# Question 5:

## Part (a)
| Answer | Mark | Guidance |
|--------|------|----------|
| Time taken depends upon temperature | B1 | OE; **not** $x$ set values |

## Part (b)
| Answer | Mark | Guidance |
|--------|------|----------|
| $b$ (gradient) $= -0.0873$ to $-0.087$ | B2 | AWFW; $(-0.0872\dot{7})$ |
| $b$ (gradient) $= -0.09$ to $-0.08$ | (B1) | AWFW; $-8.73^{-02} \Rightarrow$ B0 |
| $a$ (intercept) $= 5.94$ to $5.96$ | B2 | AWFW; $(5.950\dot{9})$ |
| $a$ (intercept) $= 5.6$ to $6.1$ | (B1) | AWFW |
| Attempt at $\sum x$, $\sum x^2$, $\sum y$ and $\sum xy$ **or** attempt at $S_{xx}$ and $S_{xy}$ | (M1) | 396, 16016, 30.9 and 958.8; 1760 and $-153.6$ |
| Attempt at correct formula for $b$ | (m1) | — |
| $b = -0.0873$ to $-0.087$ | (A1) | AWFW |
| $a = 5.94$ to $5.96$ | (A1) | AWFW |

## Part (c)(i)
| Answer | Mark | Guidance |
|--------|------|----------|
| Each $1°C$ rise in temperature results in an (average) decrease of $0.087$ m (5 s) in time taken for pellets to dissolve | B1 B1 | Quantified rise in $x$ (results in) decrease in $y$; OE |

## Part (c)(ii)
| Answer | Mark | Guidance |
|--------|------|----------|
| $a$ is $y$-value at $x = 0$ at which water is solid/ice/frozen so pellets cannot dissolve | B1 B1 | Indication that it is $y$ at $x = 0$; Mention of solid or ice or frozen |

## Part (d)(i)
| Answer | Mark | Guidance |
|--------|------|----------|
| When $x = 30$: $y = 3.3$ to $3.4$ | B2 | AWFW; $(3.332\dot{7})$ |
| $y = 2.9$ to $3.7$ | (B1) | AWFW |
| If B0, use of their equation with $x = 30$ | (M1) | — |

## Part (d)(ii)
| Answer | Mark | Guidance |
|--------|------|----------|
| When $x = 75$: $y < 0$ or negative | B1 | OE |
| which is impossible | ↑Dep↑ B1 | OE; **not** extrapolation |

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