic acid.

In an acid-carbonate reaction, $H^{+}$ from the acid reacts with $C{O}_3^{2-}$ to form water and carbon dioxide. This is not an electron-transfer process, so it is not a redox reaction. Therefore, ethanoic acid is not functioning as a strong oxidizing agent here.

For limescale to be removed, the solid $CaC{O}_3$ must be turned into something that can be washed away. If an insoluble precipitate formed, a solid would remain on the surface and the kettle would not be effectively cleaned.

After ruling out the incorrect claims, the remaining correct description is that ethanoic acid reacts with calcium carbonate to form calcium ethanoate (soluble), water, and carbon dioxide (the bubbles). The balanced equation is:

$2C{H}_{3}COOH(aq)+CaC{O}_3(s)\to (C{H}_{3}COO{)}_{2}Ca(aq)+H_{2}O(l)+C{O}_2(g)$

Because the salt is soluble, the limescale dissolves and can be rinsed away.

You’re being asked to name the scientific word for the ability of acids to chemically attack and break down materials such as metals, stone, and body tissue. This is a well-known property discussed in Topic 3: Types of chemical reaction when learning about acids and bases.

A neutral substance has a pH of 7 (for example, pure water). Neutral substances typically do not react strongly with materials and are not described as "eating away" metals or tissue, so this option doesn’t match.

Alkaline describes a base in water (pH greater than 7). Even though some bases can be hazardous, the word “alkaline” mainly tells you about pH, not specifically about damaging metals or stone. Soapy describes how many alkalis feel (slippery), which is a touch/texture clue rather than the idea of chemically destroying materials.

A substance that causes chemical destruction or permanent damage to things like metal or skin is described as corrosive. That word directly fits the description given for acids in the question.

The prompt is looking for a term used for substances that can "damage or 'eat away' materials" including metals and living tissue. This is one of the main characteristics of acids covered in Topic: Acids and bases.

Neutral solutions (like pure water) have a pH of 7. Because they are not strongly reactive, they wouldn’t normally be described as wearing away metals or stone, so this choice cannot be correct.

Alkaline means basic, which is the opposite of acidic. The question is asking for a word that describes acids, so an option meaning “base” does not fit.

A soapy or slippery feel is associated with bases/alkalis, not acids. Acids don’t have that characteristic texture (even though they can still harm skin), so this option should be eliminated.

Once the other choices are removed, the remaining term is corrosive. This word means a substance can chemically break down and damage solids through reactions—exactly what acids can do to metals, stone, and skin.

The clue is that the substance must be able to neutralizing excess acidity. To chemically reduce acidity, you need a base/alkali. Using an acid would not fix the problem—it would push the pH even lower (more acidic).

Check the first three substances:

• Ethanoic acid ($C{H}_{3}COOH$) is the acid present in vinegar.
• Sulfuric acid ($H_{2}S{O}_4$) is a very strong acid used in industry.
• Citric acid is an acid naturally found in lemons and other citrus fruits.

Because each of these is an acid, none of them can neutralize an already acidic solution in water treatment.

Sodium hydroxide ($NaOH$), often called caustic soda, is a strong alkali that matches both uses described:

1. Paper Industry: It is used during chemical processing of wood (Kraft pulping) to help remove lignin, which holds wood fibers together.
2. Water Treatment: It is commonly added to increase the $pH$ of water, neutralizing extra acidity and helping reduce corrosion in metal pipes.

Since it is a strong base and is specifically used in pulping and in controlling water $pH$, sodium hydroxide is the correct choice.

The task is neutralizing excess acidity. Neutralization happens when an acid reacts with a base, which is a key idea in the Types of chemical reaction: Acids and bases topic.

Look at the first three options—each is an acid:

• Ethanoic acid is a weak organic acid.
• Sulfuric acid is a strong mineral acid.
• Citric acid is a weak organic acid found in citrus fruits.

Putting any of these into an acidic sample would increase the amount of acid present, not neutralize it. So options A, B, and C can be eliminated.

Sodium hydroxide ($NaOH$) is a strong base (alkali). In acidic water, it provides hydroxide ions ($O{H}^{-}$), which combine with hydrogen ions ($H^{+}$) to form water ($H_{2}O$):

$H^{+}(aq)+O{H}^{-}(aq)\to H_{2}O(l)$

As $H^{+}$ ions are removed, the pH rises toward neutral (pH 7).

After eliminating the acids, sodium hydroxide is the only substance left that can carry out neutralization. It is also widely used in paper production (lignin removal) and in water treatment plants to adjust pH.