What makes reactions faster or slower

For a concentration-versus-time graph, the key information is the gradient (slope) at any point. That slope tells you the rate of reaction at that moment.

As time passes, the reaction rate often changes. We describe rate as how much concentration changes per unit time:

$\text{Rate}=\left|\frac{\Delta \text{Concentration}}{\Delta \text{time}}\right|$

If the curve actually crossed the x-axis, that would mean the concentration has become negative, which cannot happen in a real chemical system. A reactant might decrease to zero in a reaction that fully uses it up, but we don’t interpret “crossing the axis” as the sign that a reaction has ended.

A straight line with a constant positive slope shows the concentration is increasing at a steady, non-zero rate. Because the slope is not zero, the reaction is still happening and substances are still being formed, so it cannot represent completion.

The maximum gradient is when the reaction is going fastest. That usually occurs right at the start ($t=0$), when reactant concentrations are highest. So the steepest slope points to the beginning (highest rate), not to the reaction being finished.

When a reaction is complete or has reached dynamic equilibrium, the concentrations stop changing with time.

On the graph, this appears as a horizontal segment. Along a horizontal line, any two points have the same concentration value ($y$). To check what that means for rate, consider a line staying at $5{\text{ mol dm}}^{-3}$ from $t=2$ to $t=10$ and apply the gradient idea.

A horizontal line has a gradient of $0$. That means the reaction rate is zero ($0{\text{ mol dm}}^{-3}{\text{ s}}^{-1}$), showing the concentrations are no longer changing—so the reaction has reached completion or equilibrium. Therefore, the correct observation is the curve flattening out.

On a concentration-versus-time graph, the y-axis shows concentration ($[C]$) and the x-axis shows time ($t$). The gradient (slope) at a point represents the rate of reaction.

$\text{Rate}=\frac{\Delta [\text{Concentration}]}{\Delta t}$

Once a reaction is complete, the limiting reactant has been fully used up, so concentrations stop changing. That makes the reaction rate zero.

If the curve crossed the x-axis, the graph would be showing a negative concentration, which is not physically possible. A concentration can fall to zero (touch the axis), but moving below it is not a realistic outcome for a chemical concentration.

A straight line with a constant positive slope indicates the concentration keeps increasing at a constant rate. If the reaction were finished, the concentration would stay fixed rather than continue rising.

The maximum gradient corresponds to the greatest reaction rate. By Collision Theory, the rate is typically highest at the start (initial rate) because reactant concentration is greatest then. So a maximum slope signals the reaction is fastest, not that it has ended.

To identify when the reaction stops changing, we look for where the gradient becomes zero. You can picture this clearly by imagining what the curve looks like when it stops rising or falling.

When the curve levels off into a horizontal line, its gradient is zero ($m=0$). That means concentration is constant over time, which is the clearest evidence the reaction has reached completion.

To judge whether the statement is correct, we need the standard maths-based meanings used for Chemical reaction kinetics rates. The claim compares finding a reaction rate at an exact instant with finding a rate across a time interval.

The instantaneous rate describes how fast the reaction is going at one exact moment. On a concentration–time curve, we determine it by drawing a tangent that touches the curve at that single time. The slope of that tangent line gives the instantaneous rate.

${\text{Rate}}_{inst}=\text{gradient of tangent at }t$

The average rate looks at how much the concentration changes during a chosen time span ($\Delta t$). On the graph, we connect the two times with a straight line (a secant line). The slope of that secant line represents the average rate for that interval.

$\text{Average Rate}=\frac{\Delta [\text{Concentration}]}{\Delta t}$

Because the statement matches the accepted definitions (tangent for an instant, secant for an interval), the option saying the statement is incorrect cannot be right.

The statement correctly describes how instantaneous and average rates are found from concentration–time graphs, so the correct answer is True.

To decide if the statement is true, focus on how slopes on a concentration–time graph are used in Chemical reaction kinetics rates.

We are contrasting:

1. Instantaneous rate: the rate at one exact time.
2. Average rate: the rate across a time interval.

Think of a graph with concentration on the $y$-axis and time on the $x$-axis.

If you want the rate at exactly one time (for example, $t=5$ s), you draw a tangent line at that point. It touches the curve at that single point and follows its direction there.

$\text{Instantaneous Rate}=\text{Slope of the tangent line at a single point}$

If instead you want the rate between two times (for example, from $t=2$ s to $t=8$ s), you mark those two points and draw the straight line that connects them. That connecting line is a secant line.

Since it uses two points, it represents change over a span of time.

$\text{Average Rate}=\frac{\Delta [\text{Concentration}]}{\Delta t}=\text{Slope of the secant line}$

Using the graph meanings:

• Option B would mean the tangent/secant descriptions are wrong, but the graph interpretation confirms tangent = single instant and secant = interval.
• Option A matches these standard graphical definitions.

The statement correctly describes the graph-based methods for determining reaction rates, so the correct answer is True.

To answer this, start by remembering what electrical conductivity refers to in chemistry. In water-based solutions, an electric current can pass only if there are charged particles that can move around.

Those moving charged particles are ions. If a solution contains essentially no ions, it will conduct very poorly. So a conductivity meter is really sensing how easily current can flow because of the ions present.

A conductivity meter works because the current it detects increases when:

1. There are more ions in solution (higher concentration), and
2. Those ions can move effectively (their mobility depends on charge and speed in solution).

During a reaction, reactants may be converted into products that have a different number of ions in solution. As that happens, the conductivity will rise or fall. Measuring how quickly it changes lets you judge the reaction rate by following how fast the ion levels are changing.

Now compare the other choices with the tools that actually measure them:

• Option A: Changes in light absorption are measured with a colorimeter or spectrophotometer, not with a conductivity meter.
• Option C: Gas volume is typically collected and measured using a gas syringe or by displacement of water.
• Option D: Mass changes are found using an electronic balance.

Because a conductivity meter responds to the presence and amount of mobile ions carrying charge, it is used to follow the change in the concentration of ions as a reaction proceeds. That matches Option B.

Begin with what the instrument measures: a conductivity meter tells you how well an aqueous solution conducts electricity. In water, conduction happens because mobile ions move and carry charge, so conductivity is tied to the overall ion concentration and the charges of the ions present.

Option A is about how much light a solution absorbs. That is monitored using colorimetry/spectrophotometry, which is useful when a substance changes color or intensity. It is not what a conductivity meter measures.

Option C focuses on the volume of gas produced. That can show reaction rate, but it requires collecting gas with equipment like a gas syringe or a setup that measures displaced water. Conductivity equipment isn’t used for gas volume.

Option D refers to tracking mass of the flask and contents. This is done on an electronic balance, often when a gas escapes and the mass decreases. That’s a mass measurement, not an electrical one.

After eliminating the other measurement types, Option B remains. Since conductivity depends on mobile ions, a conductivity meter is ideal for following reactions where the concentration of ions changes over time (for example, precipitation or neutralization).